Track unowned relations in doubly-linked list

Relations dropped in a single transaction are tracked in a list of
unowned relations.  With large number of dropped relations this resulted
in poor performance at the end of a transaction, when the relations are
removed from the singly linked list one by one.

Commit b4166911 attempted to address this issue (particularly when it
happens during recovery) by removing the relations in a reverse order,
resulting in O(1) lookups in the list of unowned relations.  This did
not work reliably, though, and it was possible to trigger the O(N^2)
behavior in various ways.

Instead of trying to remove the relations in a specific order with
respect to the linked list, which seems rather fragile, switch to a
regular doubly linked.  That allows us to remove relations cheaply no
matter where in the list they are.

As b4166911 was a bugfix, backpatched to all supported versions, do the
same thing here.

Reviewed-by: Alvaro Herrera
Discussion: https://www.postgresql.org/message-id/flat/80c27103-99e4-1d0c-642c-d9f3b94aaa0a%402ndquadrant.com
Backpatch-through: 9.4

diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
index 2aba2dfe91763858a56b1f45e4cb76c3a4966199..6ed68185edb9fdcea69f9e01ed054813e7023644 100644 (file)
--- a/src/backend/storage/smgr/md.c
+++ b/src/backend/storage/smgr/md.c
@@ -1699,13 +1699,7 @@ DropRelationFiles(RelFileNode *delrels, int ndelrels, bool isRedo)
 
        smgrdounlinkall(srels, ndelrels, isRedo);
 
-       /*
-        * Call smgrclose() in reverse order as when smgropen() is called.
-        * This trick enables remove_from_unowned_list() in smgrclose()
-        * to search the SMgrRelation from the unowned list,
-        * with O(1) performance.
-        */
-       for (i = ndelrels - 1; i >= 0; i--)
+       for (i = 0; i < ndelrels; i++)
                smgrclose(srels[i]);
        pfree(srels);
 }

diff --git a/src/backend/storage/smgr/smgr.c b/src/backend/storage/smgr/smgr.c
index 0c0bba4ab3347ccc59ca7ed495b036d0e20de26a..f6de9df9e619e043d80e85fe638182a18860257d 100644 (file)
--- a/src/backend/storage/smgr/smgr.c
+++ b/src/backend/storage/smgr/smgr.c
@@ -18,6 +18,7 @@
 #include "postgres.h"
 
 #include "commands/tablespace.h"
+#include "lib/ilist.h"
 #include "storage/bufmgr.h"
 #include "storage/ipc.h"
 #include "storage/smgr.h"
@@ -97,12 +98,10 @@ static const int NSmgr = lengthof(smgrsw);
  */
 static HTAB *SMgrRelationHash = NULL;
 
-static SMgrRelation first_unowned_reln = NULL;
+static dlist_head      unowned_relns;
 
 /* local function prototypes */
 static void smgrshutdown(int code, Datum arg);
-static void add_to_unowned_list(SMgrRelation reln);
-static void remove_from_unowned_list(SMgrRelation reln);
 
 
 /*
@@ -165,7 +164,7 @@ smgropen(RelFileNode rnode, BackendId backend)
                ctl.entrysize = sizeof(SMgrRelationData);
                SMgrRelationHash = hash_create("smgr relation table", 400,
                                                                           &ctl, HASH_ELEM | HASH_BLOBS);
-               first_unowned_reln = NULL;
+               dlist_init(&unowned_relns);
        }
 
        /* Look up or create an entry */
@@ -192,7 +191,7 @@ smgropen(RelFileNode rnode, BackendId backend)
                        reln->md_num_open_segs[forknum] = 0;
 
                /* it has no owner yet */
-               add_to_unowned_list(reln);
+               dlist_push_tail(&unowned_relns, &reln->node);
        }
 
        return reln;
@@ -222,7 +221,7 @@ smgrsetowner(SMgrRelation *owner, SMgrRelation reln)
        if (reln->smgr_owner)
                *(reln->smgr_owner) = NULL;
        else
-               remove_from_unowned_list(reln);
+               dlist_delete(&reln->node);
 
