by Can anyone explain the following results that I am getting from "getrusage"
system function call on a DEC Alphastation running OSF1 V3.2.
I am trying to find the CPU time used to compute a function that does not
do any I/O at all inside it. It essentially scales a 2D image (floating point
array) by interpolation.
Here is the relevant code segment:
/**********************************************************/
#include <stdio.h>
#include <malloc.h>
#include <math.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/resource.h>
extern void my_func() ;
void main()
{
long sys_time_1, sys_time_2 ;
struct rusage *r_usage ;
r_usage = (struct rusage *)malloc(sizeof(struct rusage)) ;
.
.
.
/* find the system time used so far and save it in sys_time_1 */
if ( (status = getrusage(RUSAGE_SELF, r_usage)) != 0 )
{
perror("getrusage error") ;
}
else
{
sys_time_1 = r_usage->ru_stime.tv_usec ;
}
/* compute the function now */
my_func(...) ;
/* find the system time used till now and save it in sys_time_2 */
if ( (status = getrusage(RUSAGE_SELF, r_usage)) != 0 )
{
perror("getrusage error") ;
}
else
{
sys_time_2 = r_usage->ru_stime.tv_usec ;
}
/* Find the system time used by the function as (sys_time_2 - sys_tine_1) */
printf("System time used for interpolation = %d\n", (sys_time_2 - sys_time_1) ) ;
.
.
.
/**********************************************************/Quote:}
I was assuming that this would give me the actual CPU time used for the
computation of the function "my_func" in microseconds. But I seem to get
all sorts of values ranging from 3904 to as high as 11712 when I
ran the code 50 times.
Isn't this number supposed to be the same every time if it is truly
giving me the "system time" ( in terms of {(CPU cycles)/(clock freq)} ) ?
Or am I missing something?
Also, when I look at the manpage for "clock" system function, it says
"The clock() function is made obsolete by the getrusage() function; however,
it is included for compatibility with older BSD programs."
I thought this meant I could still use "clock" instead of "getrusage"
and would get similar results (in microseconds - according to the man page).
But if I change the two lines (in the above block):
sys_time_1 = r_usage->ru_stime.tv_usec ;
and
sys_time_2 = r_usage->ru_stime.tv_usec ;
to
sys_time_1 = clock() ;
and
sys_time_2 = clock() ;
and then run the code, I get very different numbers ranging from 183326
to 216658! Much larger than the CPU times I got through "getrusage".
Any clarifications/suggestions/explanations would be highly appreciated.
Thank you.
It will very helpful if you please send me a copy of your reply at my direct
on the newsgroup.
Thank you.
Sincerely,
-Pranab K. Banerjee
Crump Institute for Biological Imaging
University of California, Los Angeles