1 /*-------------------------------------------------------------------------
4 * PostgreSQL multi-transaction-log manager
6 * The pg_multixact manager is a pg_xact-like manager that stores an array of
7 * MultiXactMember for each MultiXactId. It is a fundamental part of the
8 * shared-row-lock implementation. Each MultiXactMember is comprised of a
9 * TransactionId and a set of flag bits. The name is a bit historical:
10 * originally, a MultiXactId consisted of more than one TransactionId (except
11 * in rare corner cases), hence "multi". Nowadays, however, it's perfectly
12 * legitimate to have MultiXactIds that only include a single Xid.
14 * The meaning of the flag bits is opaque to this module, but they are mostly
15 * used in heapam.c to identify lock modes that each of the member transactions
16 * is holding on any given tuple. This module just contains support to store
17 * and retrieve the arrays.
19 * We use two SLRU areas, one for storing the offsets at which the data
20 * starts for each MultiXactId in the other one. This trick allows us to
21 * store variable length arrays of TransactionIds. (We could alternatively
22 * use one area containing counts and TransactionIds, with valid MultiXactId
23 * values pointing at slots containing counts; but that way seems less robust
24 * since it would get completely confused if someone inquired about a bogus
25 * MultiXactId that pointed to an intermediate slot containing an XID.)
27 * XLOG interactions: this module generates a record whenever a new OFFSETs or
28 * MEMBERs page is initialized to zeroes, as well as an
29 * XLOG_MULTIXACT_CREATE_ID record whenever a new MultiXactId is defined.
30 * This module ignores the WAL rule "write xlog before data," because it
31 * suffices that actions recording a MultiXactId in a heap xmax do follow that
32 * rule. The only way for the MXID to be referenced from any data page is for
33 * heap_lock_tuple() or heap_update() to have put it there, and each generates
34 * an XLOG record that must follow ours. The normal LSN interlock between the
35 * data page and that XLOG record will ensure that our XLOG record reaches
36 * disk first. If the SLRU members/offsets data reaches disk sooner than the
37 * XLOG records, we do not care; after recovery, no xmax will refer to it. On
38 * the flip side, to ensure that all referenced entries _do_ reach disk, this
39 * module's XLOG records completely rebuild the data entered since the last
40 * checkpoint. We flush and sync all dirty OFFSETs and MEMBERs pages to disk
41 * before each checkpoint is considered complete.
43 * Like clog.c, and unlike subtrans.c, we have to preserve state across
44 * crashes and ensure that MXID and offset numbering increases monotonically
45 * across a crash. We do this in the same way as it's done for transaction
46 * IDs: the WAL record is guaranteed to contain evidence of every MXID we
47 * could need to worry about, and we just make sure that at the end of
48 * replay, the next-MXID and next-offset counters are at least as large as
49 * anything we saw during replay.
51 * We are able to remove segments no longer necessary by carefully tracking
52 * each table's used values: during vacuum, any multixact older than a certain
53 * value is removed; the cutoff value is stored in pg_class. The minimum value
54 * across all tables in each database is stored in pg_database, and the global
55 * minimum across all databases is part of pg_control and is kept in shared
56 * memory. Whenever that minimum is advanced, the SLRUs are truncated.
58 * When new multixactid values are to be created, care is taken that the
59 * counter does not fall within the wraparound horizon considering the global
62 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
63 * Portions Copyright (c) 1994, Regents of the University of California
65 * src/backend/access/transam/multixact.c
67 *-------------------------------------------------------------------------
71 #include "access/multixact.h"
72 #include "access/slru.h"
73 #include "access/transam.h"
74 #include "access/twophase.h"
75 #include "access/twophase_rmgr.h"
76 #include "access/xact.h"
77 #include "access/xlog.h"
78 #include "access/xloginsert.h"
79 #include "catalog/pg_type.h"
80 #include "commands/dbcommands.h"
82 #include "lib/ilist.h"
83 #include "miscadmin.h"
85 #include "postmaster/autovacuum.h"
86 #include "storage/lmgr.h"
87 #include "storage/pmsignal.h"
88 #include "storage/proc.h"
89 #include "storage/procarray.h"
90 #include "utils/builtins.h"
91 #include "utils/memutils.h"
92 #include "utils/snapmgr.h"
96 * Defines for MultiXactOffset page sizes. A page is the same BLCKSZ as is
97 * used everywhere else in Postgres.
99 * Note: because MultiXactOffsets are 32 bits and wrap around at 0xFFFFFFFF,
100 * MultiXact page numbering also wraps around at
101 * 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE, and segment numbering at
102 * 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need
103 * take no explicit notice of that fact in this module, except when comparing
104 * segment and page numbers in TruncateMultiXact (see
105 * MultiXactOffsetPagePrecedes).
108 /* We need four bytes per offset */
109 #define MULTIXACT_OFFSETS_PER_PAGE (BLCKSZ / sizeof(MultiXactOffset))
111 #define MultiXactIdToOffsetPage(xid) \
112 ((xid) / (MultiXactOffset) MULTIXACT_OFFSETS_PER_PAGE)
113 #define MultiXactIdToOffsetEntry(xid) \
114 ((xid) % (MultiXactOffset) MULTIXACT_OFFSETS_PER_PAGE)
115 #define MultiXactIdToOffsetSegment(xid) (MultiXactIdToOffsetPage(xid) / SLRU_PAGES_PER_SEGMENT)
118 * The situation for members is a bit more complex: we store one byte of
119 * additional flag bits for each TransactionId. To do this without getting
120 * into alignment issues, we store four bytes of flags, and then the
121 * corresponding 4 Xids. Each such 5-word (20-byte) set we call a "group", and
122 * are stored as a whole in pages. Thus, with 8kB BLCKSZ, we keep 409 groups
123 * per page. This wastes 12 bytes per page, but that's OK -- simplicity (and
124 * performance) trumps space efficiency here.
126 * Note that the "offset" macros work with byte offset, not array indexes, so
127 * arithmetic must be done using "char *" pointers.
129 /* We need eight bits per xact, so one xact fits in a byte */
130 #define MXACT_MEMBER_BITS_PER_XACT 8
131 #define MXACT_MEMBER_FLAGS_PER_BYTE 1
132 #define MXACT_MEMBER_XACT_BITMASK ((1 << MXACT_MEMBER_BITS_PER_XACT) - 1)
134 /* how many full bytes of flags are there in a group? */
135 #define MULTIXACT_FLAGBYTES_PER_GROUP 4
136 #define MULTIXACT_MEMBERS_PER_MEMBERGROUP \
137 (MULTIXACT_FLAGBYTES_PER_GROUP * MXACT_MEMBER_FLAGS_PER_BYTE)
138 /* size in bytes of a complete group */
139 #define MULTIXACT_MEMBERGROUP_SIZE \
140 (sizeof(TransactionId) * MULTIXACT_MEMBERS_PER_MEMBERGROUP + MULTIXACT_FLAGBYTES_PER_GROUP)
141 #define MULTIXACT_MEMBERGROUPS_PER_PAGE (BLCKSZ / MULTIXACT_MEMBERGROUP_SIZE)
142 #define MULTIXACT_MEMBERS_PER_PAGE \
143 (MULTIXACT_MEMBERGROUPS_PER_PAGE * MULTIXACT_MEMBERS_PER_MEMBERGROUP)
146 * Because the number of items per page is not a divisor of the last item
147 * number (member 0xFFFFFFFF), the last segment does not use the maximum number
148 * of pages, and moreover the last used page therein does not use the same
149 * number of items as previous pages. (Another way to say it is that the
150 * 0xFFFFFFFF member is somewhere in the middle of the last page, so the page
151 * has some empty space after that item.)
153 * This constant is the number of members in the last page of the last segment.
155 #define MAX_MEMBERS_IN_LAST_MEMBERS_PAGE \
156 ((uint32) ((0xFFFFFFFF % MULTIXACT_MEMBERS_PER_PAGE) + 1))
158 /* page in which a member is to be found */
159 #define MXOffsetToMemberPage(xid) ((xid) / (TransactionId) MULTIXACT_MEMBERS_PER_PAGE)
160 #define MXOffsetToMemberSegment(xid) (MXOffsetToMemberPage(xid) / SLRU_PAGES_PER_SEGMENT)
162 /* Location (byte offset within page) of flag word for a given member */
163 #define MXOffsetToFlagsOffset(xid) \
164 ((((xid) / (TransactionId) MULTIXACT_MEMBERS_PER_MEMBERGROUP) % \
165 (TransactionId) MULTIXACT_MEMBERGROUPS_PER_PAGE) * \
166 (TransactionId) MULTIXACT_MEMBERGROUP_SIZE)
167 #define MXOffsetToFlagsBitShift(xid) \
168 (((xid) % (TransactionId) MULTIXACT_MEMBERS_PER_MEMBERGROUP) * \
169 MXACT_MEMBER_BITS_PER_XACT)
171 /* Location (byte offset within page) of TransactionId of given member */
172 #define MXOffsetToMemberOffset(xid) \
173 (MXOffsetToFlagsOffset(xid) + MULTIXACT_FLAGBYTES_PER_GROUP + \
174 ((xid) % MULTIXACT_MEMBERS_PER_MEMBERGROUP) * sizeof(TransactionId))
176 /* Multixact members wraparound thresholds. */
177 #define MULTIXACT_MEMBER_SAFE_THRESHOLD (MaxMultiXactOffset / 2)
178 #define MULTIXACT_MEMBER_DANGER_THRESHOLD \
179 (MaxMultiXactOffset - MaxMultiXactOffset / 4)
181 #define PreviousMultiXactId(xid) \
182 ((xid) == FirstMultiXactId ? MaxMultiXactId : (xid) - 1)
185 * Links to shared-memory data structures for MultiXact control
187 static SlruCtlData MultiXactOffsetCtlData;
188 static SlruCtlData MultiXactMemberCtlData;
190 #define MultiXactOffsetCtl (&MultiXactOffsetCtlData)
191 #define MultiXactMemberCtl (&MultiXactMemberCtlData)
194 * MultiXact state shared across all backends. All this state is protected
195 * by MultiXactGenLock. (We also use MultiXactOffsetControlLock and
196 * MultiXactMemberControlLock to guard accesses to the two sets of SLRU
197 * buffers. For concurrency's sake, we avoid holding more than one of these
200 typedef struct MultiXactStateData
202 /* next-to-be-assigned MultiXactId */
203 MultiXactId nextMXact;
205 /* next-to-be-assigned offset */
206 MultiXactOffset nextOffset;
208 /* Have we completed multixact startup? */
209 bool finishedStartup;
212 * Oldest multixact that is still potentially referenced by a relation.
213 * Anything older than this should not be consulted. These values are
216 MultiXactId oldestMultiXactId;
217 Oid oldestMultiXactDB;
220 * Oldest multixact offset that is potentially referenced by a multixact
221 * referenced by a relation. We don't always know this value, so there's
222 * a flag here to indicate whether or not we currently do.
224 MultiXactOffset oldestOffset;
225 bool oldestOffsetKnown;
227 /* support for anti-wraparound measures */
228 MultiXactId multiVacLimit;
229 MultiXactId multiWarnLimit;
230 MultiXactId multiStopLimit;
231 MultiXactId multiWrapLimit;
233 /* support for members anti-wraparound measures */
234 MultiXactOffset offsetStopLimit; /* known if oldestOffsetKnown */
237 * Per-backend data starts here. We have two arrays stored in the area
238 * immediately following the MultiXactStateData struct. Each is indexed by
241 * In both arrays, there's a slot for all normal backends (1..MaxBackends)
242 * followed by a slot for max_prepared_xacts prepared transactions. Valid
243 * BackendIds start from 1; element zero of each array is never used.
245 * OldestMemberMXactId[k] is the oldest MultiXactId each backend's current
246 * transaction(s) could possibly be a member of, or InvalidMultiXactId
247 * when the backend has no live transaction that could possibly be a
248 * member of a MultiXact. Each backend sets its entry to the current
249 * nextMXact counter just before first acquiring a shared lock in a given
250 * transaction, and clears it at transaction end. (This works because only
251 * during or after acquiring a shared lock could an XID possibly become a
252 * member of a MultiXact, and that MultiXact would have to be created
253 * during or after the lock acquisition.)
255 * OldestVisibleMXactId[k] is the oldest MultiXactId each backend's
256 * current transaction(s) think is potentially live, or InvalidMultiXactId
257 * when not in a transaction or not in a transaction that's paid any
258 * attention to MultiXacts yet. This is computed when first needed in a
259 * given transaction, and cleared at transaction end. We can compute it
260 * as the minimum of the valid OldestMemberMXactId[] entries at the time
261 * we compute it (using nextMXact if none are valid). Each backend is
262 * required not to attempt to access any SLRU data for MultiXactIds older
263 * than its own OldestVisibleMXactId[] setting; this is necessary because
264 * the checkpointer could truncate away such data at any instant.
266 * The oldest valid value among all of the OldestMemberMXactId[] and
267 * OldestVisibleMXactId[] entries is considered by vacuum as the earliest
268 * possible value still having any live member transaction. Subtracting
269 * vacuum_multixact_freeze_min_age from that value we obtain the freezing
270 * point for multixacts for that table. Any value older than that is
271 * removed from tuple headers (or "frozen"; see FreezeMultiXactId. Note
272 * that multis that have member xids that are older than the cutoff point
273 * for xids must also be frozen, even if the multis themselves are newer
274 * than the multixid cutoff point). Whenever a full table vacuum happens,
275 * the freezing point so computed is used as the new pg_class.relminmxid
276 * value. The minimum of all those values in a database is stored as
277 * pg_database.datminmxid. In turn, the minimum of all of those values is
278 * stored in pg_control and used as truncation point for pg_multixact. At
279 * checkpoint or restartpoint, unneeded segments are removed.
281 MultiXactId perBackendXactIds[FLEXIBLE_ARRAY_MEMBER];
282 } MultiXactStateData;
285 * Last element of OldestMemberMXactID and OldestVisibleMXactId arrays.
286 * Valid elements are (1..MaxOldestSlot); element 0 is never used.
288 #define MaxOldestSlot (MaxBackends + max_prepared_xacts)
290 /* Pointers to the state data in shared memory */
291 static MultiXactStateData *MultiXactState;
292 static MultiXactId *OldestMemberMXactId;
293 static MultiXactId *OldestVisibleMXactId;
297 * Definitions for the backend-local MultiXactId cache.
299 * We use this cache to store known MultiXacts, so we don't need to go to
300 * SLRU areas every time.
