[postgis-devel] Re: Cost of memcmp

[Paul Ramsey]

OK, I should have been a bit more skeptical of those results.. the
compiler optimized the test out of existence. Here's one that returns
real answers on the cost of memcmp.

First the results though:

1000000 ticks per second.
Compared 1024 bytes (~64 vertices), 1000000 times, took 165666 ticks.
Compared 2048 bytes (~128 vertices), 1000000 times, took 288141 ticks.
Compared 4096 bytes (~256 vertices), 1000000 times, took 570401 ticks.
Compared 8192 bytes (~512 vertices), 1000000 times, took 1112775 ticks.
Compared 16384 bytes (~1024 vertices), 1000000 times, took 2221507 ticks.
Compared 32768 bytes (~2048 vertices), 1000000 times, took 4781961 ticks.
Compared 65536 bytes (~4096 vertices), 1000000 times, took 9576764 ticks.
Compared 131072 bytes (~8192 vertices), 1000000 times, took 19109345 ticks.
Compared 262144 bytes (~16384 vertices), 1000000 times, took 38152790 ticks.


Then the code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>

clock_t start_time, end_time;
int i, j, k;
int loop = 1000000;
size_t size = 1024;
char* memseg1;
char* memseg2;

int expn = 1;

int main() {

  printf("%d ticks per second.\n", CLOCKS_PER_SEC);

  for(j = 0; j < 18; j++ ) {

    memseg1 = malloc(size);
    memseg2 = malloc(size);
    memcpy(memseg2, memseg1, size);

    start_time = clock();
    k = 0;
    for( i = 0; i < loop; i++ )
    {
      if(memcmp(memseg1, memseg2, size))
      {
        k++;
      }
    }
    end_time = clock();

    printf("Compared %d bytes (~%d vertices), %d times, took %d
ticks.\n", size, size / 16, loop, end_time - start_time);
    size = size * 2;
    free(memseg1);
    free(memseg2);
  }
}



On Mon, Oct 6, 2008 at 9:59 PM, Paul Ramsey <pramsey at cleverelephant.ca> wrote:
> I can't see anything wrong with this test program, but it seems to
> indicate that the cost of memcmp is basically constant and tiny.
> Anyone see what I'm doing wrong? If the answer is "no", then we should
> clearly by moving to a memcmp implementation.
>
>
> #include <stdio.h>
> #include <string.h>
> #include <stdlib.h>
> #include <time.h>
>
> clock_t start_time, end_time;
> int i, j;
> int loop = 10000000;
> size_t size = 1024;
> char* memseg1;
> char* memseg2;
>
> int expn = 1;
>
> int main() {
>
>  printf("%d ticks per second.\n", CLOCKS_PER_SEC);
>
>  for(j = 0; j < 18; j++ ) {
>
>    memseg1 = malloc(size);
>    memseg2 = malloc(size);
>    memcpy(memseg2, memseg1, size);
>
>    start_time = clock();
>    for( i = 0; i < loop; i++ )
>    {
>      if(memcmp(memseg1, memseg2, size)) {}
>    }
>    end_time = clock();
>
>    printf("Compared %d bytes, %d times, took %d ticks.\n", size,
> loop, end_time - start_time);
>    size = size * 2;
>    free(memseg1);
>    free(memseg2);
>  }
> }
>