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Tweak dynahash.c to avoid wasting memory space in non-shared hash tables.

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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 2
as possible.
Back-patch to 8.1 --- the problem exists further back, but a much larger
patch would be needed and it doesn't seem worth taking any risks.
This commit is contained in:
Tom Lane 2006-06-25 18:29:49 +00:00
parent f5940e7b34
commit 2924ca6747
2 changed files with 44 additions and 11 deletions
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51
src/backend/utils/hash/dynahash.c
View File

@ -9,7 +9,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/utils/hash/dynahash.c,v 1.67 2006/03/05 15:58:46 momjian Exp $
* $PostgreSQL: pgsql/src/backend/utils/hash/dynahash.c,v 1.68 2006/06/25 18:29:49 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -68,6 +68,7 @@ static bool element_alloc(HTAB *hashp, int nelem);
static bool dir_realloc(HTAB *hashp);
static bool expand_table(HTAB *hashp);
static void hdefault(HTAB *hashp);
static int choose_nelem_alloc(Size entrysize);
static bool init_htab(HTAB *hashp, long nelem);
static void hash_corrupted(HTAB *hashp);
@ -262,8 +263,13 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
/*
* For a shared hash table, preallocate the requested number of elements.
* This reduces problems with run-time out-of-shared-memory conditions.
*
* For a non-shared hash table, preallocate the requested number of
* elements if it's less than our chosen nelem_alloc. This avoids
* wasting space if the caller correctly estimates a small table size.
*/
if (flags & HASH_SHARED_MEM)
if ((flags & HASH_SHARED_MEM) ||
nelem < hctl->nelem_alloc)
{
if (!element_alloc(hashp, (int) nelem))
{
@ -305,6 +311,37 @@ hdefault(HTAB *hashp)
hctl->freeList = NULL;
}
/*
* Given the user-specified entry size, choose nelem_alloc, ie, how many
* elements to add to the hash table when we need more.
*/
static int
choose_nelem_alloc(Size entrysize)
{
int nelem_alloc;
Size elementSize;
Size allocSize;
/* Each element has a HASHELEMENT header plus user data. */
/* NB: this had better match element_alloc() */
elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(entrysize);
/*
* The idea here is to choose nelem_alloc at least 32, but round up
* so that the allocation request will be a power of 2 or just less.
* This makes little difference for hash tables in shared memory,
* but for hash tables managed by palloc, the allocation request
* will be rounded up to a power of 2 anyway. If we fail to take
* this into account, we'll waste as much as half the allocated space.
*/
allocSize = 32 * 4; /* assume elementSize at least 8 */
do {
allocSize <<= 1;
nelem_alloc = allocSize / elementSize;
} while (nelem_alloc < 32);
return nelem_alloc;
}
static bool
init_htab(HTAB *hashp, long nelem)
@ -364,8 +401,7 @@ init_htab(HTAB *hashp, long nelem)
}
/* Choose number of entries to allocate at a time */
hctl->nelem_alloc = (int) Min(nelem, HASHELEMENT_ALLOC_MAX);
hctl->nelem_alloc = Max(hctl->nelem_alloc, 1);
hctl->nelem_alloc = choose_nelem_alloc(hctl->entrysize);
#if HASH_DEBUG
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)
/* segments */
size = add_size(size, mul_size(nSegments,
MAXALIGN(DEF_SEGSIZE * sizeof(HASHBUCKET))));
/* elements --- allocated in groups of up to HASHELEMENT_ALLOC_MAX */
elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(entrysize);
elementAllocCnt = Min(num_entries, HASHELEMENT_ALLOC_MAX);
elementAllocCnt = Max(elementAllocCnt, 1);
/* elements --- allocated in groups of choose_nelem_alloc() entries */
elementAllocCnt = choose_nelem_alloc(entrysize);
nElementAllocs = (num_entries - 1) / elementAllocCnt + 1;
elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(entrysize);
size = add_size(size,
mul_size(nElementAllocs,
mul_size(elementAllocCnt, elementSize)));

4
src/include/utils/hsearch.h
View File

@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/utils/hsearch.h,v 1.42 2006/03/05 15:59:07 momjian Exp $
* $PostgreSQL: pgsql/src/include/utils/hsearch.h,v 1.43 2006/06/25 18:29:49 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -156,8 +156,6 @@ typedef struct HASHCTL
/* max_dsize value to indicate expansible directory */
#define NO_MAX_DSIZE (-1)
/* max number of hash elements allocated at once */
#define HASHELEMENT_ALLOC_MAX (32)
/* hash_search operations */
typedef enum