302 * The cache lasts for the duration of a single transaction, the rationale
303 * for this being that most entries will contain our own TransactionId and
304 * so they will be uninteresting by the time our next transaction starts.
305 * (XXX not clear that this is correct --- other members of the MultiXact
306 * could hang around longer than we did. However, it's not clear what a
307 * better policy for flushing old cache entries would be.) FIXME actually
308 * this is plain wrong now that multixact's may contain update Xids.
310 * We allocate the cache entries in a memory context that is deleted at
311 * transaction end, so we don't need to do retail freeing of entries.
313 typedef struct mXactCacheEnt
318 MultiXactMember members[FLEXIBLE_ARRAY_MEMBER];
321 #define MAX_CACHE_ENTRIES 256
322 static dlist_head MXactCache = DLIST_STATIC_INIT(MXactCache);
323 static int MXactCacheMembers = 0;
324 static MemoryContext MXactContext = NULL;
326 #ifdef MULTIXACT_DEBUG
327 #define debug_elog2(a,b) elog(a,b)
328 #define debug_elog3(a,b,c) elog(a,b,c)
329 #define debug_elog4(a,b,c,d) elog(a,b,c,d)
330 #define debug_elog5(a,b,c,d,e) elog(a,b,c,d,e)
331 #define debug_elog6(a,b,c,d,e,f) elog(a,b,c,d,e,f)
333 #define debug_elog2(a,b)
334 #define debug_elog3(a,b,c)
335 #define debug_elog4(a,b,c,d)
336 #define debug_elog5(a,b,c,d,e)
337 #define debug_elog6(a,b,c,d,e,f)
340 /* internal MultiXactId management */
341 static void MultiXactIdSetOldestVisible(void);
342 static void RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
343 int nmembers, MultiXactMember *members);
344 static MultiXactId GetNewMultiXactId(int nmembers, MultiXactOffset *offset);
346 /* MultiXact cache management */
347 static int mxactMemberComparator(const void *arg1, const void *arg2);
348 static MultiXactId mXactCacheGetBySet(int nmembers, MultiXactMember *members);
349 static int mXactCacheGetById(MultiXactId multi, MultiXactMember **members);
350 static void mXactCachePut(MultiXactId multi, int nmembers,
351 MultiXactMember *members);
353 static char *mxstatus_to_string(MultiXactStatus status);
355 /* management of SLRU infrastructure */
356 static int ZeroMultiXactOffsetPage(int pageno, bool writeXlog);
357 static int ZeroMultiXactMemberPage(int pageno, bool writeXlog);
358 static bool MultiXactOffsetPagePrecedes(int page1, int page2);
359 static bool MultiXactMemberPagePrecedes(int page1, int page2);
360 static bool MultiXactOffsetPrecedes(MultiXactOffset offset1,
361 MultiXactOffset offset2);
362 static void ExtendMultiXactOffset(MultiXactId multi);
363 static void ExtendMultiXactMember(MultiXactOffset offset, int nmembers);
364 static bool MultiXactOffsetWouldWrap(MultiXactOffset boundary,
365 MultiXactOffset start, uint32 distance);
366 static bool SetOffsetVacuumLimit(bool is_startup);
367 static bool find_multixact_start(MultiXactId multi, MultiXactOffset *result);
368 static void WriteMZeroPageXlogRec(int pageno, uint8 info);
369 static void WriteMTruncateXlogRec(Oid oldestMultiDB,
370 MultiXactId startOff, MultiXactId endOff,
371 MultiXactOffset startMemb, MultiXactOffset endMemb);
376 * Construct a MultiXactId representing two TransactionIds.
378 * The two XIDs must be different, or be requesting different statuses.
380 * NB - we don't worry about our local MultiXactId cache here, because that
381 * is handled by the lower-level routines.
384 MultiXactIdCreate(TransactionId xid1, MultiXactStatus status1,
385 TransactionId xid2, MultiXactStatus status2)
387 MultiXactId newMulti;
388 MultiXactMember members[2];
390 AssertArg(TransactionIdIsValid(xid1));
391 AssertArg(TransactionIdIsValid(xid2));
393 Assert(!TransactionIdEquals(xid1, xid2) || (status1 != status2));
395 /* MultiXactIdSetOldestMember() must have been called already. */
396 Assert(MultiXactIdIsValid(OldestMemberMXactId[MyBackendId]));
399 * Note: unlike MultiXactIdExpand, we don't bother to check that both XIDs
400 * are still running. In typical usage, xid2 will be our own XID and the
401 * caller just did a check on xid1, so it'd be wasted effort.
404 members[0].xid = xid1;
405 members[0].status = status1;
406 members[1].xid = xid2;
407 members[1].status = status2;
409 newMulti = MultiXactIdCreateFromMembers(2, members);
411 debug_elog3(DEBUG2, "Create: %s",
412 mxid_to_string(newMulti, 2, members));
419 * Add a TransactionId to a pre-existing MultiXactId.
421 * If the TransactionId is already a member of the passed MultiXactId with the
422 * same status, just return it as-is.
424 * Note that we do NOT actually modify the membership of a pre-existing
425 * MultiXactId; instead we create a new one. This is necessary to avoid
426 * a race condition against code trying to wait for one MultiXactId to finish;
427 * see notes in heapam.c.
429 * NB - we don't worry about our local MultiXactId cache here, because that
430 * is handled by the lower-level routines.
432 * Note: It is critical that MultiXactIds that come from an old cluster (i.e.
433 * one upgraded by pg_upgrade from a cluster older than this feature) are not
437 MultiXactIdExpand(MultiXactId multi, TransactionId xid, MultiXactStatus status)
439 MultiXactId newMulti;
440 MultiXactMember *members;
441 MultiXactMember *newMembers;
446 AssertArg(MultiXactIdIsValid(multi));
447 AssertArg(TransactionIdIsValid(xid));
449 /* MultiXactIdSetOldestMember() must have been called already. */
450 Assert(MultiXactIdIsValid(OldestMemberMXactId[MyBackendId]));
452 debug_elog5(DEBUG2, "Expand: received multi %u, xid %u status %s",
453 multi, xid, mxstatus_to_string(status));
456 * Note: we don't allow for old multis here. The reason is that the only
457 * caller of this function does a check that the multixact is no longer
460 nmembers = GetMultiXactIdMembers(multi, &members, false, false);
464 MultiXactMember member;
467 * The MultiXactId is obsolete. This can only happen if all the
468 * MultiXactId members stop running between the caller checking and
469 * passing it to us. It would be better to return that fact to the
470 * caller, but it would complicate the API and it's unlikely to happen
471 * too often, so just deal with it by creating a singleton MultiXact.
474 member.status = status;
475 newMulti = MultiXactIdCreateFromMembers(1, &member);
477 debug_elog4(DEBUG2, "Expand: %u has no members, create singleton %u",
483 * If the TransactionId is already a member of the MultiXactId with the
484 * same status, just return the existing MultiXactId.
486 for (i = 0; i < nmembers; i++)
488 if (TransactionIdEquals(members[i].xid, xid) &&
489 (members[i].status == status))
491 debug_elog4(DEBUG2, "Expand: %u is already a member of %u",
499 * Determine which of the members of the MultiXactId are still of
500 * interest. This is any running transaction, and also any transaction
501 * that grabbed something stronger than just a lock and was committed. (An
502 * update that aborted is of no interest here; and having more than one
503 * update Xid in a multixact would cause errors elsewhere.)
505 * Removing dead members is not just an optimization: freezing of tuples
506 * whose Xmax are multis depends on this behavior.
508 * Note we have the same race condition here as above: j could be 0 at the
511 newMembers = (MultiXactMember *)
512 palloc(sizeof(MultiXactMember) * (nmembers + 1));
514 for (i = 0, j = 0; i < nmembers; i++)
516 if (TransactionIdIsInProgress(members[i].xid) ||
517 (ISUPDATE_from_mxstatus(members[i].status) &&
518 TransactionIdDidCommit(members[i].xid)))
520 newMembers[j].xid = members[i].xid;
521 newMembers[j++].status = members[i].status;
525 newMembers[j].xid = xid;
526 newMembers[j++].status = status;
527 newMulti = MultiXactIdCreateFromMembers(j, newMembers);
532 debug_elog3(DEBUG2, "Expand: returning new multi %u", newMulti);
538 * MultiXactIdIsRunning
539 * Returns whether a MultiXactId is "running".
541 * We return true if at least one member of the given MultiXactId is still
542 * running. Note that a "false" result is certain not to change,
543 * because it is not legal to add members to an existing MultiXactId.
545 * Caller is expected to have verified that the multixact does not come from
546 * a pg_upgraded share-locked tuple.
549 MultiXactIdIsRunning(MultiXactId multi, bool isLockOnly)
551 MultiXactMember *members;
555 debug_elog3(DEBUG2, "IsRunning %u?", multi);
558 * "false" here means we assume our callers have checked that the given
559 * multi cannot possibly come from a pg_upgraded database.
561 nmembers = GetMultiXactIdMembers(multi, &members, false, isLockOnly);
565 debug_elog2(DEBUG2, "IsRunning: no members");
570 * Checking for myself is cheap compared to looking in shared memory;
571 * return true if any live subtransaction of the current top-level
572 * transaction is a member.
574 * This is not needed for correctness, it's just a fast path.
576 for (i = 0; i < nmembers; i++)
578 if (TransactionIdIsCurrentTransactionId(members[i].xid))
580 debug_elog3(DEBUG2, "IsRunning: I (%d) am running!", i);
587 * This could be made faster by having another entry point in procarray.c,
588 * walking the PGPROC array only once for all the members. But in most
589 * cases nmembers should be small enough that it doesn't much matter.
591 for (i = 0; i < nmembers; i++)
593 if (TransactionIdIsInProgress(members[i].xid))
595 debug_elog4(DEBUG2, "IsRunning: member %d (%u) is running",
604 debug_elog3(DEBUG2, "IsRunning: %u is not running", multi);
610 * MultiXactIdSetOldestMember
611 * Save the oldest MultiXactId this transaction could be a member of.
613 * We set the OldestMemberMXactId for a given transaction the first time it's
614 * going to do some operation that might require a MultiXactId (tuple lock,
615 * update or delete). We need to do this even if we end up using a
616 * TransactionId instead of a MultiXactId, because there is a chance that
617 * another transaction would add our XID to a MultiXactId.
619 * The value to set is the next-to-be-assigned MultiXactId, so this is meant to
620 * be called just before doing any such possibly-MultiXactId-able operation.
623 MultiXactIdSetOldestMember(void)
625 if (!MultiXactIdIsValid(OldestMemberMXactId[MyBackendId]))
627 MultiXactId nextMXact;
630 * You might think we don't need to acquire a lock here, since
631 * fetching and storing of TransactionIds is probably atomic, but in
632 * fact we do: suppose we pick up nextMXact and then lose the CPU for
633 * a long time. Someone else could advance nextMXact, and then
634 * another someone else could compute an OldestVisibleMXactId that
635 * would be after the value we are going to store when we get control
636 * back. Which would be wrong.
638 * Note that a shared lock is sufficient, because it's enough to stop
639 * someone from advancing nextMXact; and nobody else could be trying
640 * to write to our OldestMember entry, only reading (and we assume
641 * storing it is atomic.)
643 LWLockAcquire(MultiXactGenLock, LW_SHARED);
646 * We have to beware of the possibility that nextMXact is in the
647 * wrapped-around state. We don't fix the counter itself here, but we
648 * must be sure to store a valid value in our array entry.
650 nextMXact = MultiXactState->nextMXact;
651 if (nextMXact < FirstMultiXactId)
652 nextMXact = FirstMultiXactId;
654 OldestMemberMXactId[MyBackendId] = nextMXact;
656 LWLockRelease(MultiXactGenLock);
658 debug_elog4(DEBUG2, "MultiXact: setting OldestMember[%d] = %u",
659 MyBackendId, nextMXact);
664 * MultiXactIdSetOldestVisible
665 * Save the oldest MultiXactId this transaction considers possibly live.
667 * We set the OldestVisibleMXactId for a given transaction the first time
668 * it's going to inspect any MultiXactId. Once we have set this, we are
669 * guaranteed that the checkpointer won't truncate off SLRU data for
670 * MultiXactIds at or after our OldestVisibleMXactId.
672 * The value to set is the oldest of nextMXact and all the valid per-backend
673 * OldestMemberMXactId[] entries. Because of the locking we do, we can be
674 * certain that no subsequent call to MultiXactIdSetOldestMember can set
675 * an OldestMemberMXactId[] entry older than what we compute here. Therefore
676 * there is no live transaction, now or later, that can be a member of any
677 * MultiXactId older than the OldestVisibleMXactId we compute here.
680 MultiXactIdSetOldestVisible(void)
682 if (!MultiXactIdIsValid(OldestVisibleMXactId[MyBackendId]))
684 MultiXactId oldestMXact;
687 LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
690 * We have to beware of the possibility that nextMXact is in the
691 * wrapped-around state. We don't fix the counter itself here, but we
692 * must be sure to store a valid value in our array entry.
694 oldestMXact = MultiXactState->nextMXact;
695 if (oldestMXact < FirstMultiXactId)
696 oldestMXact = FirstMultiXactId;
698 for (i = 1; i <= MaxOldestSlot; i++)
700 MultiXactId thisoldest = OldestMemberMXactId[i];
702 if (MultiXactIdIsValid(thisoldest) &&
703 MultiXactIdPrecedes(thisoldest, oldestMXact))
704 oldestMXact = thisoldest;
707 OldestVisibleMXactId[MyBackendId] = oldestMXact;
709 LWLockRelease(MultiXactGenLock);
711 debug_elog4(DEBUG2, "MultiXact: setting OldestVisible[%d] = %u",
712 MyBackendId, oldestMXact);
717 * ReadNextMultiXactId
718 * Return the next MultiXactId to be assigned, but don't allocate it
721 ReadNextMultiXactId(void)
725 /* XXX we could presumably do this without a lock. */
726 LWLockAcquire(MultiXactGenLock, LW_SHARED);
727 mxid = MultiXactState->nextMXact;
728 LWLockRelease(MultiXactGenLock);
730 if (mxid < FirstMultiXactId)
731 mxid = FirstMultiXactId;
737 * MultiXactIdCreateFromMembers
738 * Make a new MultiXactId from the specified set of members
740 * Make XLOG, SLRU and cache entries for a new MultiXactId, recording the
741 * given TransactionIds as members. Returns the newly created MultiXactId.
743 * NB: the passed members[] array will be sorted in-place.
