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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.
This commit is contained in:
Tom Lane 2007-04-30 00:12:08 +00:00
parent b1a1ea49da
commit 39a333aa2b
1 changed files with 7 additions and 18 deletions
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25
src/backend/utils/mmgr/aset.c
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@ -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);
/*