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" <[EMAIL PROTECTED]> 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);
}
}
}
---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
---------------------------(end of broadcast)---------------------------
TIP 6: explain analyze is your friend