746 MultiXactIdCreateFromMembers(int nmembers, MultiXactMember *members)
749 MultiXactOffset offset;
750 xl_multixact_create xlrec;
752 debug_elog3(DEBUG2, "Create: %s",
753 mxid_to_string(InvalidMultiXactId, nmembers, members));
756 * See if the same set of members already exists in our cache; if so, just
757 * re-use that MultiXactId. (Note: it might seem that looking in our
758 * cache is insufficient, and we ought to search disk to see if a
759 * duplicate definition already exists. But since we only ever create
760 * MultiXacts containing our own XID, in most cases any such MultiXacts
761 * were in fact created by us, and so will be in our cache. There are
762 * corner cases where someone else added us to a MultiXact without our
763 * knowledge, but it's not worth checking for.)
765 multi = mXactCacheGetBySet(nmembers, members);
766 if (MultiXactIdIsValid(multi))
768 debug_elog2(DEBUG2, "Create: in cache!");
772 /* Verify that there is a single update Xid among the given members. */
775 bool has_update = false;
777 for (i = 0; i < nmembers; i++)
779 if (ISUPDATE_from_mxstatus(members[i].status))
782 elog(ERROR, "new multixact has more than one updating member");
789 * Assign the MXID and offsets range to use, and make sure there is space
790 * in the OFFSETs and MEMBERs files. NB: this routine does
791 * START_CRIT_SECTION().
793 * Note: unlike MultiXactIdCreate and MultiXactIdExpand, we do not check
794 * that we've called MultiXactIdSetOldestMember here. This is because
795 * this routine is used in some places to create new MultiXactIds of which
796 * the current backend is not a member, notably during freezing of multis
797 * in vacuum. During vacuum, in particular, it would be unacceptable to
798 * keep OldestMulti set, in case it runs for long.
800 multi = GetNewMultiXactId(nmembers, &offset);
802 /* Make an XLOG entry describing the new MXID. */
805 xlrec.nmembers = nmembers;
808 * XXX Note: there's a lot of padding space in MultiXactMember. We could
809 * find a more compact representation of this Xlog record -- perhaps all
810 * the status flags in one XLogRecData, then all the xids in another one?
811 * Not clear that it's worth the trouble though.
814 XLogRegisterData((char *) (&xlrec), SizeOfMultiXactCreate);
815 XLogRegisterData((char *) members, nmembers * sizeof(MultiXactMember));
817 (void) XLogInsert(RM_MULTIXACT_ID, XLOG_MULTIXACT_CREATE_ID);
819 /* Now enter the information into the OFFSETs and MEMBERs logs */
820 RecordNewMultiXact(multi, offset, nmembers, members);
822 /* Done with critical section */
825 /* Store the new MultiXactId in the local cache, too */
826 mXactCachePut(multi, nmembers, members);
828 debug_elog2(DEBUG2, "Create: all done");
835 * Write info about a new multixact into the offsets and members files
837 * This is broken out of MultiXactIdCreateFromMembers so that xlog replay can
841 RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
842 int nmembers, MultiXactMember *members)
848 MultiXactOffset *offptr;
851 LWLockAcquire(MultiXactOffsetControlLock, LW_EXCLUSIVE);
853 pageno = MultiXactIdToOffsetPage(multi);
854 entryno = MultiXactIdToOffsetEntry(multi);
857 * Note: we pass the MultiXactId to SimpleLruReadPage as the "transaction"
858 * to complain about if there's any I/O error. This is kinda bogus, but
859 * since the errors will always give the full pathname, it should be clear
860 * enough that a MultiXactId is really involved. Perhaps someday we'll
861 * take the trouble to generalize the slru.c error reporting code.
863 slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, multi);
864 offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
869 MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
871 /* Exchange our lock */
872 LWLockRelease(MultiXactOffsetControlLock);
874 LWLockAcquire(MultiXactMemberControlLock, LW_EXCLUSIVE);
878 for (i = 0; i < nmembers; i++, offset++)
880 TransactionId *memberptr;
887 Assert(members[i].status <= MultiXactStatusUpdate);
889 pageno = MXOffsetToMemberPage(offset);
890 memberoff = MXOffsetToMemberOffset(offset);
891 flagsoff = MXOffsetToFlagsOffset(offset);
892 bshift = MXOffsetToFlagsBitShift(offset);
894 if (pageno != prev_pageno)
896 slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true, multi);
897 prev_pageno = pageno;
900 memberptr = (TransactionId *)
901 (MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);
903 *memberptr = members[i].xid;
905 flagsptr = (uint32 *)
906 (MultiXactMemberCtl->shared->page_buffer[slotno] + flagsoff);
908 flagsval = *flagsptr;
909 flagsval &= ~(((1 << MXACT_MEMBER_BITS_PER_XACT) - 1) << bshift);
910 flagsval |= (members[i].status << bshift);
911 *flagsptr = flagsval;
913 MultiXactMemberCtl->shared->page_dirty[slotno] = true;
916 LWLockRelease(MultiXactMemberControlLock);
921 * Get the next MultiXactId.
923 * Also, reserve the needed amount of space in the "members" area. The
924 * starting offset of the reserved space is returned in *offset.
926 * This may generate XLOG records for expansion of the offsets and/or members
927 * files. Unfortunately, we have to do that while holding MultiXactGenLock
928 * to avoid race conditions --- the XLOG record for zeroing a page must appear
929 * before any backend can possibly try to store data in that page!
931 * We start a critical section before advancing the shared counters. The
932 * caller must end the critical section after writing SLRU data.
935 GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
938 MultiXactOffset nextOffset;
940 debug_elog3(DEBUG2, "GetNew: for %d xids", nmembers);
942 /* safety check, we should never get this far in a HS slave */
943 if (RecoveryInProgress())
944 elog(ERROR, "cannot assign MultiXactIds during recovery");
946 LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
948 /* Handle wraparound of the nextMXact counter */
949 if (MultiXactState->nextMXact < FirstMultiXactId)
950 MultiXactState->nextMXact = FirstMultiXactId;
952 /* Assign the MXID */
953 result = MultiXactState->nextMXact;
956 * Check to see if it's safe to assign another MultiXactId. This protects
957 * against catastrophic data loss due to multixact wraparound. The basic
960 * If we're past multiVacLimit or the safe threshold for member storage
961 * space, or we don't know what the safe threshold for member storage is,
962 * start trying to force autovacuum cycles.
963 * If we're past multiWarnLimit, start issuing warnings.
964 * If we're past multiStopLimit, refuse to create new MultiXactIds.
966 * Note these are pretty much the same protections in GetNewTransactionId.
969 if (!MultiXactIdPrecedes(result, MultiXactState->multiVacLimit))
972 * For safety's sake, we release MultiXactGenLock while sending
973 * signals, warnings, etc. This is not so much because we care about
974 * preserving concurrency in this situation, as to avoid any
975 * possibility of deadlock while doing get_database_name(). First,
976 * copy all the shared values we'll need in this path.
978 MultiXactId multiWarnLimit = MultiXactState->multiWarnLimit;
979 MultiXactId multiStopLimit = MultiXactState->multiStopLimit;
980 MultiXactId multiWrapLimit = MultiXactState->multiWrapLimit;
981 Oid oldest_datoid = MultiXactState->oldestMultiXactDB;
983 LWLockRelease(MultiXactGenLock);
985 if (IsUnderPostmaster &&
986 !MultiXactIdPrecedes(result, multiStopLimit))
988 char *oldest_datname = get_database_name(oldest_datoid);
991 * Immediately kick autovacuum into action as we're already in
994 SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
996 /* complain even if that DB has disappeared */
999 (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
1000 errmsg("database is not accepting commands that generate new MultiXactIds to avoid wraparound data loss in database \"%s\"",
1002 errhint("Execute a database-wide VACUUM in that database.\n"
1003 "You might also need to commit or roll back old prepared transactions.")));
1006 (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
1007 errmsg("database is not accepting commands that generate new MultiXactIds to avoid wraparound data loss in database with OID %u",
1009 errhint("Execute a database-wide VACUUM in that database.\n"
1010 "You might also need to commit or roll back old prepared transactions.")));
1014 * To avoid swamping the postmaster with signals, we issue the autovac
1015 * request only once per 64K multis generated. This still gives
1016 * plenty of chances before we get into real trouble.
1018 if (IsUnderPostmaster && (result % 65536) == 0)
1019 SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1021 if (!MultiXactIdPrecedes(result, multiWarnLimit))
1023 char *oldest_datname = get_database_name(oldest_datoid);
1025 /* complain even if that DB has disappeared */
1028 (errmsg_plural("database \"%s\" must be vacuumed before %u more MultiXactId is used",
1029 "database \"%s\" must be vacuumed before %u more MultiXactIds are used",
1030 multiWrapLimit - result,
1032 multiWrapLimit - result),
1033 errhint("Execute a database-wide VACUUM in that database.\n"
1034 "You might also need to commit or roll back old prepared transactions.")));
1037 (errmsg_plural("database with OID %u must be vacuumed before %u more MultiXactId is used",
1038 "database with OID %u must be vacuumed before %u more MultiXactIds are used",
1039 multiWrapLimit - result,
1041 multiWrapLimit - result),
1042 errhint("Execute a database-wide VACUUM in that database.\n"
1043 "You might also need to commit or roll back old prepared transactions.")));
1046 /* Re-acquire lock and start over */
1047 LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
1048 result = MultiXactState->nextMXact;
1049 if (result < FirstMultiXactId)
1050 result = FirstMultiXactId;
1053 /* Make sure there is room for the MXID in the file. */
1054 ExtendMultiXactOffset(result);
1057 * Reserve the members space, similarly to above. Also, be careful not to
1058 * return zero as the starting offset for any multixact. See
1059 * GetMultiXactIdMembers() for motivation.
1061 nextOffset = MultiXactState->nextOffset;
1062 if (nextOffset == 0)
1065 nmembers++; /* allocate member slot 0 too */
1068 *offset = nextOffset;
1071 * Protect against overrun of the members space as well, with the
1074 * If we're past offsetStopLimit, refuse to generate more multis.
1075 * If we're close to offsetStopLimit, emit a warning.
1077 * Arbitrarily, we start emitting warnings when we're 20 segments or less
1078 * from offsetStopLimit.
1080 * Note we haven't updated the shared state yet, so if we fail at this
1081 * point, the multixact ID we grabbed can still be used by the next guy.
1083 * Note that there is no point in forcing autovacuum runs here: the
1084 * multixact freeze settings would have to be reduced for that to have any
1088 #define OFFSET_WARN_SEGMENTS 20
1089 if (MultiXactState->oldestOffsetKnown &&
1090 MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit, nextOffset,
1093 /* see comment in the corresponding offsets wraparound case */
1094 SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1097 (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
1098 errmsg("multixact \"members\" limit exceeded"),
1099 errdetail_plural("This command would create a multixact with %u members, but the remaining space is only enough for %u member.",
1100 "This command would create a multixact with %u members, but the remaining space is only enough for %u members.",
1101 MultiXactState->offsetStopLimit - nextOffset - 1,
1103 MultiXactState->offsetStopLimit - nextOffset - 1),
1104 errhint("Execute a database-wide VACUUM in database with OID %u with reduced vacuum_multixact_freeze_min_age and vacuum_multixact_freeze_table_age settings.",
1105 MultiXactState->oldestMultiXactDB)));
1109 * Check whether we should kick autovacuum into action, to prevent members
1110 * wraparound. NB we use a much larger window to trigger autovacuum than
1111 * just the warning limit. The warning is just a measure of last resort -
1112 * this is in line with GetNewTransactionId's behaviour.
1114 if (!MultiXactState->oldestOffsetKnown ||
1115 (MultiXactState->nextOffset - MultiXactState->oldestOffset
1116 > MULTIXACT_MEMBER_SAFE_THRESHOLD))
1119 * To avoid swamping the postmaster with signals, we issue the autovac
1120 * request only when crossing a segment boundary. With default
1121 * compilation settings that's roughly after 50k members. This still
1122 * gives plenty of chances before we get into real trouble.
1124 if ((MXOffsetToMemberPage(nextOffset) / SLRU_PAGES_PER_SEGMENT) !=
1125 (MXOffsetToMemberPage(nextOffset + nmembers) / SLRU_PAGES_PER_SEGMENT))
1126 SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
1129 if (MultiXactState->oldestOffsetKnown &&
1130 MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
1132 nmembers + MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT * OFFSET_WARN_SEGMENTS))
1134 (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
1135 errmsg_plural("database with OID %u must be vacuumed before %d more multixact member is used",
1136 "database with OID %u must be vacuumed before %d more multixact members are used",
1137 MultiXactState->offsetStopLimit - nextOffset + nmembers,
1138 MultiXactState->oldestMultiXactDB,
1139 MultiXactState->offsetStopLimit - nextOffset + nmembers),
1140 errhint("Execute a database-wide VACUUM in that database with reduced vacuum_multixact_freeze_min_age and vacuum_multixact_freeze_table_age settings.")));
1142 ExtendMultiXactMember(nextOffset, nmembers);
1145 * Critical section from here until caller has written the data into the
1146 * just-reserved SLRU space; we don't want to error out with a partly
1147 * written MultiXact structure. (In particular, failing to write our
1148 * start offset after advancing nextMXact would effectively corrupt the
1149 * previous MultiXact.)
1151 START_CRIT_SECTION();
1154 * Advance counters. As in GetNewTransactionId(), this must not happen
1155 * until after file extension has succeeded!
1157 * We don't care about MultiXactId wraparound here; it will be handled by
1158 * the next iteration. But note that nextMXact may be InvalidMultiXactId
1159 * or the first value on a segment-beginning page after this routine
1160 * exits, so anyone else looking at the variable must be prepared to deal
1161 * with either case. Similarly, nextOffset may be zero, but we won't use
1162 * that as the actual start offset of the next multixact.
1164 (MultiXactState->nextMXact)++;
1166 MultiXactState->nextOffset += nmembers;
1168 LWLockRelease(MultiXactGenLock);
1170 debug_elog4(DEBUG2, "GetNew: returning %u offset %u", result, *offset);
1175 * GetMultiXactIdMembers
1176 * Return the set of MultiXactMembers that make up a MultiXactId
1178 * Return value is the number of members found, or -1 if there are none,
1179 * and *members is set to a newly palloc'ed array of members. It's the
1180 * caller's responsibility to free it when done with it.
