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Commit2924ca6

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Tweak dynahash.c to avoid wasting memory space in non-shared hash tables.
palloc() will normally round allocation requests up to the next power of 2,so make dynahash choose allocation sizes that are as close to a power of 2as possible.Back-patch to 8.1 --- the problem exists further back, but a much largerpatch would be needed and it doesn't seem worth taking any risks.
1 parentf5940e7 commit2924ca6

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2 files changed

+44
-11
lines changed

2 files changed

+44
-11
lines changed

‎src/backend/utils/hash/dynahash.c

Lines changed: 43 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -9,7 +9,7 @@
99
*
1010
*
1111
* IDENTIFICATION
12-
* $PostgreSQL: pgsql/src/backend/utils/hash/dynahash.c,v 1.67 2006/03/05 15:58:46 momjian Exp $
12+
* $PostgreSQL: pgsql/src/backend/utils/hash/dynahash.c,v 1.68 2006/06/25 18:29:49 tgl Exp $
1313
*
1414
*-------------------------------------------------------------------------
1515
*/
@@ -68,6 +68,7 @@ static bool element_alloc(HTAB *hashp, int nelem);
6868
staticbooldir_realloc(HTAB*hashp);
6969
staticboolexpand_table(HTAB*hashp);
7070
staticvoidhdefault(HTAB*hashp);
71+
staticintchoose_nelem_alloc(Sizeentrysize);
7172
staticboolinit_htab(HTAB*hashp,longnelem);
7273
staticvoidhash_corrupted(HTAB*hashp);
7374

@@ -262,8 +263,13 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
262263
/*
263264
* For a shared hash table, preallocate the requested number of elements.
264265
* This reduces problems with run-time out-of-shared-memory conditions.
266+
*
267+
* For a non-shared hash table, preallocate the requested number of
268+
* elements if it's less than our chosen nelem_alloc. This avoids
269+
* wasting space if the caller correctly estimates a small table size.
265270
*/
266-
if (flags&HASH_SHARED_MEM)
271+
if ((flags&HASH_SHARED_MEM)||
272+
nelem<hctl->nelem_alloc)
267273
{
268274
if (!element_alloc(hashp, (int)nelem))
269275
{
@@ -305,6 +311,37 @@ hdefault(HTAB *hashp)
305311
hctl->freeList=NULL;
306312
}
307313

314+
/*
315+
* Given the user-specified entry size, choose nelem_alloc, ie, how many
316+
* elements to add to the hash table when we need more.
317+
*/
318+
staticint
319+
choose_nelem_alloc(Sizeentrysize)
320+
{
321+
intnelem_alloc;
322+
SizeelementSize;
323+
SizeallocSize;
324+
325+
/* Each element has a HASHELEMENT header plus user data. */
326+
/* NB: this had better match element_alloc() */
327+
elementSize=MAXALIGN(sizeof(HASHELEMENT))+MAXALIGN(entrysize);
328+
329+
/*
330+
* The idea here is to choose nelem_alloc at least 32, but round up
331+
* so that the allocation request will be a power of 2 or just less.
332+
* This makes little difference for hash tables in shared memory,
333+
* but for hash tables managed by palloc, the allocation request
334+
* will be rounded up to a power of 2 anyway. If we fail to take
335+
* this into account, we'll waste as much as half the allocated space.
336+
*/
337+
allocSize=32*4;/* assume elementSize at least 8 */
338+
do {
339+
allocSize <<=1;
340+
nelem_alloc=allocSize /elementSize;
341+
}while (nelem_alloc<32);
342+
343+
returnnelem_alloc;
344+
}
308345

309346
staticbool
310347
init_htab(HTAB*hashp,longnelem)
@@ -364,8 +401,7 @@ init_htab(HTAB *hashp, long nelem)
364401
}
365402

366403
/* Choose number of entries to allocate at a time */
367-
hctl->nelem_alloc= (int)Min(nelem,HASHELEMENT_ALLOC_MAX);
368-
hctl->nelem_alloc=Max(hctl->nelem_alloc,1);
404+
hctl->nelem_alloc=choose_nelem_alloc(hctl->entrysize);
369405

370406
#ifHASH_DEBUG
371407
fprintf(stderr,"init_htab:\n%s%p\n%s%ld\n%s%ld\n%s%d\n%s%ld\n%s%u\n%s%x\n%s%x\n%s%ld\n%s%ld\n",
@@ -417,11 +453,10 @@ hash_estimate_size(long num_entries, Size entrysize)
417453
/* segments */
418454
size=add_size(size,mul_size(nSegments,
419455
MAXALIGN(DEF_SEGSIZE*sizeof(HASHBUCKET))));
420-
/* elements --- allocated in groups of up to HASHELEMENT_ALLOC_MAX */
421-
elementSize=MAXALIGN(sizeof(HASHELEMENT))+MAXALIGN(entrysize);
422-
elementAllocCnt=Min(num_entries,HASHELEMENT_ALLOC_MAX);
423-
elementAllocCnt=Max(elementAllocCnt,1);
456+
/* elements --- allocated in groups of choose_nelem_alloc() entries */
457+
elementAllocCnt=choose_nelem_alloc(entrysize);
424458
nElementAllocs= (num_entries-1) /elementAllocCnt+1;
459+
elementSize=MAXALIGN(sizeof(HASHELEMENT))+MAXALIGN(entrysize);
425460
size=add_size(size,
426461
mul_size(nElementAllocs,
427462
mul_size(elementAllocCnt,elementSize)));

‎src/include/utils/hsearch.h

Lines changed: 1 addition & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -7,7 +7,7 @@
77
* Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
88
* Portions Copyright (c) 1994, Regents of the University of California
99
*
10-
* $PostgreSQL: pgsql/src/include/utils/hsearch.h,v 1.42 2006/03/05 15:59:07 momjian Exp $
10+
* $PostgreSQL: pgsql/src/include/utils/hsearch.h,v 1.43 2006/06/25 18:29:49 tgl Exp $
1111
*
1212
*-------------------------------------------------------------------------
1313
*/
@@ -156,8 +156,6 @@ typedef struct HASHCTL
156156

157157
/* max_dsize value to indicate expansible directory */
158158
#defineNO_MAX_DSIZE(-1)
159-
/* max number of hash elements allocated at once */
160-
#defineHASHELEMENT_ALLOC_MAX(32)
161159

162160
/* hash_search operations */
163161
typedefenum

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