* reading in or writing out a page buffer does not hold the control lock,
* only the per-buffer lock for the buffer it is working on.
*
+ * "Holding the control lock" means exclusive lock in all cases except for
+ * SimpleLruReadPage_ReadOnly(); see comments for SlruRecentlyUsed() for
+ * the implications of that.
+ *
* When initiating I/O on a buffer, we acquire the per-buffer lock exclusively
* before releasing the control lock. The per-buffer lock is released after
* completing the I/O, re-acquiring the control lock, and updating the shared
* Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
- * $PostgreSQL: pgsql/src/backend/access/transam/slru.c,v 1.31 2005/11/22 18:17:07 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/slru.c,v 1.32 2005/12/06 18:10:06 tgl Exp $
*
*-------------------------------------------------------------------------
*/
} SlruFlushData;
/*
- * Macro to mark a buffer slot "most recently used".
+ * Macro to mark a buffer slot "most recently used". Note multiple evaluation
+ * of arguments!
+ *
+ * The reason for the if-test is that there are often many consecutive
+ * accesses to the same page (particularly the latest page). By suppressing
+ * useless increments of cur_lru_count, we reduce the probability that old
+ * pages' counts will "wrap around" and make them appear recently used.
+ *
+ * We allow this code to be executed concurrently by multiple processes within
+ * SimpleLruReadPage_ReadOnly(). As long as int reads and writes are atomic,
+ * this should not cause any completely-bogus values to enter the computation.
+ * However, it is possible for either cur_lru_count or individual
+ * page_lru_count entries to be "reset" to lower values than they should have,
+ * in case a process is delayed while it executes this macro. With care in
+ * SlruSelectLRUPage(), this does little harm, and in any case the absolute
+ * worst possible consequence is a nonoptimal choice of page to evict. The
+ * gain from allowing concurrent reads of SLRU pages seems worth it.
*/
#define SlruRecentlyUsed(shared, slotno) \
do { \
- if ((shared)->page_lru_count[slotno] != 0) { \
- int iilru; \
- for (iilru = 0; iilru < NUM_SLRU_BUFFERS; iilru++) \
- (shared)->page_lru_count[iilru]++; \
- (shared)->page_lru_count[slotno] = 0; \
+ int new_lru_count = (shared)->cur_lru_count; \
+ if (new_lru_count != (shared)->page_lru_count[slotno]) { \
+ (shared)->cur_lru_count = ++new_lru_count; \
+ (shared)->page_lru_count[slotno] = new_lru_count; \
} \
} while (0)
shared->page_buffer[slotno] = bufptr;
shared->page_status[slotno] = SLRU_PAGE_EMPTY;
shared->page_dirty[slotno] = false;
- shared->page_lru_count[slotno] = 1;
+ shared->page_lru_count[slotno] = 0;
shared->buffer_locks[slotno] = LWLockAssign();
bufptr += BLCKSZ;
}
}
}
+/*
+ * Find a page in a shared buffer, reading it in if necessary.
+ * The page number must correspond to an already-initialized page.
+ * The caller must intend only read-only access to the page.
+ *
+ * The passed-in xid is used only for error reporting, and may be
+ * InvalidTransactionId if no specific xid is associated with the action.
+ *
+ * Return value is the shared-buffer slot number now holding the page.
+ * The buffer's LRU access info is updated.
+ *
+ * Control lock must NOT be held at entry, but will be held at exit.
+ * It is unspecified whether the lock will be shared or exclusive.
+ */
+int
+SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno, TransactionId xid)
+{
+ SlruShared shared = ctl->shared;
+ int slotno;
+
+ /* Try to find the page while holding only shared lock */
+ LWLockAcquire(shared->ControlLock, LW_SHARED);
+
+ /* See if page is already in a buffer */
+ for (slotno = 0; slotno < NUM_SLRU_BUFFERS; slotno++)
+ {
+ if (shared->page_number[slotno] == pageno &&
+ shared->page_status[slotno] != SLRU_PAGE_EMPTY &&
+ shared->page_status[slotno] != SLRU_PAGE_READ_IN_PROGRESS)
+ {
+ /* See comments for SlruRecentlyUsed macro */
+ SlruRecentlyUsed(shared, slotno);
+ return slotno;
+ }
+ }
+
+ /* No luck, so switch to normal exclusive lock and do regular read */
+ LWLockRelease(shared->ControlLock);
+ LWLockAcquire(shared->ControlLock, LW_EXCLUSIVE);
+
+ return SimpleLruReadPage(ctl, pageno, xid);
+}
+
/*
* Write a page from a shared buffer, if necessary.
