mirror of https://github.com/postgres/postgres
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 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
parent
f5940e7b34
commit
2924ca6747
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@ -9,7 +9,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/utils/hash/dynahash.c,v 1.67 2006/03/05 15:58:46 momjian Exp $
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* $PostgreSQL: pgsql/src/backend/utils/hash/dynahash.c,v 1.68 2006/06/25 18:29:49 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -68,6 +68,7 @@ static bool element_alloc(HTAB *hashp, int nelem);
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static bool dir_realloc(HTAB *hashp);
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static bool expand_table(HTAB *hashp);
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static void hdefault(HTAB *hashp);
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static int choose_nelem_alloc(Size entrysize);
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static bool init_htab(HTAB *hashp, long nelem);
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static void hash_corrupted(HTAB *hashp);
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@ -262,8 +263,13 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
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/*
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* For a shared hash table, preallocate the requested number of elements.
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* This reduces problems with run-time out-of-shared-memory conditions.
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*
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* For a non-shared hash table, preallocate the requested number of
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* elements if it's less than our chosen nelem_alloc. This avoids
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* wasting space if the caller correctly estimates a small table size.
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*/
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if (flags & HASH_SHARED_MEM)
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if ((flags & HASH_SHARED_MEM) ||
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nelem < hctl->nelem_alloc)
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{
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if (!element_alloc(hashp, (int) nelem))
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{
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@ -305,6 +311,37 @@ hdefault(HTAB *hashp)
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hctl->freeList = NULL;
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}
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/*
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* Given the user-specified entry size, choose nelem_alloc, ie, how many
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* elements to add to the hash table when we need more.
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*/
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static int
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choose_nelem_alloc(Size entrysize)
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{
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int nelem_alloc;
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Size elementSize;
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Size allocSize;
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/* Each element has a HASHELEMENT header plus user data. */
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/* NB: this had better match element_alloc() */
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elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(entrysize);
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/*
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* The idea here is to choose nelem_alloc at least 32, but round up
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* so that the allocation request will be a power of 2 or just less.
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* This makes little difference for hash tables in shared memory,
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* but for hash tables managed by palloc, the allocation request
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* will be rounded up to a power of 2 anyway. If we fail to take
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* this into account, we'll waste as much as half the allocated space.
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*/
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allocSize = 32 * 4; /* assume elementSize at least 8 */
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do {
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allocSize <<= 1;
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nelem_alloc = allocSize / elementSize;
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} while (nelem_alloc < 32);
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return nelem_alloc;
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}
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static bool
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init_htab(HTAB *hashp, long nelem)
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@ -364,8 +401,7 @@ init_htab(HTAB *hashp, long nelem)
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}
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/* Choose number of entries to allocate at a time */
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hctl->nelem_alloc = (int) Min(nelem, HASHELEMENT_ALLOC_MAX);
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hctl->nelem_alloc = Max(hctl->nelem_alloc, 1);
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hctl->nelem_alloc = choose_nelem_alloc(hctl->entrysize);
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#if HASH_DEBUG
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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",
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@ -417,11 +453,10 @@ hash_estimate_size(long num_entries, Size entrysize)
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/* segments */
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size = add_size(size, mul_size(nSegments,
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MAXALIGN(DEF_SEGSIZE * sizeof(HASHBUCKET))));
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/* elements --- allocated in groups of up to HASHELEMENT_ALLOC_MAX */
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elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(entrysize);
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elementAllocCnt = Min(num_entries, HASHELEMENT_ALLOC_MAX);
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elementAllocCnt = Max(elementAllocCnt, 1);
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/* elements --- allocated in groups of choose_nelem_alloc() entries */
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elementAllocCnt = choose_nelem_alloc(entrysize);
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nElementAllocs = (num_entries - 1) / elementAllocCnt + 1;
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elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(entrysize);
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size = add_size(size,
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mul_size(nElementAllocs,
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mul_size(elementAllocCnt, elementSize)));
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@ -7,7 +7,7 @@
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* Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* $PostgreSQL: pgsql/src/include/utils/hsearch.h,v 1.42 2006/03/05 15:59:07 momjian Exp $
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* $PostgreSQL: pgsql/src/include/utils/hsearch.h,v 1.43 2006/06/25 18:29:49 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -156,8 +156,6 @@ typedef struct HASHCTL
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/* max_dsize value to indicate expansible directory */
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#define NO_MAX_DSIZE (-1)
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/* max number of hash elements allocated at once */
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#define HASHELEMENT_ALLOC_MAX (32)
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/* hash_search operations */
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typedef enum
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