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Commit65c6cab

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committed
Avoid O(N^2) behavior in SyncPostCheckpoint().
As in commits6301c3a ande9d9ba2, avoid doing repetitivelist_delete_first() operations, since that would be expensive whenthere are many files waiting to be unlinked. This is a slightlylarger change than in those cases. We have to keep the list statevalid for calls to AbsorbSyncRequests(), so it's necessary to invent a"canceled" field instead of immediately deleting PendingUnlinkEntryentries. Also, because we might not be able to process all theentries, we need a new list primitive list_delete_first_n().list_delete_first_n() is almost list_copy_tail(), but it modifies theinput List instead of making a new copy. I found a couple of existinguses of the latter that could profitably use the new function. (Theremight be more, but the other callers look like they probably shouldn'toverwrite the input List.)As before, back-patch to v13.Discussion:https://postgr.es/m/CD2F0E7F-9822-45EC-A411-AE56F14DEA9F@amazon.com
1 parentd8dba4d commit65c6cab

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5 files changed

+103
-14
lines changed

5 files changed

+103
-14
lines changed

‎src/backend/nodes/list.c

Lines changed: 66 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -906,6 +906,72 @@ list_delete_last(List *list)
906906
returnlist_truncate(list,list_length(list)-1);
907907
}
908908

909+
/*
910+
* Delete the first N cells of the list.
911+
*
912+
* The List is pfree'd if the request causes all cells to be deleted.
913+
*/
914+
List*
915+
list_delete_first_n(List*list,intn)
916+
{
917+
check_list_invariants(list);
918+
919+
/* No-op request? */
920+
if (n <=0)
921+
returnlist;
922+
923+
/* Delete whole list? */
924+
if (n >=list_length(list))
925+
{
926+
list_free(list);
927+
returnNIL;
928+
}
929+
930+
/*
931+
* Otherwise, we normally just collapse out the removed elements. But for
932+
* debugging purposes, move the whole list contents someplace else.
933+
*
934+
* (Note that we *must* keep the contents in the same memory context.)
935+
*/
936+
#ifndefDEBUG_LIST_MEMORY_USAGE
937+
memmove(&list->elements[0],&list->elements[n],
938+
(list->length-n)*sizeof(ListCell));
939+
list->length-=n;
940+
#else
941+
{
942+
ListCell*newelems;
943+
intnewmaxlen=list->length-n;
944+
945+
newelems= (ListCell*)
946+
MemoryContextAlloc(GetMemoryChunkContext(list),
947+
newmaxlen*sizeof(ListCell));
948+
memcpy(newelems,&list->elements[n],newmaxlen*sizeof(ListCell));
949+
if (list->elements!=list->initial_elements)
950+
pfree(list->elements);
951+
else
952+
{
953+
/*
954+
* As in enlarge_list(), clear the initial_elements[] space and/or
955+
* mark it inaccessible.
956+
*/
957+
#ifdefCLOBBER_FREED_MEMORY
958+
wipe_mem(list->initial_elements,
959+
list->max_length*sizeof(ListCell));
960+
#else
961+
VALGRIND_MAKE_MEM_NOACCESS(list->initial_elements,
962+
list->max_length*sizeof(ListCell));
963+
#endif
964+
}
965+
list->elements=newelems;
966+
list->max_length=newmaxlen;
967+
list->length=newmaxlen;
968+
check_list_invariants(list);
969+
}
970+
#endif
971+
972+
returnlist;
973+
}
974+
909975
/*
910976
* Generate the union of two lists. This is calculated by copying
911977
* list1 via list_copy(), then adding to it all the members of list2

‎src/backend/optimizer/util/clauses.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -4139,7 +4139,7 @@ add_function_defaults(List *args, int pronargs, HeapTuple func_tuple)
41394139
if (ndelete<0)
41404140
elog(ERROR,"not enough default arguments");
41414141
if (ndelete>0)
4142-
defaults=list_copy_tail(defaults,ndelete);
4142+
defaults=list_delete_first_n(defaults,ndelete);
41434143

41444144
/* And form the combined argument list, not modifying the input list */
41454145
returnlist_concat_copy(args,defaults);

‎src/backend/parser/parse_func.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1691,7 +1691,7 @@ func_get_detail(List *funcname,
16911691

16921692
ndelete=list_length(defaults)-best_candidate->ndargs;
16931693
if (ndelete>0)
1694-
defaults=list_copy_tail(defaults,ndelete);
1694+
defaults=list_delete_first_n(defaults,ndelete);
16951695
*argdefaults=defaults;
16961696
}
16971697
}

