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Commitfb0b5b0

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Track unowned relations in doubly-linked list
Relations dropped in a single transaction are tracked in a list ofunowned relations. With large number of dropped relations this resultedin poor performance at the end of a transaction, when the relations areremoved from the singly linked list one by one.Commitb416691 attempted to address this issue (particularly when ithappens during recovery) by removing the relations in a reverse order,resulting in O(1) lookups in the list of unowned relations. This didnot 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 withrespect to the linked list, which seems rather fragile, switch to aregular doubly linked. That allows us to remove relations cheaply nomatter where in the list they are.Asb416691 was a bugfix, backpatched to all supported versions, do thesame thing here.Reviewed-by: Alvaro HerreraDiscussion:https://www.postgresql.org/message-id/flat/80c27103-99e4-1d0c-642c-d9f3b94aaa0a%402ndquadrant.comBackpatch-through: 9.4
1 parent7009f1a commitfb0b5b0

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3 files changed

+20
-65
lines changed

3 files changed

+20
-65
lines changed

‎src/backend/storage/smgr/md.c

Lines changed: 1 addition & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -1743,13 +1743,7 @@ DropRelationFiles(RelFileNode *delrels, int ndelrels, bool isRedo)
17431743

17441744
smgrdounlinkall(srels,ndelrels,isRedo);
17451745

1746-
/*
1747-
* Call smgrclose() in reverse order as when smgropen() is called.
1748-
* This trick enables remove_from_unowned_list() in smgrclose()
1749-
* to search the SMgrRelation from the unowned list,
1750-
* with O(1) performance.
1751-
*/
1752-
for (i=ndelrels-1;i >=0;i--)
1746+
for (i=0;i<ndelrels;i++)
17531747
smgrclose(srels[i]);
17541748
pfree(srels);
17551749
}

‎src/backend/storage/smgr/smgr.c

Lines changed: 17 additions & 57 deletions
Original file line numberDiff line numberDiff line change
@@ -18,6 +18,7 @@
1818
#include"postgres.h"
1919

2020
#include"commands/tablespace.h"
21+
#include"lib/ilist.h"
2122
#include"storage/bufmgr.h"
2223
#include"storage/ipc.h"
2324
#include"storage/smgr.h"
@@ -82,12 +83,10 @@ static const int NSmgr = lengthof(smgrsw);
8283
*/
8384
staticHTAB*SMgrRelationHash=NULL;
8485

85-
staticSMgrRelationfirst_unowned_reln=NULL;
86+
staticdlist_headunowned_relns;
8687

8788
/* local function prototypes */
8889
staticvoidsmgrshutdown(intcode,Datumarg);
89-
staticvoidadd_to_unowned_list(SMgrRelationreln);
90-
staticvoidremove_from_unowned_list(SMgrRelationreln);
9190

9291

9392
/*
@@ -150,7 +149,7 @@ smgropen(RelFileNode rnode, BackendId backend)
150149
ctl.entrysize=sizeof(SMgrRelationData);
151150
SMgrRelationHash=hash_create("smgr relation table",400,
152151
&ctl,HASH_ELEM |HASH_BLOBS);
153-
first_unowned_reln=NULL;
152+
dlist_init(&unowned_relns);
154153
}
155154

156155
/* Look up or create an entry */
@@ -177,7 +176,7 @@ smgropen(RelFileNode rnode, BackendId backend)
177176
reln->md_num_open_segs[forknum]=0;
178177

179178
/* it has no owner yet */
180-
add_to_unowned_list(reln);
179+
dlist_push_tail(&unowned_relns,&reln->node);
181180
}
182181

183182
returnreln;
@@ -207,7 +206,7 @@ smgrsetowner(SMgrRelation *owner, SMgrRelation reln)
207206
if (reln->smgr_owner)
208207
*(reln->smgr_owner)=NULL;
209208
else
210-
remove_from_unowned_list(reln);
209+
dlist_delete(&reln->node);
211210

