Tom Lane wrote:
Jeremy Kerr <j...@ozlabs.org> writes:
Rather than testing single bits in a loop, change AllocSetFreeIndex to
use the __builtin_clz() function to calculate the chunk index.
This requires a new check for __builtin_clz in the configure script.
Results in a ~2% performance increase on sysbench on PowerPC.
I did some performance testing on this by extracting the
AllocSetFreeIndex function into a standalone test program that just
executed it a lot of times in a loop. And there's a problem: on
x86_64 it is not much of a win. The code sequence that gcc generates
for __builtin_clz is basically
bsrl %eax, %eax
xorl $31, %eax
and it turns out that Intel hasn't seen fit to put a lot of effort into
the BSR instruction. It's constant time, all right, but on most of
their CPUs that constant time is like 8 or 16 times slower than an ADD;
cf http://www.intel.com/Assets/PDF/manual/248966.pdf
hmm interesting - I don't have the exact numbers any more but that
patch(or a previous version of it) definitly showed a noticable
improvement when I tested with sysbench on a current generation Intel
Nehalem...
Stefan
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