1182 * from_pgupgrade must be passed as true if and only if only the multixact
1183 * corresponds to a value from a tuple that was locked in a 9.2-or-older
1184 * installation and later pg_upgrade'd (that is, the infomask is
1185 * HEAP_LOCKED_UPGRADED). In this case, we know for certain that no members
1186 * can still be running, so we return -1 just like for an empty multixact
1187 * without any further checking. It would be wrong to try to resolve such a
1188 * multixact: either the multixact is within the current valid multixact
1189 * range, in which case the returned result would be bogus, or outside that
1190 * range, in which case an error would be raised.
1192 * In all other cases, the passed multixact must be within the known valid
1193 * range, that is, greater to or equal than oldestMultiXactId, and less than
1194 * nextMXact. Otherwise, an error is raised.
1196 * onlyLock must be set to true if caller is certain that the given multi
1197 * is used only to lock tuples; can be false without loss of correctness,
1198 * but passing a true means we can return quickly without checking for
1202 GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
1203 bool from_pgupgrade, bool onlyLock)
1209 MultiXactOffset *offptr;
1210 MultiXactOffset offset;
1214 MultiXactId oldestMXact;
1215 MultiXactId nextMXact;
1216 MultiXactId tmpMXact;
1217 MultiXactOffset nextOffset;
1218 MultiXactMember *ptr;
1220 debug_elog3(DEBUG2, "GetMembers: asked for %u", multi);
1222 if (!MultiXactIdIsValid(multi) || from_pgupgrade)
1225 /* See if the MultiXactId is in the local cache */
1226 length = mXactCacheGetById(multi, members);
1229 debug_elog3(DEBUG2, "GetMembers: found %s in the cache",
1230 mxid_to_string(multi, length, *members));
1234 /* Set our OldestVisibleMXactId[] entry if we didn't already */
1235 MultiXactIdSetOldestVisible();
1238 * If we know the multi is used only for locking and not for updates, then
1239 * we can skip checking if the value is older than our oldest visible
1240 * multi. It cannot possibly still be running.
1243 MultiXactIdPrecedes(multi, OldestVisibleMXactId[MyBackendId]))
1245 debug_elog2(DEBUG2, "GetMembers: a locker-only multi is too old");
1251 * We check known limits on MultiXact before resorting to the SLRU area.
1253 * An ID older than MultiXactState->oldestMultiXactId cannot possibly be
1254 * useful; it has already been removed, or will be removed shortly, by
1255 * truncation. If one is passed, an error is raised.
1257 * Also, an ID >= nextMXact shouldn't ever be seen here; if it is seen, it
1258 * implies undetected ID wraparound has occurred. This raises a hard
1261 * Shared lock is enough here since we aren't modifying any global state.
1262 * Acquire it just long enough to grab the current counter values. We may
1263 * need both nextMXact and nextOffset; see below.
1265 LWLockAcquire(MultiXactGenLock, LW_SHARED);
1267 oldestMXact = MultiXactState->oldestMultiXactId;
1268 nextMXact = MultiXactState->nextMXact;
1269 nextOffset = MultiXactState->nextOffset;
1271 LWLockRelease(MultiXactGenLock);
1273 if (MultiXactIdPrecedes(multi, oldestMXact))
1276 (errcode(ERRCODE_INTERNAL_ERROR),
1277 errmsg("MultiXactId %u does no longer exist -- apparent wraparound",
1282 if (!MultiXactIdPrecedes(multi, nextMXact))
1284 (errcode(ERRCODE_INTERNAL_ERROR),
1285 errmsg("MultiXactId %u has not been created yet -- apparent wraparound",
1289 * Find out the offset at which we need to start reading MultiXactMembers
1290 * and the number of members in the multixact. We determine the latter as
1291 * the difference between this multixact's starting offset and the next
1292 * one's. However, there are some corner cases to worry about:
1294 * 1. This multixact may be the latest one created, in which case there is
1295 * no next one to look at. In this case the nextOffset value we just
1296 * saved is the correct endpoint.
1298 * 2. The next multixact may still be in process of being filled in: that
1299 * is, another process may have done GetNewMultiXactId but not yet written
1300 * the offset entry for that ID. In that scenario, it is guaranteed that
1301 * the offset entry for that multixact exists (because GetNewMultiXactId
1302 * won't release MultiXactGenLock until it does) but contains zero
1303 * (because we are careful to pre-zero offset pages). Because
1304 * GetNewMultiXactId will never return zero as the starting offset for a
1305 * multixact, when we read zero as the next multixact's offset, we know we
1306 * have this case. We sleep for a bit and try again.
1308 * 3. Because GetNewMultiXactId increments offset zero to offset one to
1309 * handle case #2, there is an ambiguity near the point of offset
1310 * wraparound. If we see next multixact's offset is one, is that our
1311 * multixact's actual endpoint, or did it end at zero with a subsequent
1312 * increment? We handle this using the knowledge that if the zero'th
1313 * member slot wasn't filled, it'll contain zero, and zero isn't a valid
1314 * transaction ID so it can't be a multixact member. Therefore, if we
1315 * read a zero from the members array, just ignore it.
1317 * This is all pretty messy, but the mess occurs only in infrequent corner
1318 * cases, so it seems better than holding the MultiXactGenLock for a long
1319 * time on every multixact creation.
1322 LWLockAcquire(MultiXactOffsetControlLock, LW_EXCLUSIVE);
1324 pageno = MultiXactIdToOffsetPage(multi);
1325 entryno = MultiXactIdToOffsetEntry(multi);
1327 slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, multi);
1328 offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
1332 Assert(offset != 0);
1335 * Use the same increment rule as GetNewMultiXactId(), that is, don't
1336 * handle wraparound explicitly until needed.
1338 tmpMXact = multi + 1;
1340 if (nextMXact == tmpMXact)
1342 /* Corner case 1: there is no next multixact */
1343 length = nextOffset - offset;
1347 MultiXactOffset nextMXOffset;
1349 /* handle wraparound if needed */
1350 if (tmpMXact < FirstMultiXactId)
1351 tmpMXact = FirstMultiXactId;
1353 prev_pageno = pageno;
1355 pageno = MultiXactIdToOffsetPage(tmpMXact);
1356 entryno = MultiXactIdToOffsetEntry(tmpMXact);
1358 if (pageno != prev_pageno)
1359 slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, tmpMXact);
1361 offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
1363 nextMXOffset = *offptr;
1365 if (nextMXOffset == 0)
1367 /* Corner case 2: next multixact is still being filled in */
1368 LWLockRelease(MultiXactOffsetControlLock);
1369 CHECK_FOR_INTERRUPTS();
1374 length = nextMXOffset - offset;
1377 LWLockRelease(MultiXactOffsetControlLock);
1379 ptr = (MultiXactMember *) palloc(length * sizeof(MultiXactMember));
1382 /* Now get the members themselves. */
1383 LWLockAcquire(MultiXactMemberControlLock, LW_EXCLUSIVE);
1387 for (i = 0; i < length; i++, offset++)
1389 TransactionId *xactptr;
1395 pageno = MXOffsetToMemberPage(offset);
1396 memberoff = MXOffsetToMemberOffset(offset);
1398 if (pageno != prev_pageno)
1400 slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true, multi);
1401 prev_pageno = pageno;
1404 xactptr = (TransactionId *)
1405 (MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);
1407 if (!TransactionIdIsValid(*xactptr))
1409 /* Corner case 3: we must be looking at unused slot zero */
1410 Assert(offset == 0);
1414 flagsoff = MXOffsetToFlagsOffset(offset);
1415 bshift = MXOffsetToFlagsBitShift(offset);
1416 flagsptr = (uint32 *) (MultiXactMemberCtl->shared->page_buffer[slotno] + flagsoff);
1418 ptr[truelength].xid = *xactptr;
1419 ptr[truelength].status = (*flagsptr >> bshift) & MXACT_MEMBER_XACT_BITMASK;
1423 LWLockRelease(MultiXactMemberControlLock);
1426 * Copy the result into the local cache.
1428 mXactCachePut(multi, truelength, ptr);
1430 debug_elog3(DEBUG2, "GetMembers: no cache for %s",
1431 mxid_to_string(multi, truelength, ptr));
1436 * mxactMemberComparator
1437 * qsort comparison function for MultiXactMember
1439 * We can't use wraparound comparison for XIDs because that does not respect
1440 * the triangle inequality! Any old sort order will do.
1443 mxactMemberComparator(const void *arg1, const void *arg2)
1445 MultiXactMember member1 = *(const MultiXactMember *) arg1;
1446 MultiXactMember member2 = *(const MultiXactMember *) arg2;
1448 if (member1.xid > member2.xid)
1450 if (member1.xid < member2.xid)
1452 if (member1.status > member2.status)
1454 if (member1.status < member2.status)
1460 * mXactCacheGetBySet
1461 * returns a MultiXactId from the cache based on the set of
1462 * TransactionIds that compose it, or InvalidMultiXactId if
1465 * This is helpful, for example, if two transactions want to lock a huge
1466 * table. By using the cache, the second will use the same MultiXactId
1467 * for the majority of tuples, thus keeping MultiXactId usage low (saving
1468 * both I/O and wraparound issues).
1470 * NB: the passed members array will be sorted in-place.
1473 mXactCacheGetBySet(int nmembers, MultiXactMember *members)
1477 debug_elog3(DEBUG2, "CacheGet: looking for %s",
1478 mxid_to_string(InvalidMultiXactId, nmembers, members));
1480 /* sort the array so comparison is easy */
1481 qsort(members, nmembers, sizeof(MultiXactMember), mxactMemberComparator);
1483 dlist_foreach(iter, &MXactCache)
1485 mXactCacheEnt *entry = dlist_container(mXactCacheEnt, node, iter.cur);
1487 if (entry->nmembers != nmembers)
1491 * We assume the cache entries are sorted, and that the unused bits in
1492 * "status" are zeroed.
1494 if (memcmp(members, entry->members, nmembers * sizeof(MultiXactMember)) == 0)
1496 debug_elog3(DEBUG2, "CacheGet: found %u", entry->multi);
1497 dlist_move_head(&MXactCache, iter.cur);
1498 return entry->multi;
1502 debug_elog2(DEBUG2, "CacheGet: not found :-(");
1503 return InvalidMultiXactId;
1508 * returns the composing MultiXactMember set from the cache for a
1509 * given MultiXactId, if present.
1511 * If successful, *xids is set to the address of a palloc'd copy of the
1512 * MultiXactMember set. Return value is number of members, or -1 on failure.
1515 mXactCacheGetById(MultiXactId multi, MultiXactMember **members)
1519 debug_elog3(DEBUG2, "CacheGet: looking for %u", multi);
1521 dlist_foreach(iter, &MXactCache)
1523 mXactCacheEnt *entry = dlist_container(mXactCacheEnt, node, iter.cur);
1525 if (entry->multi == multi)
1527 MultiXactMember *ptr;
1530 size = sizeof(MultiXactMember) * entry->nmembers;
1531 ptr = (MultiXactMember *) palloc(size);
1534 memcpy(ptr, entry->members, size);
1536 debug_elog3(DEBUG2, "CacheGet: found %s",
1537 mxid_to_string(multi,
1542 * Note we modify the list while not using a modifiable iterator.
1543 * This is acceptable only because we exit the iteration
1544 * immediately afterwards.
1546 dlist_move_head(&MXactCache, iter.cur);
1548 return entry->nmembers;
1552 debug_elog2(DEBUG2, "CacheGet: not found");
1558 * Add a new MultiXactId and its composing set into the local cache.
1561 mXactCachePut(MultiXactId multi, int nmembers, MultiXactMember *members)
1563 mXactCacheEnt *entry;
1565 debug_elog3(DEBUG2, "CachePut: storing %s",
1566 mxid_to_string(multi, nmembers, members));
1568 if (MXactContext == NULL)
1570 /* The cache only lives as long as the current transaction */
1571 debug_elog2(DEBUG2, "CachePut: initializing memory context");
1572 MXactContext = AllocSetContextCreate(TopTransactionContext,
1573 "MultiXact cache context",
1574 ALLOCSET_SMALL_SIZES);
1577 entry = (mXactCacheEnt *)
1578 MemoryContextAlloc(MXactContext,
1579 offsetof(mXactCacheEnt, members) +
1580 nmembers * sizeof(MultiXactMember));
1582 entry->multi = multi;
1583 entry->nmembers = nmembers;
1584 memcpy(entry->members, members, nmembers * sizeof(MultiXactMember));
1586 /* mXactCacheGetBySet assumes the entries are sorted, so sort them */
1587 qsort(entry->members, nmembers, sizeof(MultiXactMember), mxactMemberComparator);
1589 dlist_push_head(&MXactCache, &entry->node);
1590 if (MXactCacheMembers++ >= MAX_CACHE_ENTRIES)
1593 mXactCacheEnt *entry;
1595 node = dlist_tail_node(&MXactCache);
1597 MXactCacheMembers--;
1599 entry = dlist_container(mXactCacheEnt, node, node);
1600 debug_elog3(DEBUG2, "CachePut: pruning cached multi %u",
1608 mxstatus_to_string(MultiXactStatus status)
1612 case MultiXactStatusForKeyShare:
1614 case MultiXactStatusForShare:
1616 case MultiXactStatusForNoKeyUpdate:
1617 return "fornokeyupd";
1618 case MultiXactStatusForUpdate:
1620 case MultiXactStatusNoKeyUpdate:
1622 case MultiXactStatusUpdate:
1625 elog(ERROR, "unrecognized multixact status %d", status);
1631 mxid_to_string(MultiXactId multi, int nmembers, MultiXactMember *members)
1633 static char *str = NULL;
1640 initStringInfo(&buf);
1642 appendStringInfo(&buf, "%u %d[%u (%s)", multi, nmembers, members[0].xid,
1643 mxstatus_to_string(members[0].status));
1645 for (i = 1; i < nmembers; i++)
1646 appendStringInfo(&buf, ", %u (%s)", members[i].xid,
1647 mxstatus_to_string(members[i].status));
1649 appendStringInfoChar(&buf, ']');
1650 str = MemoryContextStrdup(TopMemoryContext, buf.data);
1656 * AtEOXact_MultiXact
1657 * Handle transaction end for MultiXact
1659 * This is called at top transaction commit or abort (we don't care which).
1662 AtEOXact_MultiXact(void)
1665 * Reset our OldestMemberMXactId and OldestVisibleMXactId values, both of
1666 * which should only be valid while within a transaction.
1668 * We assume that storing a MultiXactId is atomic and so we need not take
1669 * MultiXactGenLock to do this.
1671 OldestMemberMXactId[MyBackendId] = InvalidMultiXactId;
1672 OldestVisibleMXactId[MyBackendId] = InvalidMultiXactId;
1675 * Discard the local MultiXactId cache. Since MXactContext was created as
1676 * a child of TopTransactionContext, we needn't delete it explicitly.
