From 2924ca67475afa3cc3c1a814b170b61803caeee9 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sun, 25 Jun 2006 18:29:49 +0000
Subject: [PATCH] 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.
---
src/backend/utils/hash/dynahash.c | 51 ++++++++++++++++++++++++++-----
src/include/utils/hsearch.h | 4 +--
2 files changed, 44 insertions(+), 11 deletions(-)
diff --git a/src/backend/utils/hash/dynahash.c b/src/backend/utils/hash/dynahash.c
index ccd8ed5c486..75f9d0eaf1b 100644
--- a/src/backend/utils/hash/dynahash.c
+++ b/src/backend/utils/hash/dynahash.c
@@ -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)));
diff --git a/src/include/utils/hsearch.h b/src/include/utils/hsearch.h
index 97bab5b85b1..a89be514d90 100644
--- a/src/include/utils/hsearch.h
+++ b/src/include/utils/hsearch.h
@@ -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
--
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