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 b416691 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 b416691 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

Author: tvondra

src/backend/storage/smgr/md.c

@@ -1743,13 +1743,7 @@ 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);
 }

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"
@@ -82,12 +83,10 @@ 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);
 
 
 /*
@@ -150,7 +149,7 @@ 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 */
@@ -177,7 +176,7 @@ 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;
@@ -207,7 +206,7 @@ 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;
@@ -231,53 +230,8 @@ 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);
 }
 
 /*
@@ -304,7 +258,7 @@ 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),
@@ -797,13 +751,19 @@ 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);
 	}
 }

src/include/storage/smgr.h

@@ -15,6 +15,7 @@
 #define SMGR_H
 
 #include "fmgr.h"
+#include "lib/ilist.h"
 #include "storage/block.h"
 #include "storage/relfilenode.h"
 
@@ -72,7 +73,7 @@ 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;