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Speed up AllocSetFreeIndex, which is a significant cost in palloc and pfree,

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by using a lookup table instead of a naive shift-and-count loop.  Based on
code originally posted by Sean Eron Anderson at
http://graphics.stanford.edu/%7eseander/bithacks.html.
Greg Stark did the research and benchmarking to show that this is what
we should use.  Jeremy Kerr first noticed that this is a hotspot that
could be optimized, though we ended up not using his suggestion of
platform-specific bit-searching code.
This commit is contained in:
Tom Lane 2009-07-21 19:53:12 +00:00
parent f3f45c87d2
commit ab5b4e2f9e
1 changed files with 32 additions and 9 deletions
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41
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 1.79 2009/06/11 14:49:06 momjian Exp $
* $PostgreSQL: pgsql/src/backend/utils/mmgr/aset.c,v 1.80 2009/07/21 19:53:12 tgl Exp $
*
* NOTE:
* This is a new (Feb. 05, 1999) implementation of the allocation set
@ -238,6 +238,17 @@ static MemoryContextMethods AllocSetMethods = {
#endif
};
/*
* Table for AllocSetFreeIndex
*/
#define LT16(n) n, n, n, n, n, n, n, n, n, n, n, n, n, n, n, n
static const unsigned char LogTable256[256] =
{
0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4,
LT16(5), LT16(6), LT16(6), LT16(7), LT16(7), LT16(7), LT16(7),
LT16(8), LT16(8), LT16(8), LT16(8), LT16(8), LT16(8), LT16(8), LT16(8)
};
/* ----------
* Debug macros
@ -266,18 +277,30 @@ static MemoryContextMethods AllocSetMethods = {
static inline int
AllocSetFreeIndex(Size size)
{
int idx = 0;
int idx;
unsigned int t,
tsize;
if (size > 0)
if (size > (1 << ALLOC_MINBITS))
{
size = (size - 1) >> ALLOC_MINBITS;
while (size != 0)
{
idx++;
size >>= 1;
}
tsize = (size - 1) >> ALLOC_MINBITS;
/*
* At this point we need to obtain log2(tsize)+1, ie, the number
* of not-all-zero bits at the right. We used to do this with a
* shift-and-count loop, but this function is enough of a hotspot
* to justify micro-optimization effort. The best approach seems
* to be to use a lookup table. Note that this code assumes that
* ALLOCSET_NUM_FREELISTS <= 17, since we only cope with two bytes
* of the tsize value.
*/
t = tsize >> 8;
idx = t ? LogTable256[t] + 8 : LogTable256[tsize];
Assert(idx < ALLOCSET_NUM_FREELISTS);
}
else
idx = 0;
return idx;
}