1678 MXactContext = NULL;
1679 dlist_init(&MXactCache);
1680 MXactCacheMembers = 0;
1684 * AtPrepare_MultiXact
1685 * Save multixact state at 2PC transaction prepare
1687 * In this phase, we only store our OldestMemberMXactId value in the two-phase
1691 AtPrepare_MultiXact(void)
1693 MultiXactId myOldestMember = OldestMemberMXactId[MyBackendId];
1695 if (MultiXactIdIsValid(myOldestMember))
1696 RegisterTwoPhaseRecord(TWOPHASE_RM_MULTIXACT_ID, 0,
1697 &myOldestMember, sizeof(MultiXactId));
1701 * PostPrepare_MultiXact
1702 * Clean up after successful PREPARE TRANSACTION
1705 PostPrepare_MultiXact(TransactionId xid)
1707 MultiXactId myOldestMember;
1710 * Transfer our OldestMemberMXactId value to the slot reserved for the
1711 * prepared transaction.
1713 myOldestMember = OldestMemberMXactId[MyBackendId];
1714 if (MultiXactIdIsValid(myOldestMember))
1716 BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid);
1719 * Even though storing MultiXactId is atomic, acquire lock to make
1720 * sure others see both changes, not just the reset of the slot of the
1721 * current backend. Using a volatile pointer might suffice, but this
1724 LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
1726 OldestMemberMXactId[dummyBackendId] = myOldestMember;
1727 OldestMemberMXactId[MyBackendId] = InvalidMultiXactId;
1729 LWLockRelease(MultiXactGenLock);
1733 * We don't need to transfer OldestVisibleMXactId value, because the
1734 * transaction is not going to be looking at any more multixacts once it's
1737 * We assume that storing a MultiXactId is atomic and so we need not take
1738 * MultiXactGenLock to do this.
1740 OldestVisibleMXactId[MyBackendId] = InvalidMultiXactId;
1743 * Discard the local MultiXactId cache like in AtEOX_MultiXact
1745 MXactContext = NULL;
1746 dlist_init(&MXactCache);
1747 MXactCacheMembers = 0;
1751 * multixact_twophase_recover
1752 * Recover the state of a prepared transaction at startup
1755 multixact_twophase_recover(TransactionId xid, uint16 info,
1756 void *recdata, uint32 len)
1758 BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid);
1759 MultiXactId oldestMember;
1762 * Get the oldest member XID from the state file record, and set it in the
1763 * OldestMemberMXactId slot reserved for this prepared transaction.
1765 Assert(len == sizeof(MultiXactId));
1766 oldestMember = *((MultiXactId *) recdata);
1768 OldestMemberMXactId[dummyBackendId] = oldestMember;
1772 * multixact_twophase_postcommit
1773 * Similar to AtEOX_MultiXact but for COMMIT PREPARED
1776 multixact_twophase_postcommit(TransactionId xid, uint16 info,
1777 void *recdata, uint32 len)
1779 BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid);
1781 Assert(len == sizeof(MultiXactId));
1783 OldestMemberMXactId[dummyBackendId] = InvalidMultiXactId;
1787 * multixact_twophase_postabort
1788 * This is actually just the same as the COMMIT case.
1791 multixact_twophase_postabort(TransactionId xid, uint16 info,
1792 void *recdata, uint32 len)
1794 multixact_twophase_postcommit(xid, info, recdata, len);
1798 * Initialization of shared memory for MultiXact. We use two SLRU areas,
1799 * thus double memory. Also, reserve space for the shared MultiXactState
1800 * struct and the per-backend MultiXactId arrays (two of those, too).
1803 MultiXactShmemSize(void)
1807 /* We need 2*MaxOldestSlot + 1 perBackendXactIds[] entries */
1808 #define SHARED_MULTIXACT_STATE_SIZE \
1809 add_size(offsetof(MultiXactStateData, perBackendXactIds) + sizeof(MultiXactId), \
1810 mul_size(sizeof(MultiXactId) * 2, MaxOldestSlot))
1812 size = SHARED_MULTIXACT_STATE_SIZE;
1813 size = add_size(size, SimpleLruShmemSize(NUM_MXACTOFFSET_BUFFERS, 0));
1814 size = add_size(size, SimpleLruShmemSize(NUM_MXACTMEMBER_BUFFERS, 0));
1820 MultiXactShmemInit(void)
1824 debug_elog2(DEBUG2, "Shared Memory Init for MultiXact");
1826 MultiXactOffsetCtl->PagePrecedes = MultiXactOffsetPagePrecedes;
1827 MultiXactMemberCtl->PagePrecedes = MultiXactMemberPagePrecedes;
1829 SimpleLruInit(MultiXactOffsetCtl,
1830 "multixact_offset", NUM_MXACTOFFSET_BUFFERS, 0,
1831 MultiXactOffsetControlLock, "pg_multixact/offsets",
1832 LWTRANCHE_MXACTOFFSET_BUFFERS);
1833 SimpleLruInit(MultiXactMemberCtl,
1834 "multixact_member", NUM_MXACTMEMBER_BUFFERS, 0,
1835 MultiXactMemberControlLock, "pg_multixact/members",
1836 LWTRANCHE_MXACTMEMBER_BUFFERS);
1838 /* Initialize our shared state struct */
1839 MultiXactState = ShmemInitStruct("Shared MultiXact State",
1840 SHARED_MULTIXACT_STATE_SIZE,
1842 if (!IsUnderPostmaster)
1846 /* Make sure we zero out the per-backend state */
1847 MemSet(MultiXactState, 0, SHARED_MULTIXACT_STATE_SIZE);
1853 * Set up array pointers. Note that perBackendXactIds[0] is wasted space
1854 * since we only use indexes 1..MaxOldestSlot in each array.
1856 OldestMemberMXactId = MultiXactState->perBackendXactIds;
1857 OldestVisibleMXactId = OldestMemberMXactId + MaxOldestSlot;
1861 * This func must be called ONCE on system install. It creates the initial
1862 * MultiXact segments. (The MultiXacts directories are assumed to have been
1863 * created by initdb, and MultiXactShmemInit must have been called already.)
1866 BootStrapMultiXact(void)
1870 LWLockAcquire(MultiXactOffsetControlLock, LW_EXCLUSIVE);
1872 /* Create and zero the first page of the offsets log */
1873 slotno = ZeroMultiXactOffsetPage(0, false);
1875 /* Make sure it's written out */
1876 SimpleLruWritePage(MultiXactOffsetCtl, slotno);
1877 Assert(!MultiXactOffsetCtl->shared->page_dirty[slotno]);
1879 LWLockRelease(MultiXactOffsetControlLock);
1881 LWLockAcquire(MultiXactMemberControlLock, LW_EXCLUSIVE);
1883 /* Create and zero the first page of the members log */
1884 slotno = ZeroMultiXactMemberPage(0, false);
1886 /* Make sure it's written out */
1887 SimpleLruWritePage(MultiXactMemberCtl, slotno);
1888 Assert(!MultiXactMemberCtl->shared->page_dirty[slotno]);
1890 LWLockRelease(MultiXactMemberControlLock);
1894 * Initialize (or reinitialize) a page of MultiXactOffset to zeroes.
1895 * If writeXlog is TRUE, also emit an XLOG record saying we did this.
1897 * The page is not actually written, just set up in shared memory.
1898 * The slot number of the new page is returned.
1900 * Control lock must be held at entry, and will be held at exit.
1903 ZeroMultiXactOffsetPage(int pageno, bool writeXlog)
1907 slotno = SimpleLruZeroPage(MultiXactOffsetCtl, pageno);
1910 WriteMZeroPageXlogRec(pageno, XLOG_MULTIXACT_ZERO_OFF_PAGE);
1916 * Ditto, for MultiXactMember
1919 ZeroMultiXactMemberPage(int pageno, bool writeXlog)
1923 slotno = SimpleLruZeroPage(MultiXactMemberCtl, pageno);
1926 WriteMZeroPageXlogRec(pageno, XLOG_MULTIXACT_ZERO_MEM_PAGE);
1932 * MaybeExtendOffsetSlru
1933 * Extend the offsets SLRU area, if necessary
1935 * After a binary upgrade from <= 9.2, the pg_multixact/offset SLRU area might
1936 * contain files that are shorter than necessary; this would occur if the old
1937 * installation had used multixacts beyond the first page (files cannot be
1938 * copied, because the on-disk representation is different). pg_upgrade would
1939 * update pg_control to set the next offset value to be at that position, so
1940 * that tuples marked as locked by such MultiXacts would be seen as visible
1941 * without having to consult multixact. However, trying to create and use a
1942 * new MultiXactId would result in an error because the page on which the new
1943 * value would reside does not exist. This routine is in charge of creating
1947 MaybeExtendOffsetSlru(void)
1951 pageno = MultiXactIdToOffsetPage(MultiXactState->nextMXact);
1953 LWLockAcquire(MultiXactOffsetControlLock, LW_EXCLUSIVE);
1955 if (!SimpleLruDoesPhysicalPageExist(MultiXactOffsetCtl, pageno))
1960 * Fortunately for us, SimpleLruWritePage is already prepared to deal
1961 * with creating a new segment file even if the page we're writing is
1962 * not the first in it, so this is enough.
1964 slotno = ZeroMultiXactOffsetPage(pageno, false);
1965 SimpleLruWritePage(MultiXactOffsetCtl, slotno);
1968 LWLockRelease(MultiXactOffsetControlLock);
1972 * This must be called ONCE during postmaster or standalone-backend startup.
1974 * StartupXLOG has already established nextMXact/nextOffset by calling
1975 * MultiXactSetNextMXact and/or MultiXactAdvanceNextMXact, and the oldestMulti
1976 * info from pg_control and/or MultiXactAdvanceOldest, but we haven't yet
1980 StartupMultiXact(void)
1982 MultiXactId multi = MultiXactState->nextMXact;
1983 MultiXactOffset offset = MultiXactState->nextOffset;
1987 * Initialize offset's idea of the latest page number.
1989 pageno = MultiXactIdToOffsetPage(multi);
1990 MultiXactOffsetCtl->shared->latest_page_number = pageno;
1993 * Initialize member's idea of the latest page number.
1995 pageno = MXOffsetToMemberPage(offset);
1996 MultiXactMemberCtl->shared->latest_page_number = pageno;
2000 * This must be called ONCE at the end of startup/recovery.
2005 MultiXactId nextMXact;
2006 MultiXactOffset offset;
2007 MultiXactId oldestMXact;
2013 LWLockAcquire(MultiXactGenLock, LW_SHARED);
2014 nextMXact = MultiXactState->nextMXact;
2015 offset = MultiXactState->nextOffset;
2016 oldestMXact = MultiXactState->oldestMultiXactId;
2017 oldestMXactDB = MultiXactState->oldestMultiXactDB;
2018 LWLockRelease(MultiXactGenLock);
2020 /* Clean up offsets state */
2021 LWLockAcquire(MultiXactOffsetControlLock, LW_EXCLUSIVE);
2024 * (Re-)Initialize our idea of the latest page number for offsets.
2026 pageno = MultiXactIdToOffsetPage(nextMXact);
2027 MultiXactOffsetCtl->shared->latest_page_number = pageno;
2030 * Zero out the remainder of the current offsets page. See notes in
2031 * TrimCLOG() for background. Unlike CLOG, some WAL record covers every
2032 * pg_multixact SLRU mutation. Since, also unlike CLOG, we ignore the WAL
2033 * rule "write xlog before data," nextMXact successors may carry obsolete,
2034 * nonzero offset values. Zero those so case 2 of GetMultiXactIdMembers()
2035 * operates normally.
2037 entryno = MultiXactIdToOffsetEntry(nextMXact);
2041 MultiXactOffset *offptr;
2043 slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, nextMXact);
2044 offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
2047 MemSet(offptr, 0, BLCKSZ - (entryno * sizeof(MultiXactOffset)));
2049 MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
2052 LWLockRelease(MultiXactOffsetControlLock);
2054 /* And the same for members */
2055 LWLockAcquire(MultiXactMemberControlLock, LW_EXCLUSIVE);
2058 * (Re-)Initialize our idea of the latest page number for members.
2060 pageno = MXOffsetToMemberPage(offset);
2061 MultiXactMemberCtl->shared->latest_page_number = pageno;
2064 * Zero out the remainder of the current members page. See notes in
2065 * TrimCLOG() for motivation.
2067 flagsoff = MXOffsetToFlagsOffset(offset);
2071 TransactionId *xidptr;
2074 memberoff = MXOffsetToMemberOffset(offset);
2075 slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true, offset);
2076 xidptr = (TransactionId *)
2077 (MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);
2079 MemSet(xidptr, 0, BLCKSZ - memberoff);
2082 * Note: we don't need to zero out the flag bits in the remaining
2083 * members of the current group, because they are always reset before
2087 MultiXactMemberCtl->shared->page_dirty[slotno] = true;
2090 LWLockRelease(MultiXactMemberControlLock);
2092 /* signal that we're officially up */
2093 LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
2094 MultiXactState->finishedStartup = true;
2095 LWLockRelease(MultiXactGenLock);
2097 /* Now compute how far away the next members wraparound is. */
2098 SetMultiXactIdLimit(oldestMXact, oldestMXactDB, true);
2102 * This must be called ONCE during postmaster or standalone-backend shutdown
2105 ShutdownMultiXact(void)
2107 /* Flush dirty MultiXact pages to disk */
2108 TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_START(false);
2109 SimpleLruFlush(MultiXactOffsetCtl, false);
2110 SimpleLruFlush(MultiXactMemberCtl, false);
2111 TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_DONE(false);
2115 * Get the MultiXact data to save in a checkpoint record
2118 MultiXactGetCheckptMulti(bool is_shutdown,
2119 MultiXactId *nextMulti,
2120 MultiXactOffset *nextMultiOffset,
2121 MultiXactId *oldestMulti,
2124 LWLockAcquire(MultiXactGenLock, LW_SHARED);
2125 *nextMulti = MultiXactState->nextMXact;
2126 *nextMultiOffset = MultiXactState->nextOffset;
2127 *oldestMulti = MultiXactState->oldestMultiXactId;
2128 *oldestMultiDB = MultiXactState->oldestMultiXactDB;
2129 LWLockRelease(MultiXactGenLock);
2132 "MultiXact: checkpoint is nextMulti %u, nextOffset %u, oldestMulti %u in DB %u",
2133 *nextMulti, *nextMultiOffset, *oldestMulti, *oldestMultiDB);
2137 * Perform a checkpoint --- either during shutdown, or on-the-fly
2140 CheckPointMultiXact(void)
2142 TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_START(true);
2144 /* Flush dirty MultiXact pages to disk */
2145 SimpleLruFlush(MultiXactOffsetCtl, true);
2146 SimpleLruFlush(MultiXactMemberCtl, true);
2148 TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_DONE(true);
2152 * Set the next-to-be-assigned MultiXactId and offset
2154 * This is used when we can determine the correct next ID/offset exactly
2155 * from a checkpoint record. Although this is only called during bootstrap
2156 * and XLog replay, we take the lock in case any hot-standby backends are
2157 * examining the values.