212211
/* Now establish the ownership relationship. */
213212
reln->smgr_owner=owner;
@@ -231,53 +230,8 @@ smgrclearowner(SMgrRelation *owner, SMgrRelation reln)
231230
/* unset our reference to the owner */
232231
reln->smgr_owner=NULL;
233232

234-
add_to_unowned_list(reln);
235-
}
236-
237-
/*
238-
* add_to_unowned_list -- link an SMgrRelation onto the unowned list
239-
*
240-
* Check remove_from_unowned_list()'s comments for performance
241-
* considerations.
242-
*/
243-
staticvoid
244-
add_to_unowned_list(SMgrRelationreln)
245-
{
246-
/* place it at head of the list (to make smgrsetowner cheap) */
247-
reln->next_unowned_reln=first_unowned_reln;
248-
first_unowned_reln=reln;
249-
}
250-
251-
/*
252-
* remove_from_unowned_list -- unlink an SMgrRelation from the unowned list
253-
*
254-
* If the reln is not present in the list, nothing happens. Typically this
255-
* would be caller error, but there seems no reason to throw an error.
256-
*
257-
* In the worst case this could be rather slow; but in all the cases that seem
258-
* likely to be performance-critical, the reln being sought will actually be
259-
* first in the list. Furthermore, the number of unowned relns touched in any
260-
* one transaction shouldn't be all that high typically. So it doesn't seem
261-
* worth expending the additional space and management logic needed for a
262-
* doubly-linked list.
263-
*/
264-
staticvoid
265-
remove_from_unowned_list(SMgrRelationreln)
266-
{
267-
SMgrRelation*link;
268-
SMgrRelationcur;
269-
270-
for (link=&first_unowned_reln,cur=*link;
271-
cur!=NULL;
272-
link=&cur->next_unowned_reln,cur=*link)
273-
{
274-
if (cur==reln)
275-
{
276-
*link=cur->next_unowned_reln;
277-
cur->next_unowned_reln=NULL;
278-
break;
279-
}
280-
}
233+
/* add to list of unowned relations */
234+
dlist_push_tail(&unowned_relns,&reln->node);
281235
}
282236

283237
/*
@@ -304,7 +258,7 @@ smgrclose(SMgrRelation reln)
304258
owner=reln->smgr_owner;
305259

306260
if (!owner)
307-
remove_from_unowned_list(reln);
261+
dlist_delete(&reln->node);
308262

309263
if (hash_search(SMgrRelationHash,
310264
(void*)&(reln->smgr_rnode),
@@ -797,13 +751,19 @@ smgrpostckpt(void)
797751
void
798752
AtEOXact_SMgr(void)
799753
{
754+
dlist_mutable_iteriter;
755+
800756
/*
801757
* Zap all unowned SMgrRelations. We rely on smgrclose() to remove each
802758
* one from the list.
803759
*/
804-
while (first_unowned_reln!=NULL)
760+
dlist_foreach_modify(iter,&unowned_relns)
805761
{
806-
Assert(first_unowned_reln->smgr_owner==NULL);
807-
smgrclose(first_unowned_reln);
762+
SMgrRelationrel=dlist_container(SMgrRelationData,node,
763+
iter.cur);
764+
765+
Assert(rel->smgr_owner==NULL);
766+
767+
smgrclose(rel);
808768
}
809769
}

‎src/include/storage/smgr.h

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -15,6 +15,7 @@
1515
#defineSMGR_H
1616

1717
#include"fmgr.h"
18+
#include"lib/ilist.h"
1819
#include"storage/block.h"
1920
#include"storage/relfilenode.h"
2021

@@ -72,7 +73,7 @@ typedef struct SMgrRelationData
7273
struct_MdfdVec*md_seg_fds[MAX_FORKNUM+1];
7374

7475
/* if unowned, list link in list of all unowned SMgrRelations */
75-
structSMgrRelationData*next_unowned_reln;
76+
dlist_nodenode;
7677
}SMgrRelationData;
7778

7879
typedefSMgrRelationData*SMgrRelation;

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