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Use simplehash for backend-private buffer pin refcounts.

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Replace dynahash with simplehash for the per-backend PrivateRefCountHash
overflow table.  Simplehash generates inlined, open-addressed lookup
code, avoiding the per-call overhead of dynahash that becomes noticeable
when many buffers are pinned with a CPU-bound workload.

Motivated by testing of the index prefetching patch, which pins many
more buffers concurrently than typical index scans.

Author: Peter Geoghegan <pg@bowt.ie>
Suggested-by: Andres Freund <andres@anarazel.de>
Reviewed-By: Tomas Vondra <tomas@vondra.me>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAH2-Wz=g=JTSyDB4UtB5su2ZcvsS7VbP+ZMvvaG6ABoCb+s8Lw@mail.gmail.com
This commit is contained in:
Peter Geoghegan 2026-03-12 13:26:16 -04:00
parent d071e1cfec
commit a367c433ad
2 changed files with 37 additions and 33 deletions
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68
src/backend/storage/buffer/bufmgr.c
View file

@ -45,6 +45,7 @@
#endif
#include "catalog/storage.h"
#include "catalog/storage_xlog.h"
#include "common/hashfn.h"
#include "executor/instrument.h"
#include "lib/binaryheap.h"
#include "miscadmin.h"
@ -124,9 +125,22 @@ typedef struct PrivateRefCountEntry
*/
Buffer buffer;
char status;
PrivateRefCountData data;
} PrivateRefCountEntry;
#define SH_PREFIX refcount
#define SH_ELEMENT_TYPE PrivateRefCountEntry
#define SH_KEY_TYPE Buffer
#define SH_KEY buffer
#define SH_HASH_KEY(tb, key) murmurhash32((uint32) (key))
#define SH_EQUAL(tb, a, b) ((a) == (b))
#define SH_SCOPE static inline
#define SH_DECLARE
#define SH_DEFINE
#include "lib/simplehash.h"
/* 64 bytes, about the size of a cache line on common systems */
#define REFCOUNT_ARRAY_ENTRIES 8
@ -248,7 +262,7 @@ static BufferDesc *PinCountWaitBuf = NULL;
*/
static Buffer PrivateRefCountArrayKeys[REFCOUNT_ARRAY_ENTRIES];
static struct PrivateRefCountEntry PrivateRefCountArray[REFCOUNT_ARRAY_ENTRIES];
static HTAB *PrivateRefCountHash = NULL;
static refcount_hash *PrivateRefCountHash = NULL;
static int32 PrivateRefCountOverflowed = 0;
static uint32 PrivateRefCountClock = 0;
static int ReservedRefCountSlot = -1;
@ -347,10 +361,9 @@ ReservePrivateRefCountEntry(void)
Assert(PrivateRefCountArrayKeys[victim_slot] == PrivateRefCountArray[victim_slot].buffer);
/* enter victim array entry into hashtable */
hashent = hash_search(PrivateRefCountHash,
&PrivateRefCountArrayKeys[victim_slot],
HASH_ENTER,
&found);
hashent = refcount_insert(PrivateRefCountHash,
PrivateRefCountArrayKeys[victim_slot],
&found);
Assert(!found);
/* move data from the entry in the array to the hash entry */
hashent->data = victim_entry->data;
@ -440,7 +453,7 @@ GetPrivateRefCountEntrySlow(Buffer buffer, bool do_move)
if (PrivateRefCountOverflowed == 0)
return NULL;
res = hash_search(PrivateRefCountHash, &buffer, HASH_FIND, NULL);
res = refcount_lookup(PrivateRefCountHash, buffer);
if (res == NULL)
return NULL;
@ -452,8 +465,14 @@ GetPrivateRefCountEntrySlow(Buffer buffer, bool do_move)
else
{
/* move buffer from hashtable into the free array slot */
bool found;
