From 39a333aa2b2d659f830a78a7d3c6bd43d4caa692 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Mon, 30 Apr 2007 00:12:08 +0000
Subject: [PATCH] Marginal performance hack: remove the loop that used to be
needed to look through a freelist for a chunk of adequate size. For a long
time now, all elements of a given freelist have been exactly the same
allocated size, so we don't need a loop. Since the loop never iterated more
than once, you'd think this wouldn't matter much, but it makes a noticeable
savings in a simple test --- perhaps because the compiler isn't optimizing on
a mistaken assumption that the loop would repeat. AllocSetAlloc is called
often enough that saving even a couple of instructions is worthwhile.
---
src/backend/utils/mmgr/aset.c | 25 +++++++------------------
1 file changed, 7 insertions(+), 18 deletions(-)
diff --git a/src/backend/utils/mmgr/aset.c b/src/backend/utils/mmgr/aset.c
index c6608751a8..71b6da1e49 100644
--- a/src/backend/utils/mmgr/aset.c
+++ b/src/backend/utils/mmgr/aset.c
@@ -11,7 +11,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/utils/mmgr/aset.c,v 1.71 2007/01/05 22:19:47 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/mmgr/aset.c,v 1.72 2007/04/30 00:12:08 tgl Exp $
*
* NOTE:
* This is a new (Feb. 05, 1999) implementation of the allocation set
@@ -516,7 +516,6 @@ AllocSetAlloc(MemoryContext context, Size size)
AllocSet set = (AllocSet) context;
AllocBlock block;
AllocChunk chunk;
- AllocChunk priorfree;
int fidx;
Size chunk_size;
Size blksize;
@@ -578,26 +577,16 @@ AllocSetAlloc(MemoryContext context, Size size)
/*
* Request is small enough to be treated as a chunk. Look in the
* corresponding free list to see if there is a free chunk we could reuse.
- */
- fidx = AllocSetFreeIndex(size);
- priorfree = NULL;
- for (chunk = set->freelist[fidx]; chunk; chunk = (AllocChunk) chunk->aset)
- {
- if (chunk->size >= size)
- break;
- priorfree = chunk;
- }
-
- /*
* If one is found, remove it from the free list, make it again a member
* of the alloc set and return its data address.
*/
+ fidx = AllocSetFreeIndex(size);
+ chunk = set->freelist[fidx];
if (chunk != NULL)
{
- if (priorfree == NULL)
- set->freelist[fidx] = (AllocChunk) chunk->aset;
- else
- priorfree->aset = chunk->aset;
+ Assert(chunk->size >= size);
+
+ set->freelist[fidx] = (AllocChunk) chunk->aset;
chunk->aset = (void *) set;
@@ -618,7 +607,7 @@ AllocSetAlloc(MemoryContext context, Size size)
/*
* Choose the actual chunk size to allocate.
*/
- chunk_size = 1 << (fidx + ALLOC_MINBITS);
+ chunk_size = (1 << ALLOC_MINBITS) << fidx;
Assert(chunk_size >= size);
/*
--
2.49.0