Re: remove BufferBlockPointers for speed and space

FreeBSD 5.4 RELEASE gcc 3.4.2 on Intel dual PIII 1Ghz

[postgres:~/develop/c/testbuf]$ gcc -Wall -o testbuf testbuf.c
[postgres:~/develop/c/testbuf]$ ./testbuf
duration round 1 of array method: 1.737 ms
duration round 2 of array method: 1.676 ms
duration round 3 of array method: 1.527 ms
duration round 1 of mul method: 0.548 ms
duration round 2 of mul method: 0.548 ms
duration round 3 of mul method: 0.546 ms
duration round 1 of shift method: 0.593 ms
duration round 2 of shift method: 0.592 ms
duration round 3 of shift method: 0.575 ms
[postgres:~/develop/c/testbuf]$ gcc -Wall -O2 -o testbuf testbuf.c
[postgres:~/develop/c/testbuf]$ ./testbuf
duration round 1 of array method: 0.169 ms
duration round 2 of array method: 0.165 ms
duration round 3 of array method: 0.165 ms
duration round 1 of mul method: 0.164 ms
duration round 2 of mul method: 0.164 ms
duration round 3 of mul method: 0.165 ms
duration round 1 of shift method: 0.164 ms
duration round 2 of shift method: 0.164 ms
duration round 3 of shift method: 0.165 ms

Looks to me like -O2 makes the difference very small (on this
platform/gcc combo) - is 5/169 worth doing?

BTW - I patched the program to stop gcc whining:

--- testbuf.c.orig Thu Aug 11 18:41:29 2005
+++ testbuf.c Thu Aug 11 18:37:43 2005
@@ -2,6 +2,7 @@
* testbuf.c
*/
#include <stdio.h>
+#include <stdlib.h>
#include <sys/file.h>
#include <sys/param.h>
#include <sys/stat.h>
@@ -66,6 +67,8 @@
(long) (stop_t.tv_usec - start_t.tv_usec) % 1000);
}
}
+
+ return 0;
}

Qingqing Zhou wrote:
> "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>
>>Also, I would like to see the actual test code. I wonder whether what
>>you measured is the ability of the compiler to optimize references to
>>successive elements of an array inside a loop; that has little or
>>nothing to do with the typical usage of BufferGetBlock().
>>
>
>
> The source code is attached.
>
> compiled with "gcc testbuf.c". I tried -O2 actually, and it turns out that
> the timing is reduced a lot so not believable.
> ---
>
> /*
> * testbuf.c
> */
> #include <stdio.h>
> #include <sys/file.h>
> #include <sys/param.h>
> #include <sys/stat.h>
> #include <sys/time.h>
> #include <unistd.h>
> #include <fcntl.h>
>
> #define BLCKSZ 8192
> #define NBuffers 80000
>
> typedef void* Block;
>
> int main(void)
> {
> int i, round, method;
> Block k, start;
> struct timeval start_t, stop_t;
> long usecs;
> Block *array = (Block *) calloc(NBuffers, sizeof(Block));
>
> start = (Block)0xff3386;
> for (i = 0; i < NBuffers; i++)
> array[i] = start + BLCKSZ*i;
>
> for (method = 0; method < 3; method ++)
> {
> start = (Block)0xff3386;
> for (round = 0; round < 3; round ++)
> {
> gettimeofday(&start_t, NULL);
> if (method == 0)
> {
> for (i = 0; i < NBuffers; i++)
> k = array[i];
> }
> if (method == 1)
> {
> for (i = 0; i < NBuffers; i++)
> k = start + i*BLCKSZ;
> }
> if (method == 2)
> {
> for (i = 0; i < NBuffers; i++)
> k = start + (i<<13);
> }
> gettimeofday(&stop_t, NULL);
>
> if (stop_t.tv_usec < start_t.tv_usec)
> {
> stop_t.tv_sec--;
> stop_t.tv_usec += 1000000;
> }
>
> usecs = (long) (stop_t.tv_sec - start_t.tv_sec) * 1000000
> + (long) (stop_t.tv_usec - start_t.tv_usec);
>
> fprintf (stdout, "duration round %d of %s method: %ld.%03ld ms\n",
> round + 1,
> method==0?"array":method==1?"mul":"shift",
> (long) ((stop_t.tv_sec - start_t.tv_sec) * 1000 +
> (stop_t.tv_usec - start_t.tv_usec) / 1000),
> (long) (stop_t.tv_usec - start_t.tv_usec) % 1000);
> }
> }
> }
>
>
>
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