2160 MultiXactSetNextMXact(MultiXactId nextMulti,
2161 MultiXactOffset nextMultiOffset)
2163 debug_elog4(DEBUG2, "MultiXact: setting next multi to %u offset %u",
2164 nextMulti, nextMultiOffset);
2165 LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
2166 MultiXactState->nextMXact = nextMulti;
2167 MultiXactState->nextOffset = nextMultiOffset;
2168 LWLockRelease(MultiXactGenLock);
2171 * During a binary upgrade, make sure that the offsets SLRU is large
2172 * enough to contain the next value that would be created.
2174 * We need to do this pretty early during the first startup in binary
2175 * upgrade mode: before StartupMultiXact() in fact, because this routine
2176 * is called even before that by StartupXLOG(). And we can't do it
2177 * earlier than at this point, because during that first call of this
2178 * routine we determine the MultiXactState->nextMXact value that
2179 * MaybeExtendOffsetSlru needs.
2181 if (IsBinaryUpgrade)
2182 MaybeExtendOffsetSlru();
2186 * Determine the last safe MultiXactId to allocate given the currently oldest
2187 * datminmxid (ie, the oldest MultiXactId that might exist in any database
2188 * of our cluster), and the OID of the (or a) database with that value.
2190 * is_startup is true when we are just starting the cluster, false when we
2191 * are updating state in a running cluster. This only affects log messages.
2194 SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid,
2197 MultiXactId multiVacLimit;
2198 MultiXactId multiWarnLimit;
2199 MultiXactId multiStopLimit;
2200 MultiXactId multiWrapLimit;
2201 MultiXactId curMulti;
2202 bool needs_offset_vacuum;
2204 Assert(MultiXactIdIsValid(oldest_datminmxid));
2207 * We pretend that a wrap will happen halfway through the multixact ID
2208 * space, but that's not really true, because multixacts wrap differently
2209 * from transaction IDs. Note that, separately from any concern about
2210 * multixact IDs wrapping, we must ensure that multixact members do not
2211 * wrap. Limits for that are set in DetermineSafeOldestOffset, not here.
2213 multiWrapLimit = oldest_datminmxid + (MaxMultiXactId >> 1);
2214 if (multiWrapLimit < FirstMultiXactId)
2215 multiWrapLimit += FirstMultiXactId;
2218 * We'll refuse to continue assigning MultiXactIds once we get within 100
2219 * multi of data loss.
2221 * Note: This differs from the magic number used in
2222 * SetTransactionIdLimit() since vacuum itself will never generate new
2223 * multis. XXX actually it does, if it needs to freeze old multis.
2225 multiStopLimit = multiWrapLimit - 100;
2226 if (multiStopLimit < FirstMultiXactId)
2227 multiStopLimit -= FirstMultiXactId;
2230 * We'll start complaining loudly when we get within 10M multis of the
2231 * stop point. This is kind of arbitrary, but if you let your gas gauge
2232 * get down to 1% of full, would you be looking for the next gas station?
2233 * We need to be fairly liberal about this number because there are lots
2234 * of scenarios where most transactions are done by automatic clients that
2235 * won't pay attention to warnings. (No, we're not gonna make this
2236 * configurable. If you know enough to configure it, you know enough to
2237 * not get in this kind of trouble in the first place.)
2239 multiWarnLimit = multiStopLimit - 10000000;
2240 if (multiWarnLimit < FirstMultiXactId)
2241 multiWarnLimit -= FirstMultiXactId;
2244 * We'll start trying to force autovacuums when oldest_datminmxid gets to
2245 * be more than autovacuum_multixact_freeze_max_age mxids old.
2247 * Note: autovacuum_multixact_freeze_max_age is a PGC_POSTMASTER parameter
2248 * so that we don't have to worry about dealing with on-the-fly changes in
2249 * its value. See SetTransactionIdLimit.
2251 multiVacLimit = oldest_datminmxid + autovacuum_multixact_freeze_max_age;
2252 if (multiVacLimit < FirstMultiXactId)
2253 multiVacLimit += FirstMultiXactId;
2255 /* Grab lock for just long enough to set the new limit values */
2256 LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
2257 MultiXactState->oldestMultiXactId = oldest_datminmxid;
2258 MultiXactState->oldestMultiXactDB = oldest_datoid;
2259 MultiXactState->multiVacLimit = multiVacLimit;
2260 MultiXactState->multiWarnLimit = multiWarnLimit;
2261 MultiXactState->multiStopLimit = multiStopLimit;
2262 MultiXactState->multiWrapLimit = multiWrapLimit;
2263 curMulti = MultiXactState->nextMXact;
2264 LWLockRelease(MultiXactGenLock);
2268 (errmsg("MultiXactId wrap limit is %u, limited by database with OID %u",
2269 multiWrapLimit, oldest_datoid)));
2272 * Computing the actual limits is only possible once the data directory is
2273 * in a consistent state. There's no need to compute the limits while
2274 * still replaying WAL - no decisions about new multis are made even
2275 * though multixact creations might be replayed. So we'll only do further
2276 * checks after TrimMultiXact() has been called.
2278 if (!MultiXactState->finishedStartup)
2281 Assert(!InRecovery);
2283 /* Set limits for offset vacuum. */
2284 needs_offset_vacuum = SetOffsetVacuumLimit(is_startup);
2287 * If past the autovacuum force point, immediately signal an autovac
2288 * request. The reason for this is that autovac only processes one
2289 * database per invocation. Once it's finished cleaning up the oldest
2290 * database, it'll call here, and we'll signal the postmaster to start
2291 * another iteration immediately if there are still any old databases.
2293 if ((MultiXactIdPrecedes(multiVacLimit, curMulti) ||
2294 needs_offset_vacuum) && IsUnderPostmaster)
2295 SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
2297 /* Give an immediate warning if past the wrap warn point */
2298 if (MultiXactIdPrecedes(multiWarnLimit, curMulti))
2300 char *oldest_datname;
2303 * We can be called when not inside a transaction, for example during
2304 * StartupXLOG(). In such a case we cannot do database access, so we
2305 * must just report the oldest DB's OID.
2307 * Note: it's also possible that get_database_name fails and returns
2308 * NULL, for example because the database just got dropped. We'll
2309 * still warn, even though the warning might now be unnecessary.
2311 if (IsTransactionState())
2312 oldest_datname = get_database_name(oldest_datoid);
2314 oldest_datname = NULL;
2318 (errmsg_plural("database \"%s\" must be vacuumed before %u more MultiXactId is used",
2319 "database \"%s\" must be vacuumed before %u more MultiXactIds are used",
2320 multiWrapLimit - curMulti,
2322 multiWrapLimit - curMulti),
2323 errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
2324 "You might also need to commit or roll back old prepared transactions.")));
2327 (errmsg_plural("database with OID %u must be vacuumed before %u more MultiXactId is used",
2328 "database with OID %u must be vacuumed before %u more MultiXactIds are used",
2329 multiWrapLimit - curMulti,
2331 multiWrapLimit - curMulti),
2332 errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
2333 "You might also need to commit or roll back old prepared transactions.")));
2338 * Ensure the next-to-be-assigned MultiXactId is at least minMulti,
2339 * and similarly nextOffset is at least minMultiOffset.
2341 * This is used when we can determine minimum safe values from an XLog
2342 * record (either an on-line checkpoint or an mxact creation log entry).
2343 * Although this is only called during XLog replay, we take the lock in case
2344 * any hot-standby backends are examining the values.
2347 MultiXactAdvanceNextMXact(MultiXactId minMulti,
2348 MultiXactOffset minMultiOffset)
2350 LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
2351 if (MultiXactIdPrecedes(MultiXactState->nextMXact, minMulti))
2353 debug_elog3(DEBUG2, "MultiXact: setting next multi to %u", minMulti);
2354 MultiXactState->nextMXact = minMulti;
2356 if (MultiXactOffsetPrecedes(MultiXactState->nextOffset, minMultiOffset))
2358 debug_elog3(DEBUG2, "MultiXact: setting next offset to %u",
2360 MultiXactState->nextOffset = minMultiOffset;
2362 LWLockRelease(MultiXactGenLock);
2366 * Update our oldestMultiXactId value, but only if it's more recent than what
2369 * This may only be called during WAL replay.
2372 MultiXactAdvanceOldest(MultiXactId oldestMulti, Oid oldestMultiDB)
2376 if (MultiXactIdPrecedes(MultiXactState->oldestMultiXactId, oldestMulti))
2377 SetMultiXactIdLimit(oldestMulti, oldestMultiDB, false);
2381 * Make sure that MultiXactOffset has room for a newly-allocated MultiXactId.
2383 * NB: this is called while holding MultiXactGenLock. We want it to be very
2384 * fast most of the time; even when it's not so fast, no actual I/O need
2385 * happen unless we're forced to write out a dirty log or xlog page to make
2386 * room in shared memory.
2389 ExtendMultiXactOffset(MultiXactId multi)
2394 * No work except at first MultiXactId of a page. But beware: just after
2395 * wraparound, the first MultiXactId of page zero is FirstMultiXactId.
2397 if (MultiXactIdToOffsetEntry(multi) != 0 &&
2398 multi != FirstMultiXactId)
2401 pageno = MultiXactIdToOffsetPage(multi);
2403 LWLockAcquire(MultiXactOffsetControlLock, LW_EXCLUSIVE);
2405 /* Zero the page and make an XLOG entry about it */
2406 ZeroMultiXactOffsetPage(pageno, true);
2408 LWLockRelease(MultiXactOffsetControlLock);
2412 * Make sure that MultiXactMember has room for the members of a newly-
2413 * allocated MultiXactId.
2415 * Like the above routine, this is called while holding MultiXactGenLock;
2416 * same comments apply.
2419 ExtendMultiXactMember(MultiXactOffset offset, int nmembers)
2422 * It's possible that the members span more than one page of the members
2423 * file, so we loop to ensure we consider each page. The coding is not
2424 * optimal if the members span several pages, but that seems unusual
2425 * enough to not worry much about.
2427 while (nmembers > 0)
2434 * Only zero when at first entry of a page.
2436 flagsoff = MXOffsetToFlagsOffset(offset);
2437 flagsbit = MXOffsetToFlagsBitShift(offset);
2438 if (flagsoff == 0 && flagsbit == 0)
2442 pageno = MXOffsetToMemberPage(offset);
2444 LWLockAcquire(MultiXactMemberControlLock, LW_EXCLUSIVE);
2446 /* Zero the page and make an XLOG entry about it */
2447 ZeroMultiXactMemberPage(pageno, true);
2449 LWLockRelease(MultiXactMemberControlLock);
2453 * Compute the number of items till end of current page. Careful: if
2454 * addition of unsigned ints wraps around, we're at the last page of
2455 * the last segment; since that page holds a different number of items
2456 * than other pages, we need to do it differently.
2458 if (offset + MAX_MEMBERS_IN_LAST_MEMBERS_PAGE < offset)
2461 * This is the last page of the last segment; we can compute the
2462 * number of items left to allocate in it without modulo
2465 difference = MaxMultiXactOffset - offset + 1;
2468 difference = MULTIXACT_MEMBERS_PER_PAGE - offset % MULTIXACT_MEMBERS_PER_PAGE;
2471 * Advance to next page, taking care to properly handle the wraparound
2472 * case. OK if nmembers goes negative.
2474 nmembers -= difference;
2475 offset += difference;
2480 * GetOldestMultiXactId
2482 * Return the oldest MultiXactId that's still possibly still seen as live by
2483 * any running transaction. Older ones might still exist on disk, but they no
2484 * longer have any running member transaction.
2486 * It's not safe to truncate MultiXact SLRU segments on the value returned by
2487 * this function; however, it can be used by a full-table vacuum to set the
2488 * point at which it will be possible to truncate SLRU for that table.
2491 GetOldestMultiXactId(void)
2493 MultiXactId oldestMXact;
2494 MultiXactId nextMXact;
2498 * This is the oldest valid value among all the OldestMemberMXactId[] and
2499 * OldestVisibleMXactId[] entries, or nextMXact if none are valid.
2501 LWLockAcquire(MultiXactGenLock, LW_SHARED);
2504 * We have to beware of the possibility that nextMXact is in the
2505 * wrapped-around state. We don't fix the counter itself here, but we
2506 * must be sure to use a valid value in our calculation.
2508 nextMXact = MultiXactState->nextMXact;
2509 if (nextMXact < FirstMultiXactId)
2510 nextMXact = FirstMultiXactId;
2512 oldestMXact = nextMXact;
2513 for (i = 1; i <= MaxOldestSlot; i++)
2515 MultiXactId thisoldest;
2517 thisoldest = OldestMemberMXactId[i];
2518 if (MultiXactIdIsValid(thisoldest) &&
2519 MultiXactIdPrecedes(thisoldest, oldestMXact))
2520 oldestMXact = thisoldest;
2521 thisoldest = OldestVisibleMXactId[i];
2522 if (MultiXactIdIsValid(thisoldest) &&
2523 MultiXactIdPrecedes(thisoldest, oldestMXact))
2524 oldestMXact = thisoldest;
2527 LWLockRelease(MultiXactGenLock);
2533 * Determine how aggressively we need to vacuum in order to prevent member
2536 * To do so determine what's the oldest member offset and install the limit
2537 * info in MultiXactState, where it can be used to prevent overrun of old data
2538 * in the members SLRU area.
2540 * The return value is true if emergency autovacuum is required and false
2544 SetOffsetVacuumLimit(bool is_startup)
2546 MultiXactId oldestMultiXactId;
2547 MultiXactId nextMXact;
2548 MultiXactOffset oldestOffset = 0; /* placate compiler */
2549 MultiXactOffset prevOldestOffset;
2550 MultiXactOffset nextOffset;
2551 bool oldestOffsetKnown = false;
2552 bool prevOldestOffsetKnown;
2553 MultiXactOffset offsetStopLimit = 0;
2554 MultiXactOffset prevOffsetStopLimit;
2557 * NB: Have to prevent concurrent truncation, we might otherwise try to
2558 * lookup a oldestMulti that's concurrently getting truncated away.
