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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. Commitb4166911
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. Asb4166911
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
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7009f1a2df
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@ -1743,13 +1743,7 @@ DropRelationFiles(RelFileNode *delrels, int ndelrels, bool isRedo)
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smgrdounlinkall(srels, ndelrels, isRedo);
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/*
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* Call smgrclose() in reverse order as when smgropen() is called.
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* This trick enables remove_from_unowned_list() in smgrclose()
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* to search the SMgrRelation from the unowned list,
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* with O(1) performance.
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*/
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for (i = ndelrels - 1; i >= 0; i--)
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for (i = 0; i < ndelrels; i++)
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smgrclose(srels[i]);
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pfree(srels);
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}
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@ -18,6 +18,7 @@
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#include "postgres.h"
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#include "commands/tablespace.h"
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#include "lib/ilist.h"
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#include "storage/bufmgr.h"
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#include "storage/ipc.h"
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#include "storage/smgr.h"
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@ -82,12 +83,10 @@ static const int NSmgr = lengthof(smgrsw);
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*/
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static HTAB *SMgrRelationHash = NULL;
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static SMgrRelation first_unowned_reln = NULL;
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static dlist_head unowned_relns;
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/* local function prototypes */
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static void smgrshutdown(int code, Datum arg);
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static void add_to_unowned_list(SMgrRelation reln);
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static void remove_from_unowned_list(SMgrRelation reln);
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/*
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@ -150,7 +149,7 @@ smgropen(RelFileNode rnode, BackendId backend)
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ctl.entrysize = sizeof(SMgrRelationData);
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SMgrRelationHash = hash_create("smgr relation table", 400,
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&ctl, HASH_ELEM | HASH_BLOBS);
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first_unowned_reln = NULL;
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dlist_init(&unowned_relns);
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}
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/* Look up or create an entry */
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@ -177,7 +176,7 @@ smgropen(RelFileNode rnode, BackendId backend)
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reln->md_num_open_segs[forknum] = 0;
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/* it has no owner yet */
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add_to_unowned_list(reln);
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dlist_push_tail(&unowned_relns, &reln->node);
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}
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return reln;
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@ -207,7 +206,7 @@ smgrsetowner(SMgrRelation *owner, SMgrRelation reln)
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if (reln->smgr_owner)
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*(reln->smgr_owner) = NULL;
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else
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remove_from_unowned_list(reln);
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dlist_delete(&reln->node);
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/* Now establish the ownership relationship. */
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reln->smgr_owner = owner;
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@ -231,53 +230,8 @@ smgrclearowner(SMgrRelation *owner, SMgrRelation reln)
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/* unset our reference to the owner */
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reln->smgr_owner = NULL;
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add_to_unowned_list(reln);
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}
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/*
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* add_to_unowned_list -- link an SMgrRelation onto the unowned list
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*
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* Check remove_from_unowned_list()'s comments for performance
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* considerations.
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*/
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static void
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add_to_unowned_list(SMgrRelation reln)
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{
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/* place it at head of the list (to make smgrsetowner cheap) */
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reln->next_unowned_reln = first_unowned_reln;
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first_unowned_reln = reln;
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}
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/*
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* remove_from_unowned_list -- unlink an SMgrRelation from the unowned list
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*
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* If the reln is not present in the list, nothing happens. Typically this
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* would be caller error, but there seems no reason to throw an error.
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*
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* In the worst case this could be rather slow; but in all the cases that seem
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* likely to be performance-critical, the reln being sought will actually be
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* first in the list. Furthermore, the number of unowned relns touched in any
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* one transaction shouldn't be all that high typically. So it doesn't seem
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* worth expending the additional space and management logic needed for a
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* doubly-linked list.
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*/
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static void
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remove_from_unowned_list(SMgrRelation reln)
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{
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SMgrRelation *link;
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SMgrRelation cur;
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for (link = &first_unowned_reln, cur = *link;
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cur != NULL;
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link = &cur->next_unowned_reln, cur = *link)
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{
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if (cur == reln)
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{
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*link = cur->next_unowned_reln;
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cur->next_unowned_reln = NULL;
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break;
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}
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}
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/* add to list of unowned relations */
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dlist_push_tail(&unowned_relns, &reln->node);
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}
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/*
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@ -304,7 +258,7 @@ smgrclose(SMgrRelation reln)
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owner = reln->smgr_owner;
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if (!owner)
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remove_from_unowned_list(reln);
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dlist_delete(&reln->node);
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if (hash_search(SMgrRelationHash,
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(void *) &(reln->smgr_rnode),
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@ -797,13 +751,19 @@ smgrpostckpt(void)
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void
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AtEOXact_SMgr(void)
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{
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dlist_mutable_iter iter;
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/*
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* Zap all unowned SMgrRelations. We rely on smgrclose() to remove each
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* one from the list.
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*/
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while (first_unowned_reln != NULL)
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dlist_foreach_modify(iter, &unowned_relns)
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{
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Assert(first_unowned_reln->smgr_owner == NULL);
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smgrclose(first_unowned_reln);
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SMgrRelation rel = dlist_container(SMgrRelationData, node,
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iter.cur);
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Assert(rel->smgr_owner == NULL);
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smgrclose(rel);
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}
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}
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@ -15,6 +15,7 @@
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#define SMGR_H
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#include "fmgr.h"
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#include "lib/ilist.h"
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#include "storage/block.h"
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#include "storage/relfilenode.h"
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@ -72,7 +73,7 @@ typedef struct SMgrRelationData
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struct _MdfdVec *md_seg_fds[MAX_FORKNUM + 1];
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/* if unowned, list link in list of all unowned SMgrRelations */
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struct SMgrRelationData *next_unowned_reln;
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dlist_node node;
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} SMgrRelationData;
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typedef SMgrRelationData *SMgrRelation;
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