        /* Now establish the ownership relationship. */
        reln->smgr_owner = owner;
@@ -246,53 +245,8 @@ smgrclearowner(SMgrRelation *owner, SMgrRelation reln)
        /* unset our reference to the owner */
        reln->smgr_owner = NULL;
 
-       add_to_unowned_list(reln);
-}
-
-/*
- * add_to_unowned_list -- link an SMgrRelation onto the unowned list
- *
- * Check remove_from_unowned_list()'s comments for performance
- * considerations.
- */
-static void
-add_to_unowned_list(SMgrRelation reln)
-{
-       /* place it at head of the list (to make smgrsetowner cheap) */
-       reln->next_unowned_reln = first_unowned_reln;
-       first_unowned_reln = reln;
-}
-
-/*
- * remove_from_unowned_list -- unlink an SMgrRelation from the unowned list
- *
- * If the reln is not present in the list, nothing happens.  Typically this
- * would be caller error, but there seems no reason to throw an error.
- *
- * In the worst case this could be rather slow; but in all the cases that seem
- * likely to be performance-critical, the reln being sought will actually be
- * first in the list.  Furthermore, the number of unowned relns touched in any
- * one transaction shouldn't be all that high typically.  So it doesn't seem
- * worth expending the additional space and management logic needed for a
- * doubly-linked list.
- */
-static void
-remove_from_unowned_list(SMgrRelation reln)
-{
-       SMgrRelation *link;
-       SMgrRelation cur;
-
-       for (link = &first_unowned_reln, cur = *link;
-                cur != NULL;
-                link = &cur->next_unowned_reln, cur = *link)
-       {
-               if (cur == reln)
-               {
-                       *link = cur->next_unowned_reln;
-                       cur->next_unowned_reln = NULL;
-                       break;
-               }
-       }
+       /* add to list of unowned relations */
+       dlist_push_tail(&unowned_relns, &reln->node);
 }
 
 /*
@@ -319,7 +273,7 @@ smgrclose(SMgrRelation reln)
        owner = reln->smgr_owner;
 
        if (!owner)
-               remove_from_unowned_list(reln);
+               dlist_delete(&reln->node);
 
        if (hash_search(SMgrRelationHash,
                                        (void *) &(reln->smgr_rnode),
@@ -812,13 +766,19 @@ smgrpostckpt(void)
 void
 AtEOXact_SMgr(void)
 {
+       dlist_mutable_iter      iter;
+
        /*
         * Zap all unowned SMgrRelations.  We rely on smgrclose() to remove each
         * one from the list.
         */
-       while (first_unowned_reln != NULL)
+       dlist_foreach_modify(iter, &unowned_relns)
        {
-               Assert(first_unowned_reln->smgr_owner == NULL);
-               smgrclose(first_unowned_reln);
+               SMgrRelation    rel = dlist_container(SMgrRelationData, node,
+                                                                                         iter.cur);
+
+               Assert(rel->smgr_owner == NULL);
+
+               smgrclose(rel);
        }
 }

diff --git a/src/include/storage/smgr.h b/src/include/storage/smgr.h
index 4d60b28dac3b099e49bcf71314920982ed30258a..8e982738789dae0ccb13f72e3d0311e65837210d 100644 (file)
--- a/src/include/storage/smgr.h
+++ b/src/include/storage/smgr.h
@@ -14,6 +14,7 @@
 #ifndef SMGR_H
 #define SMGR_H
 
+#include "lib/ilist.h"
 #include "storage/block.h"
 #include "storage/relfilenode.h"
 
@@ -71,7 +72,7 @@ typedef struct SMgrRelationData
        struct _MdfdVec *md_seg_fds[MAX_FORKNUM + 1];
 
        /* if unowned, list link in list of all unowned SMgrRelations */
-       struct SMgrRelationData *next_unowned_reln;
+       dlist_node      node;
 } SMgrRelationData;
 
 typedef SMgrRelationData *SMgrRelation;