PrivateRefCountEntry *free;
PrivateRefCountData data;
/* Save data and delete from hashtable while res is still valid */
data = res->data;
refcount_delete_item(PrivateRefCountHash, res);
Assert(PrivateRefCountOverflowed > 0);
PrivateRefCountOverflowed--;
/* Ensure there's a free array slot */
ReservePrivateRefCountEntry();
@ -466,20 +485,13 @@ GetPrivateRefCountEntrySlow(Buffer buffer, bool do_move)
/* and fill it */
free->buffer = buffer;
free->data = res->data;
free->data = data;
PrivateRefCountArrayKeys[ReservedRefCountSlot] = buffer;
/* update cache for the next lookup */
PrivateRefCountEntryLast = ReservedRefCountSlot;
ReservedRefCountSlot = -1;
/* delete from hashtable */
hash_search(PrivateRefCountHash, &buffer, HASH_REMOVE, &found);
Assert(found);
Assert(PrivateRefCountOverflowed > 0);
PrivateRefCountOverflowed--;
return free;
}
}
@ -571,11 +583,7 @@ ForgetPrivateRefCountEntry(PrivateRefCountEntry *ref)
}
else
{
bool found;
Buffer buffer = ref->buffer;
hash_search(PrivateRefCountHash, &buffer, HASH_REMOVE, &found);
Assert(found);
refcount_delete_item(PrivateRefCountHash, ref);
Assert(PrivateRefCountOverflowed > 0);
PrivateRefCountOverflowed--;
}
@ -4118,8 +4126,6 @@ AtEOXact_Buffers(bool isCommit)
void
InitBufferManagerAccess(void)
{
HASHCTL hash_ctl;
/*
* An advisory limit on the number of pins each backend should hold, based
* on shared_buffers and the maximum number of connections possible.
@ -4132,11 +4138,7 @@ InitBufferManagerAccess(void)
memset(&PrivateRefCountArray, 0, sizeof(PrivateRefCountArray));
memset(&PrivateRefCountArrayKeys, 0, sizeof(PrivateRefCountArrayKeys));
hash_ctl.keysize = sizeof(Buffer);
hash_ctl.entrysize = sizeof(PrivateRefCountEntry);
PrivateRefCountHash = hash_create("PrivateRefCount", 100, &hash_ctl,
HASH_ELEM | HASH_BLOBS);
PrivateRefCountHash = refcount_create(CurrentMemoryContext, 100, NULL);
/*
* AtProcExit_Buffers needs LWLock access, and thereby has to be called at
@ -4195,10 +4197,10 @@ CheckForBufferLeaks(void)
/* if necessary search the hash */
if (PrivateRefCountOverflowed)
{
HASH_SEQ_STATUS hstat;
refcount_iterator iter;
hash_seq_init(&hstat, PrivateRefCountHash);
while ((res = (PrivateRefCountEntry *) hash_seq_search(&hstat)) != NULL)
refcount_start_iterate(PrivateRefCountHash, &iter);
while ((res = refcount_iterate(PrivateRefCountHash, &iter)) != NULL)
{
s = DebugPrintBufferRefcount(res->buffer);
elog(WARNING, "buffer refcount leak: %s", s);
@ -4251,10 +4253,10 @@ AssertBufferLocksPermitCatalogRead(void)
/* if necessary search the hash */
if (PrivateRefCountOverflowed)
{
HASH_SEQ_STATUS hstat;
refcount_iterator iter;
hash_seq_init(&hstat, PrivateRefCountHash);
while ((res = (PrivateRefCountEntry *) hash_seq_search(&hstat)) != NULL)
refcount_start_iterate(PrivateRefCountHash, &iter);
while ((res = refcount_iterate(PrivateRefCountHash, &iter)) != NULL)
{
AssertNotCatalogBufferLock(res->buffer, res->data.lockmode);
}

2
src/tools/pgindent/typedefs.list
View file

@ -4114,6 +4114,8 @@ rbt_freefunc
reduce_outer_joins_partial_state
reduce_outer_joins_pass1_state
reduce_outer_joins_pass2_state
refcount_hash
refcount_iterator
reference
regc_wc_probefunc
regex_arc_t