‎src/backend/storage/sync/sync.c

Lines changed: 34 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -69,6 +69,7 @@ typedef struct
6969
{
7070
FileTagtag;/* identifies handler and file */
7171
CycleCtrcycle_ctr;/* checkpoint_cycle_ctr when request was made */
72+
boolcanceled;/* true if request has been canceled */
7273
}PendingUnlinkEntry;
7374

7475
staticHTAB*pendingOps=NULL;
@@ -195,13 +196,18 @@ void
195196
SyncPostCheckpoint(void)
196197
{
197198
intabsorb_counter;
199+
ListCell*lc;
198200

199201
absorb_counter=UNLINKS_PER_ABSORB;
200-
while (pendingUnlinks!=NIL)
202+
foreach(lc,pendingUnlinks)
201203
{
202-
PendingUnlinkEntry*entry= (PendingUnlinkEntry*)linitial(pendingUnlinks);
204+
PendingUnlinkEntry*entry= (PendingUnlinkEntry*)lfirst(lc);
203205
charpath[MAXPGPATH];
204206

207+
/* Skip over any canceled entries */
208+
if (entry->canceled)
209+
continue;
210+
205211
/*
206212
* New entries are appended to the end, so if the entry is new we've
207213
* reached the end of old entries.
@@ -231,22 +237,40 @@ SyncPostCheckpoint(void)
231237
errmsg("could not remove file \"%s\": %m",path)));
232238
}
233239

234-
/* And remove the list entry */
235-
pendingUnlinks=list_delete_first(pendingUnlinks);
236-
pfree(entry);
240+
/* Mark the list entry as canceled, just in case */
241+
entry->canceled= true;
237242

238243
/*
239244
* As in ProcessSyncRequests, we don't want to stop absorbing fsync
240245
* requests for a long time when there are many deletions to be done.
241-
* We can safely call AbsorbSyncRequests() at this point in the loop
242-
* (note it might try to delete list entries).
246+
* We can safely call AbsorbSyncRequests() at this point in the loop.
243247
*/
244248
if (--absorb_counter <=0)
245249
{
246250
AbsorbSyncRequests();
247251
absorb_counter=UNLINKS_PER_ABSORB;
248252
}
249253
}
254+
255+
/*
256+
* If we reached the end of the list, we can just remove the whole list
257+
* (remembering to pfree all the PendingUnlinkEntry objects). Otherwise,
258+
* we must keep the entries at or after "lc".
259+
*/
260+
if (lc==NULL)
261+
{
262+
list_free_deep(pendingUnlinks);
263+
pendingUnlinks=NIL;
264+
}
265+
else
266+
{
267+
intntodelete=list_cell_number(pendingUnlinks,lc);
268+
269+
for (inti=0;i<ntodelete;i++)
270+
pfree(list_nth(pendingUnlinks,i));
271+
272+
pendingUnlinks=list_delete_first_n(pendingUnlinks,ntodelete);
273+
}
250274
}
251275

252276
/*
@@ -486,17 +510,14 @@ RememberSyncRequest(const FileTag *ftag, SyncRequestType type)
486510
entry->canceled= true;
487511
}
488512

489-
/*Remove matching unlink requests */
513+
/*Cancel matching unlink requests */
490514
foreach(cell,pendingUnlinks)
491515
{
492516
PendingUnlinkEntry*entry= (PendingUnlinkEntry*)lfirst(cell);
493517

494518
if (entry->tag.handler==ftag->handler&&
495519
syncsw[ftag->handler].sync_filetagmatches(ftag,&entry->tag))
496-
{
497-
pendingUnlinks=foreach_delete_current(pendingUnlinks,cell);
498-
pfree(entry);
499-
}
520+
entry->canceled= true;
500521
}
501522
}
502523
elseif (type==SYNC_UNLINK_REQUEST)
@@ -508,6 +529,7 @@ RememberSyncRequest(const FileTag *ftag, SyncRequestType type)
508529
entry=palloc(sizeof(PendingUnlinkEntry));
509530
entry->tag=*ftag;
510531
entry->cycle_ctr=checkpoint_cycle_ctr;
532+
entry->canceled= false;
511533

512534
pendingUnlinks=lappend(pendingUnlinks,entry);
513535

‎src/include/nodes/pg_list.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -564,6 +564,7 @@ extern pg_nodiscard List *list_delete_int(List *list, int datum);
564564
externpg_nodiscardList*list_delete_oid(List*list,Oiddatum);
565565
externpg_nodiscardList*list_delete_first(List*list);
566566
externpg_nodiscardList*list_delete_last(List*list);
567+
externpg_nodiscardList*list_delete_first_n(List*list,intn);
567568
externpg_nodiscardList*list_delete_nth_cell(List*list,intn);
568569
externpg_nodiscardList*list_delete_cell(List*list,ListCell*cell);
569570

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