2560 LWLockAcquire(MultiXactTruncationLock, LW_SHARED);
2562 /* Read relevant fields from shared memory. */
2563 LWLockAcquire(MultiXactGenLock, LW_SHARED);
2564 oldestMultiXactId = MultiXactState->oldestMultiXactId;
2565 nextMXact = MultiXactState->nextMXact;
2566 nextOffset = MultiXactState->nextOffset;
2567 prevOldestOffsetKnown = MultiXactState->oldestOffsetKnown;
2568 prevOldestOffset = MultiXactState->oldestOffset;
2569 prevOffsetStopLimit = MultiXactState->offsetStopLimit;
2570 Assert(MultiXactState->finishedStartup);
2571 LWLockRelease(MultiXactGenLock);
2574 * Determine the offset of the oldest multixact. Normally, we can read
2575 * the offset from the multixact itself, but there's an important special
2576 * case: if there are no multixacts in existence at all, oldestMXact
2577 * obviously can't point to one. It will instead point to the multixact
2578 * ID that will be assigned the next time one is needed.
2580 if (oldestMultiXactId == nextMXact)
2583 * When the next multixact gets created, it will be stored at the next
2586 oldestOffset = nextOffset;
2587 oldestOffsetKnown = true;
2592 * Figure out where the oldest existing multixact's offsets are
2593 * stored. Due to bugs in early release of PostgreSQL 9.3.X and 9.4.X,
2594 * the supposedly-earliest multixact might not really exist. We are
2595 * careful not to fail in that case.
2598 find_multixact_start(oldestMultiXactId, &oldestOffset);
2600 if (oldestOffsetKnown)
2602 (errmsg("oldest MultiXactId member is at offset %u",
2606 (errmsg("MultiXact member wraparound protections are disabled because oldest checkpointed MultiXact %u does not exist on disk",
2607 oldestMultiXactId)));
2610 LWLockRelease(MultiXactTruncationLock);
2613 * If we can, compute limits (and install them MultiXactState) to prevent
2614 * overrun of old data in the members SLRU area. We can only do so if the
2615 * oldest offset is known though.
2617 if (oldestOffsetKnown)
2619 /* move back to start of the corresponding segment */
2620 offsetStopLimit = oldestOffset - (oldestOffset %
2621 (MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT));
2623 /* always leave one segment before the wraparound point */
2624 offsetStopLimit -= (MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);
2626 if (!prevOldestOffsetKnown && !is_startup)
2628 (errmsg("MultiXact member wraparound protections are now enabled")));
2631 (errmsg("MultiXact member stop limit is now %u based on MultiXact %u",
2632 offsetStopLimit, oldestMultiXactId)));
2634 else if (prevOldestOffsetKnown)
2637 * If we failed to get the oldest offset this time, but we have a
2638 * value from a previous pass through this function, use the old
2639 * values rather than automatically forcing an emergency autovacuum
2642 oldestOffset = prevOldestOffset;
2643 oldestOffsetKnown = true;
2644 offsetStopLimit = prevOffsetStopLimit;
2647 /* Install the computed values */
2648 LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
2649 MultiXactState->oldestOffset = oldestOffset;
2650 MultiXactState->oldestOffsetKnown = oldestOffsetKnown;
2651 MultiXactState->offsetStopLimit = offsetStopLimit;
2652 LWLockRelease(MultiXactGenLock);
2655 * Do we need an emergency autovacuum? If we're not sure, assume yes.
2657 return !oldestOffsetKnown ||
2658 (nextOffset - oldestOffset > MULTIXACT_MEMBER_SAFE_THRESHOLD);
2662 * Return whether adding "distance" to "start" would move past "boundary".
2664 * We use this to determine whether the addition is "wrapping around" the
2665 * boundary point, hence the name. The reason we don't want to use the regular
2666 * 2^31-modulo arithmetic here is that we want to be able to use the whole of
2667 * the 2^32-1 space here, allowing for more multixacts that would fit
2671 MultiXactOffsetWouldWrap(MultiXactOffset boundary, MultiXactOffset start,
2674 MultiXactOffset finish;
2677 * Note that offset number 0 is not used (see GetMultiXactIdMembers), so
2678 * if the addition wraps around the UINT_MAX boundary, skip that value.
2680 finish = start + distance;
2684 /*-----------------------------------------------------------------------
2685 * When the boundary is numerically greater than the starting point, any
2686 * value numerically between the two is not wrapped:
2689 * [---) = F wrapped past B (and UINT_MAX)
2690 * [---) = F not wrapped
2691 * [----] = F wrapped past B
2693 * When the boundary is numerically less than the starting point (i.e. the
2694 * UINT_MAX wraparound occurs somewhere in between) then all values in
2695 * between are wrapped:
2698 * [---) = F not wrapped past B (but wrapped past UINT_MAX)
2699 * [---) = F wrapped past B (and UINT_MAX)
2700 * [----] = F not wrapped
2701 *-----------------------------------------------------------------------
2703 if (start < boundary)
2704 return finish >= boundary || finish < start;
2706 return finish >= boundary && finish < start;
2710 * Find the starting offset of the given MultiXactId.
2712 * Returns false if the file containing the multi does not exist on disk.
2713 * Otherwise, returns true and sets *result to the starting member offset.
2715 * This function does not prevent concurrent truncation, so if that's
2716 * required, the caller has to protect against that.
2719 find_multixact_start(MultiXactId multi, MultiXactOffset *result)
2721 MultiXactOffset offset;
2725 MultiXactOffset *offptr;
2727 Assert(MultiXactState->finishedStartup);
2729 pageno = MultiXactIdToOffsetPage(multi);
2730 entryno = MultiXactIdToOffsetEntry(multi);
2733 * Flush out dirty data, so PhysicalPageExists can work correctly.
2734 * SimpleLruFlush() is a pretty big hammer for that. Alternatively we
2735 * could add a in-memory version of page exists, but find_multixact_start
2736 * is called infrequently, and it doesn't seem bad to flush buffers to
2737 * disk before truncation.
2739 SimpleLruFlush(MultiXactOffsetCtl, true);
2740 SimpleLruFlush(MultiXactMemberCtl, true);
2742 if (!SimpleLruDoesPhysicalPageExist(MultiXactOffsetCtl, pageno))
2745 /* lock is acquired by SimpleLruReadPage_ReadOnly */
2746 slotno = SimpleLruReadPage_ReadOnly(MultiXactOffsetCtl, pageno, multi);
2747 offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
2750 LWLockRelease(MultiXactOffsetControlLock);
2757 * Determine how many multixacts, and how many multixact members, currently
2758 * exist. Return false if unable to determine.
2761 ReadMultiXactCounts(uint32 *multixacts, MultiXactOffset *members)
2763 MultiXactOffset nextOffset;
2764 MultiXactOffset oldestOffset;
2765 MultiXactId oldestMultiXactId;
2766 MultiXactId nextMultiXactId;
2767 bool oldestOffsetKnown;
2769 LWLockAcquire(MultiXactGenLock, LW_SHARED);
2770 nextOffset = MultiXactState->nextOffset;
2771 oldestMultiXactId = MultiXactState->oldestMultiXactId;
2772 nextMultiXactId = MultiXactState->nextMXact;
2773 oldestOffset = MultiXactState->oldestOffset;
2774 oldestOffsetKnown = MultiXactState->oldestOffsetKnown;
2775 LWLockRelease(MultiXactGenLock);
2777 if (!oldestOffsetKnown)
2780 *members = nextOffset - oldestOffset;
2781 *multixacts = nextMultiXactId - oldestMultiXactId;
2786 * Multixact members can be removed once the multixacts that refer to them
2787 * are older than every datminxmid. autovacuum_multixact_freeze_max_age and
2788 * vacuum_multixact_freeze_table_age work together to make sure we never have
2789 * too many multixacts; we hope that, at least under normal circumstances,
2790 * this will also be sufficient to keep us from using too many offsets.
2791 * However, if the average multixact has many members, we might exhaust the
2792 * members space while still using few enough members that these limits fail
2793 * to trigger full table scans for relminmxid advancement. At that point,
2794 * we'd have no choice but to start failing multixact-creating operations
2797 * To prevent that, if more than a threshold portion of the members space is
2798 * used, we effectively reduce autovacuum_multixact_freeze_max_age and
2799 * to a value just less than the number of multixacts in use. We hope that
2800 * this will quickly trigger autovacuuming on the table or tables with the
2801 * oldest relminmxid, thus allowing datminmxid values to advance and removing
2804 * As the fraction of the member space currently in use grows, we become
2805 * more aggressive in clamping this value. That not only causes autovacuum
2806 * to ramp up, but also makes any manual vacuums the user issues more
2807 * aggressive. This happens because vacuum_set_xid_limits() clamps the
2808 * freeze table and the minimum freeze age based on the effective
2809 * autovacuum_multixact_freeze_max_age this function returns. In the worst
2810 * case, we'll claim the freeze_max_age to zero, and every vacuum of any
2811 * table will try to freeze every multixact.
2813 * It's possible that these thresholds should be user-tunable, but for now
2814 * we keep it simple.
2817 MultiXactMemberFreezeThreshold(void)
2819 MultiXactOffset members;
2821 uint32 victim_multixacts;
2824 /* If we can't determine member space utilization, assume the worst. */
2825 if (!ReadMultiXactCounts(&multixacts, &members))
2828 /* If member space utilization is low, no special action is required. */
2829 if (members <= MULTIXACT_MEMBER_SAFE_THRESHOLD)
2830 return autovacuum_multixact_freeze_max_age;
2833 * Compute a target for relminmxid advancement. The number of multixacts
2834 * we try to eliminate from the system is based on how far we are past
2835 * MULTIXACT_MEMBER_SAFE_THRESHOLD.
2837 fraction = (double) (members - MULTIXACT_MEMBER_SAFE_THRESHOLD) /
2838 (MULTIXACT_MEMBER_DANGER_THRESHOLD - MULTIXACT_MEMBER_SAFE_THRESHOLD);
2839 victim_multixacts = multixacts * fraction;
2841 /* fraction could be > 1.0, but lowest possible freeze age is zero */
2842 if (victim_multixacts > multixacts)
2844 return multixacts - victim_multixacts;
2847 typedef struct mxtruncinfo
2849 int earliestExistingPage;
2853 * SlruScanDirectory callback
2854 * This callback determines the earliest existing page number.
2857 SlruScanDirCbFindEarliest(SlruCtl ctl, char *filename, int segpage, void *data)
2859 mxtruncinfo *trunc = (mxtruncinfo *) data;
2861 if (trunc->earliestExistingPage == -1 ||
2862 ctl->PagePrecedes(segpage, trunc->earliestExistingPage))
2864 trunc->earliestExistingPage = segpage;
2867 return false; /* keep going */
2872 * Delete members segments [oldest, newOldest)
2874 * The members SLRU can, in contrast to the offsets one, be filled to almost
2875 * the full range at once. This means SimpleLruTruncate() can't trivially be
2876 * used - instead the to-be-deleted range is computed using the offsets
2877 * SLRU. C.f. TruncateMultiXact().
2880 PerformMembersTruncation(MultiXactOffset oldestOffset, MultiXactOffset newOldestOffset)
2882 const int maxsegment = MXOffsetToMemberSegment(MaxMultiXactOffset);
2883 int startsegment = MXOffsetToMemberSegment(oldestOffset);
2884 int endsegment = MXOffsetToMemberSegment(newOldestOffset);
2885 int segment = startsegment;
2888 * Delete all the segments but the last one. The last segment can still
2889 * contain, possibly partially, valid data.
2891 while (segment != endsegment)
2893 elog(DEBUG2, "truncating multixact members segment %x", segment);
2894 SlruDeleteSegment(MultiXactMemberCtl, segment);
2896 /* move to next segment, handling wraparound correctly */
2897 if (segment == maxsegment)
2905 * Delete offsets segments [oldest, newOldest)
2908 PerformOffsetsTruncation(MultiXactId oldestMulti, MultiXactId newOldestMulti)
2911 * We step back one multixact to avoid passing a cutoff page that hasn't
2912 * been created yet in the rare case that oldestMulti would be the first
2913 * item on a page and oldestMulti == nextMulti. In that case, if we
2914 * didn't subtract one, we'd trigger SimpleLruTruncate's wraparound
2917 SimpleLruTruncate(MultiXactOffsetCtl,
2918 MultiXactIdToOffsetPage(PreviousMultiXactId(newOldestMulti)));
2922 * Remove all MultiXactOffset and MultiXactMember segments before the oldest
2923 * ones still of interest.
2925 * This is only called on a primary as part of vacuum (via
2926 * vac_truncate_clog()). During recovery truncation is done by replaying
2927 * truncation WAL records logged here.
2929 * newOldestMulti is the oldest currently required multixact, newOldestMultiDB
2930 * is one of the databases preventing newOldestMulti from increasing.
2933 TruncateMultiXact(MultiXactId newOldestMulti, Oid newOldestMultiDB)
2935 MultiXactId oldestMulti;
2936 MultiXactId nextMulti;
2937 MultiXactOffset newOldestOffset;
2938 MultiXactOffset oldestOffset;
2939 MultiXactOffset nextOffset;
2941 MultiXactId earliest;
2943 Assert(!RecoveryInProgress());
2944 Assert(MultiXactState->finishedStartup);
2947 * We can only allow one truncation to happen at once. Otherwise parts of
2948 * members might vanish while we're doing lookups or similar. There's no
2949 * need to have an interlock with creating new multis or such, since those
2950 * are constrained by the limits (which only grow, never shrink).
2952 LWLockAcquire(MultiXactTruncationLock, LW_EXCLUSIVE);
2954 LWLockAcquire(MultiXactGenLock, LW_SHARED);
2955 nextMulti = MultiXactState->nextMXact;
2956 nextOffset = MultiXactState->nextOffset;
2957 oldestMulti = MultiXactState->oldestMultiXactId;
2958 LWLockRelease(MultiXactGenLock);
2959 Assert(MultiXactIdIsValid(oldestMulti));
2962 * Make sure to only attempt truncation if there's values to truncate
2963 * away. In normal processing values shouldn't go backwards, but there's
2964 * some corner cases (due to bugs) where that's possible.