* Does nothing if the specified slot is not dirty.
for (;;)
{
int slotno;
- int bestslot = 0;
- unsigned int bestcount = 0;
+ int cur_count;
+ int bestslot;
+ int best_delta;
+ int best_page_number;
/* See if page already has a buffer assigned */
for (slotno = 0; slotno < NUM_SLRU_BUFFERS; slotno++)
/*
* If we find any EMPTY slot, just select that one. Else locate the
- * least-recently-used slot that isn't the latest page.
+ * least-recently-used slot to replace.
+ *
+ * Normally the page_lru_count values will all be different and so
+ * there will be a well-defined LRU page. But since we allow
+ * concurrent execution of SlruRecentlyUsed() within
+ * SimpleLruReadPage_ReadOnly(), it is possible that multiple pages
+ * acquire the same lru_count values. In that case we break ties by
+ * choosing the furthest-back page.
+ *
+ * In no case will we select the slot containing latest_page_number
+ * for replacement, even if it appears least recently used.
+ *
+ * Notice that this next line forcibly advances cur_lru_count to a
+ * value that is certainly beyond any value that will be in the
+ * page_lru_count array after the loop finishes. This ensures that
+ * the next execution of SlruRecentlyUsed will mark the page newly
+ * used, even if it's for a page that has the current counter value.
+ * That gets us back on the path to having good data when there are
+ * multiple pages with the same lru_count.
*/
+ cur_count = (shared->cur_lru_count)++;
+ best_delta = -1;
+ bestslot = 0; /* no-op, just keeps compiler quiet */
+ best_page_number = 0; /* ditto */
for (slotno = 0; slotno < NUM_SLRU_BUFFERS; slotno++)
{
+ int this_delta;
+ int this_page_number;
+
if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
return slotno;
- if (shared->page_lru_count[slotno] > bestcount &&
- shared->page_number[slotno] != shared->latest_page_number)
+ this_delta = cur_count - shared->page_lru_count[slotno];
+ if (this_delta < 0)
+ {
+ /*
+ * Clean up in case shared updates have caused cur_count
+ * increments to get "lost". We back off the page counts,
+ * rather than trying to increase cur_count, to avoid any
+ * question of infinite loops or failure in the presence of
+ * wrapped-around counts.
+ */
+ shared->page_lru_count[slotno] = cur_count;
+ this_delta = 0;
+ }
+ this_page_number = shared->page_number[slotno];
+ if ((this_delta > best_delta ||
+ (this_delta == best_delta &&
+ ctl->PagePrecedes(this_page_number, best_page_number))) &&
+ this_page_number != shared->latest_page_number)
{
bestslot = slotno;
- bestcount = shared->page_lru_count[slotno];
+ best_delta = this_delta;
+ best_page_number = this_page_number;
}
}
* Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
- * $PostgreSQL: pgsql/src/include/access/slru.h,v 1.15 2005/11/05 21:19:47 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/slru.h,v 1.16 2005/12/06 18:10:06 tgl Exp $
*
*-------------------------------------------------------------------------
*/
/*
* Info for each buffer slot. Page number is undefined when status is
- * EMPTY. lru_count is essentially the number of page switches since last
- * use of this page; the page with highest lru_count is the best candidate
- * to replace.
+ * EMPTY.
*/
char *page_buffer[NUM_SLRU_BUFFERS];
SlruPageStatus page_status[NUM_SLRU_BUFFERS];
bool page_dirty[NUM_SLRU_BUFFERS];
int page_number[NUM_SLRU_BUFFERS];
- unsigned int page_lru_count[NUM_SLRU_BUFFERS];
+ int page_lru_count[NUM_SLRU_BUFFERS];
LWLockId buffer_locks[NUM_SLRU_BUFFERS];
+ /*----------
+ * We mark a page "most recently used" by setting
+ * page_lru_count[slotno] = ++cur_lru_count;
+ * The oldest page is therefore the one with the highest value of
+ * cur_lru_count - page_lru_count[slotno]
+ * The counts will eventually wrap around, but this calculation still
+ * works as long as no page's age exceeds INT_MAX counts.
+ *----------
+ */
+ int cur_lru_count;
+
/*
* latest_page_number is the page number of the current end of the log;
* this is not critical data, since we use it only to avoid swapping out
LWLockId ctllock, const char *subdir);
extern int SimpleLruZeroPage(SlruCtl ctl, int pageno);
extern int SimpleLruReadPage(SlruCtl ctl, int pageno, TransactionId xid);
+extern int SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno,
+ TransactionId xid);
extern void SimpleLruWritePage(SlruCtl ctl, int slotno, SlruFlush fdata);
extern void SimpleLruFlush(SlruCtl ctl, bool checkpoint);
extern void SimpleLruTruncate(SlruCtl ctl, int cutoffPage);
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