tweaking MemSet() performance

In include/c.h, MemSet() is defined to be different than the stock
function memset() only when copying less than or equal to
MEMSET_LOOP_LIMIT bytes (currently 64). The comments above the macro
definition note:

* We got the 64 number by testing this against the stock memset() on
* BSD/OS 3.0. Larger values were slower. bjm 1997/09/11
*
* I think the crossover point could be a good deal higher for
* most platforms, actually. tgl 2000-03-19

I decided to investigate Tom's suggestion and determine the
performance of MemSet() versus memset() on my machine, for various
values of MEMSET_LOOP_LIMIT. The machine this is being tested on is a
Pentium 4 1.8 Ghz with RDRAM, running Linux 2.4.19pre8 with GCC 3.1.1
and glibc 2.2.5 -- the results may or may not apply to other
machines.

The test program was:

#include <string.h>
#include "postgres.h"

#undef MEMSET_LOOP_LIMIT
#define MEMSET_LOOP_LIMIT BUFFER_SIZE

int
main(void)
{
char buffer[BUFFER_SIZE];
long long i;

for (i = 0; i < 99000000; i++)
{
MemSet(buffer, 0, sizeof(buffer));
}

return 0;
}

(I manually changed MemSet() to memset() when testing the performance
of the latter function.)

It was compiled like so:

gcc -O2 -DBUFFER_SIZE=xxx -Ipgsql/src/include memset.c

(The -O2 optimization flag is important: the results are significantly
different if it is not used.)

Here are the results (each timing is the 'total' listing from 'time
./a.out'):

BUFFER_SIZE = 64
MemSet() -> 2.756, 2.810, 2.789
memset() -> 13.844, 13.782, 13.778

BUFFER_SIZE = 128
MemSet() -> 5.848, 5.989, 5.861
memset() -> 15.637, 15.631, 15.631

BUFFER_SIZE = 256
MemSet() -> 9.602, 9.652, 9.633
memset() -> 19.305, 19.370, 19.302

BUFFER_SIZE = 512
MemSet() -> 17.416, 17.462, 17.353
memset() -> 26.657, 26.658, 26.678

BUFFER_SIZE = 1024
MemSet() -> 32.144, 32.179, 32.086
memset() -> 41.186, 41.115, 41.176

BUFFER_SIZE = 2048
MemSet() -> 60.39, 60.48, 60.32
memset() -> 71.19, 71.18, 71.17

BUFFER_SIZE = 4096
MemSet() -> 118.29, 120.07, 118.69
memset() -> 131.40, 131.41

... at which point I stopped benchmarking.

Is the benchmark above a reasonable assessment of memset() / MemSet()
performance when copying word-aligned amounts of memory? If so, what's
a good value for MEMSET_LOOP_LIMIT (perhaps 512)?

Also, if anyone would like to contribute the results of doing the
benchmark on their particular system, that might provide some useful
additional data points.

Cheers,

Neil

--
Neil Conway <neilc(at)samurai(dot)com> || PGP Key ID: DB3C29FC