2966 if (MultiXactIdPrecedesOrEquals(newOldestMulti, oldestMulti))
2968 LWLockRelease(MultiXactTruncationLock);
2973 * Note we can't just plow ahead with the truncation; it's possible that
2974 * there are no segments to truncate, which is a problem because we are
2975 * going to attempt to read the offsets page to determine where to
2976 * truncate the members SLRU. So we first scan the directory to determine
2977 * the earliest offsets page number that we can read without error.
2979 * NB: It's also possible that the page that oldestMulti is on has already
2980 * been truncated away, and we crashed before updating oldestMulti.
2982 trunc.earliestExistingPage = -1;
2983 SlruScanDirectory(MultiXactOffsetCtl, SlruScanDirCbFindEarliest, &trunc);
2984 earliest = trunc.earliestExistingPage * MULTIXACT_OFFSETS_PER_PAGE;
2985 if (earliest < FirstMultiXactId)
2986 earliest = FirstMultiXactId;
2988 /* If there's nothing to remove, we can bail out early. */
2989 if (MultiXactIdPrecedes(oldestMulti, earliest))
2991 LWLockRelease(MultiXactTruncationLock);
2996 * First, compute the safe truncation point for MultiXactMember. This is
2997 * the starting offset of the oldest multixact.
2999 * Hopefully, find_multixact_start will always work here, because we've
3000 * already checked that it doesn't precede the earliest MultiXact on disk.
3001 * But if it fails, don't truncate anything, and log a message.
3003 if (oldestMulti == nextMulti)
3005 /* there are NO MultiXacts */
3006 oldestOffset = nextOffset;
3008 else if (!find_multixact_start(oldestMulti, &oldestOffset))
3011 (errmsg("oldest MultiXact %u not found, earliest MultiXact %u, skipping truncation",
3012 oldestMulti, earliest)));
3013 LWLockRelease(MultiXactTruncationLock);
3018 * Secondly compute up to where to truncate. Lookup the corresponding
3019 * member offset for newOldestMulti for that.
3021 if (newOldestMulti == nextMulti)
3023 /* there are NO MultiXacts */
3024 newOldestOffset = nextOffset;
3026 else if (!find_multixact_start(newOldestMulti, &newOldestOffset))
3029 (errmsg("cannot truncate up to MultiXact %u because it does not exist on disk, skipping truncation",
3031 LWLockRelease(MultiXactTruncationLock);
3035 elog(DEBUG1, "performing multixact truncation: "
3036 "offsets [%u, %u), offsets segments [%x, %x), "
3037 "members [%u, %u), members segments [%x, %x)",
3038 oldestMulti, newOldestMulti,
3039 MultiXactIdToOffsetSegment(oldestMulti),
3040 MultiXactIdToOffsetSegment(newOldestMulti),
3041 oldestOffset, newOldestOffset,
3042 MXOffsetToMemberSegment(oldestOffset),
3043 MXOffsetToMemberSegment(newOldestOffset));
3046 * Do truncation, and the WAL logging of the truncation, in a critical
3047 * section. That way offsets/members cannot get out of sync anymore, i.e.
3048 * once consistent the newOldestMulti will always exist in members, even
3049 * if we crashed in the wrong moment.
3051 START_CRIT_SECTION();
3054 * Prevent checkpoints from being scheduled concurrently. This is critical
3055 * because otherwise a truncation record might not be replayed after a
3056 * crash/basebackup, even though the state of the data directory would
3059 Assert(!MyPgXact->delayChkpt);
3060 MyPgXact->delayChkpt = true;
3062 /* WAL log truncation */
3063 WriteMTruncateXlogRec(newOldestMultiDB,
3064 oldestMulti, newOldestMulti,
3065 oldestOffset, newOldestOffset);
3068 * Update in-memory limits before performing the truncation, while inside
3069 * the critical section: Have to do it before truncation, to prevent
3070 * concurrent lookups of those values. Has to be inside the critical
3071 * section as otherwise a future call to this function would error out,
3072 * while looking up the oldest member in offsets, if our caller crashes
3073 * before updating the limits.
3075 LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
3076 MultiXactState->oldestMultiXactId = newOldestMulti;
3077 MultiXactState->oldestMultiXactDB = newOldestMultiDB;
3078 LWLockRelease(MultiXactGenLock);
3080 /* First truncate members */
3081 PerformMembersTruncation(oldestOffset, newOldestOffset);
3084 PerformOffsetsTruncation(oldestMulti, newOldestMulti);
3086 MyPgXact->delayChkpt = false;
3089 LWLockRelease(MultiXactTruncationLock);
3093 * Decide which of two MultiXactOffset page numbers is "older" for truncation
3096 * We need to use comparison of MultiXactId here in order to do the right
3097 * thing with wraparound. However, if we are asked about page number zero, we
3098 * don't want to hand InvalidMultiXactId to MultiXactIdPrecedes: it'll get
3099 * weird. So, offset both multis by FirstMultiXactId to avoid that.
3100 * (Actually, the current implementation doesn't do anything weird with
3101 * InvalidMultiXactId, but there's no harm in leaving this code like this.)
3104 MultiXactOffsetPagePrecedes(int page1, int page2)
3109 multi1 = ((MultiXactId) page1) * MULTIXACT_OFFSETS_PER_PAGE;
3110 multi1 += FirstMultiXactId;
3111 multi2 = ((MultiXactId) page2) * MULTIXACT_OFFSETS_PER_PAGE;
3112 multi2 += FirstMultiXactId;
3114 return MultiXactIdPrecedes(multi1, multi2);
3118 * Decide which of two MultiXactMember page numbers is "older" for truncation
3119 * purposes. There is no "invalid offset number" so use the numbers verbatim.
3122 MultiXactMemberPagePrecedes(int page1, int page2)
3124 MultiXactOffset offset1;
3125 MultiXactOffset offset2;
3127 offset1 = ((MultiXactOffset) page1) * MULTIXACT_MEMBERS_PER_PAGE;
3128 offset2 = ((MultiXactOffset) page2) * MULTIXACT_MEMBERS_PER_PAGE;
3130 return MultiXactOffsetPrecedes(offset1, offset2);
3134 * Decide which of two MultiXactIds is earlier.
3136 * XXX do we need to do something special for InvalidMultiXactId?
3137 * (Doesn't look like it.)
3140 MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2)
3142 int32 diff = (int32) (multi1 - multi2);
3148 * MultiXactIdPrecedesOrEquals -- is multi1 logically <= multi2?
3150 * XXX do we need to do something special for InvalidMultiXactId?
3151 * (Doesn't look like it.)
3154 MultiXactIdPrecedesOrEquals(MultiXactId multi1, MultiXactId multi2)
3156 int32 diff = (int32) (multi1 - multi2);
3163 * Decide which of two offsets is earlier.
3166 MultiXactOffsetPrecedes(MultiXactOffset offset1, MultiXactOffset offset2)
3168 int32 diff = (int32) (offset1 - offset2);
3174 * Write an xlog record reflecting the zeroing of either a MEMBERs or
3175 * OFFSETs page (info shows which)
3178 WriteMZeroPageXlogRec(int pageno, uint8 info)
3181 XLogRegisterData((char *) (&pageno), sizeof(int));
3182 (void) XLogInsert(RM_MULTIXACT_ID, info);
3186 * Write a TRUNCATE xlog record
3188 * We must flush the xlog record to disk before returning --- see notes in
3192 WriteMTruncateXlogRec(Oid oldestMultiDB,
3193 MultiXactId startTruncOff, MultiXactId endTruncOff,
3194 MultiXactOffset startTruncMemb, MultiXactOffset endTruncMemb)
3197 xl_multixact_truncate xlrec;
3199 xlrec.oldestMultiDB = oldestMultiDB;
3201 xlrec.startTruncOff = startTruncOff;
3202 xlrec.endTruncOff = endTruncOff;
3204 xlrec.startTruncMemb = startTruncMemb;
3205 xlrec.endTruncMemb = endTruncMemb;
3208 XLogRegisterData((char *) (&xlrec), SizeOfMultiXactTruncate);
3209 recptr = XLogInsert(RM_MULTIXACT_ID, XLOG_MULTIXACT_TRUNCATE_ID);
3214 * MULTIXACT resource manager's routines
3217 multixact_redo(XLogReaderState *record)
3219 uint8 info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
3221 /* Backup blocks are not used in multixact records */
3222 Assert(!XLogRecHasAnyBlockRefs(record));
3224 if (info == XLOG_MULTIXACT_ZERO_OFF_PAGE)
3229 memcpy(&pageno, XLogRecGetData(record), sizeof(int));
3231 LWLockAcquire(MultiXactOffsetControlLock, LW_EXCLUSIVE);
3233 slotno = ZeroMultiXactOffsetPage(pageno, false);
3234 SimpleLruWritePage(MultiXactOffsetCtl, slotno);
3235 Assert(!MultiXactOffsetCtl->shared->page_dirty[slotno]);
3237 LWLockRelease(MultiXactOffsetControlLock);
3239 else if (info == XLOG_MULTIXACT_ZERO_MEM_PAGE)
3244 memcpy(&pageno, XLogRecGetData(record), sizeof(int));
3246 LWLockAcquire(MultiXactMemberControlLock, LW_EXCLUSIVE);
3248 slotno = ZeroMultiXactMemberPage(pageno, false);
3249 SimpleLruWritePage(MultiXactMemberCtl, slotno);
3250 Assert(!MultiXactMemberCtl->shared->page_dirty[slotno]);
3252 LWLockRelease(MultiXactMemberControlLock);
3254 else if (info == XLOG_MULTIXACT_CREATE_ID)
3256 xl_multixact_create *xlrec =
3257 (xl_multixact_create *) XLogRecGetData(record);
3258 TransactionId max_xid;
3261 /* Store the data back into the SLRU files */
3262 RecordNewMultiXact(xlrec->mid, xlrec->moff, xlrec->nmembers,
3265 /* Make sure nextMXact/nextOffset are beyond what this record has */
3266 MultiXactAdvanceNextMXact(xlrec->mid + 1,
3267 xlrec->moff + xlrec->nmembers);
3270 * Make sure nextXid is beyond any XID mentioned in the record. This
3271 * should be unnecessary, since any XID found here ought to have other
3272 * evidence in the XLOG, but let's be safe.
3274 max_xid = XLogRecGetXid(record);
3275 for (i = 0; i < xlrec->nmembers; i++)
3277 if (TransactionIdPrecedes(max_xid, xlrec->members[i].xid))
3278 max_xid = xlrec->members[i].xid;
3282 * We don't expect anyone else to modify nextXid, hence startup
3283 * process doesn't need to hold a lock while checking this. We still
3284 * acquire the lock to modify it, though.
3286 if (TransactionIdFollowsOrEquals(max_xid,
3287 ShmemVariableCache->nextXid))
3289 LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
3290 ShmemVariableCache->nextXid = max_xid;
3291 TransactionIdAdvance(ShmemVariableCache->nextXid);
3292 LWLockRelease(XidGenLock);
3295 else if (info == XLOG_MULTIXACT_TRUNCATE_ID)
3297 xl_multixact_truncate xlrec;
3300 memcpy(&xlrec, XLogRecGetData(record),
3301 SizeOfMultiXactTruncate);
3303 elog(DEBUG1, "replaying multixact truncation: "
3304 "offsets [%u, %u), offsets segments [%x, %x), "
3305 "members [%u, %u), members segments [%x, %x)",
3306 xlrec.startTruncOff, xlrec.endTruncOff,
3307 MultiXactIdToOffsetSegment(xlrec.startTruncOff),
3308 MultiXactIdToOffsetSegment(xlrec.endTruncOff),
3309 xlrec.startTruncMemb, xlrec.endTruncMemb,
3310 MXOffsetToMemberSegment(xlrec.startTruncMemb),
3311 MXOffsetToMemberSegment(xlrec.endTruncMemb));
3313 /* should not be required, but more than cheap enough */
3314 LWLockAcquire(MultiXactTruncationLock, LW_EXCLUSIVE);
3317 * Advance the horizon values, so they're current at the end of
3320 SetMultiXactIdLimit(xlrec.endTruncOff, xlrec.oldestMultiDB, false);
3322 PerformMembersTruncation(xlrec.startTruncMemb, xlrec.endTruncMemb);
3325 * During XLOG replay, latest_page_number isn't necessarily set up
3326 * yet; insert a suitable value to bypass the sanity test in
3327 * SimpleLruTruncate.
3329 pageno = MultiXactIdToOffsetPage(xlrec.endTruncOff);
3330 MultiXactOffsetCtl->shared->latest_page_number = pageno;
3331 PerformOffsetsTruncation(xlrec.startTruncOff, xlrec.endTruncOff);
3333 LWLockRelease(MultiXactTruncationLock);
3336 elog(PANIC, "multixact_redo: unknown op code %u", info);
3340 pg_get_multixact_members(PG_FUNCTION_ARGS)
3344 MultiXactMember *members;
3348 MultiXactId mxid = PG_GETARG_UINT32(0);
3350 FuncCallContext *funccxt;
3352 if (mxid < FirstMultiXactId)
3354 (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
3355 errmsg("invalid MultiXactId: %u", mxid)));
3357 if (SRF_IS_FIRSTCALL())
3359 MemoryContext oldcxt;
3362 funccxt = SRF_FIRSTCALL_INIT();
3363 oldcxt = MemoryContextSwitchTo(funccxt->multi_call_memory_ctx);
3365 multi = palloc(sizeof(mxact));
3366 /* no need to allow for old values here */
3367 multi->nmembers = GetMultiXactIdMembers(mxid, &multi->members, false,
3371 tupdesc = CreateTemplateTupleDesc(2, false);
3372 TupleDescInitEntry(tupdesc, (AttrNumber) 1, "xid",
3374 TupleDescInitEntry(tupdesc, (AttrNumber) 2, "mode",
3377 funccxt->attinmeta = TupleDescGetAttInMetadata(tupdesc);
3378 funccxt->user_fctx = multi;
3380 MemoryContextSwitchTo(oldcxt);
3383 funccxt = SRF_PERCALL_SETUP();
3384 multi = (mxact *) funccxt->user_fctx;
3386 while (multi->iter < multi->nmembers)
3391 values[0] = psprintf("%u", multi->members[multi->iter].xid);
3392 values[1] = mxstatus_to_string(multi->members[multi->iter].status);
3394 tuple = BuildTupleFromCStrings(funccxt->attinmeta, values);
3398 SRF_RETURN_NEXT(funccxt, HeapTupleGetDatum(tuple));
3401 if (multi->nmembers > 0)
3402 pfree(multi->members);
3405 SRF_RETURN_DONE(funccxt);