Movatterモバイル変換

[0]ホーム
URL:

Skip to content

Navigation Menu

Sign in
Appearance settings

Search code, repositories, users, issues, pull requests...

Provide feedback

We read every piece of feedback, and take your input very seriously.

Saved searches

Use saved searches to filter your results more quickly

 Sign up
Appearance settings

Commit6016005

Browse files
committed
Fix performance problem when building a lossy tidbitmap.
As pointed out by Sergey Koposov, repeated invocations of tbm_lossify canmake building a large tidbitmap into an O(N^2) operation. To fix, makesure we remove more than the minimum amount of information per call, andadd a fallback path to behave sanely if we're unable to fit the bitmapwithin the requested amount of memory.This has been wrong since the tidbitmap code was written, so back-patchto all supported branches.
1 parent44631ee commit6016005

File tree

1 file changed

+19
-3
lines changed

1 file changed

+19
-3
lines changed

‎src/backend/nodes/tidbitmap.c

Lines changed: 19 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -884,8 +884,11 @@ tbm_lossify(TIDBitmap *tbm)
884884
/*
885885
* XXX Really stupid implementation: this just lossifies pages in
886886
* essentially random order. We should be paying some attention to the
887-
* number of bits set in each page, instead. Also it might be a good idea
888-
* to lossify more than the minimum number of pages during each call.
887+
* number of bits set in each page, instead.
888+
*
889+
* Since we are called as soon as nentries exceeds maxentries, we should
890+
* push nentries down to significantly less than maxentries, or else we'll
891+
* just end up doing this again very soon. We shoot for maxentries/2.
889892
*/
890893
Assert(!tbm->iterating);
891894
Assert(tbm->status==TBM_HASH);
@@ -906,7 +909,7 @@ tbm_lossify(TIDBitmap *tbm)
906909
/* This does the dirty work ... */
907910
tbm_mark_page_lossy(tbm,page->blockno);
908911

909-
if (tbm->nentries <=tbm->maxentries)
912+
if (tbm->nentries <=tbm->maxentries /2)
910913
{
911914
/* we have done enough */
912915
hash_seq_term(&status);
@@ -919,6 +922,19 @@ tbm_lossify(TIDBitmap *tbm)
919922
* not care whether we visit lossy chunks or not.
920923
*/
921924
}
925+
926+
/*
927+
* With a big bitmap and small work_mem, it's possible that we cannot
928+
* get under maxentries. Again, if that happens, we'd end up uselessly
929+
* calling tbm_lossify over and over. To prevent this from becoming a
930+
* performance sink, force maxentries up to at least double the current
931+
* number of entries. (In essence, we're admitting inability to fit
932+
* within work_mem when we do this.) Note that this test will not fire
933+
* if we broke out of the loop early; and if we didn't, the current
934+
* number of entries is simply not reducible any further.
935+
*/
936+
if (tbm->nentries>tbm->maxentries /2)
937+
tbm->maxentries=Min(tbm->nentries, (INT_MAX-1) /2)*2;
922938
}
923939

924940
/*

0 commit comments

Comments
 (0)
[8]ページ先頭
©2009-2025 Movatter.jp