Execution Time of a Program

[CSMG Sarma]

Hi all,

I need to calculate the time of execution of a program in seconds and microseconds. I found "times" function as the right one. I used it as follows:

#include <sys/times.h>
#include <time.h>

function() {

struct tms start;
struct tms stop;
clock_t res;

res = times(&start);
if (res == -1)
error(0, errno, "TIMES failed");
/*
* my application Code here
*/

res = times(&stop);
if (res == -1)
error(0, errno, "TIMES failed");
res = (stop.tms_utime + stop.tms_stime - start.tms_utime - start.tms_utime);
printf("\ntime= %lf seconds\n", size, (double)res/CLOCKS_PER_SEC);

}

I tested the program as follows:

$ time a.out <args>

The calculated time(-0.000864 seconds) and the time displayed by "time" utility [real 0m10.398s,user 0m0.052s,sys 0m0.012s] didn't match.

Can any one tell me what's wrong with this program?

Thanks in advance,
Sarma

[Rubberman]

You don't want to do the math as you are doing it... Basically, according to the man page for the times(struct tms*) function, it returns the timing information from some arbitrary point in time. The return value is the real time involved, so you want to use two variables for res (res1, res2) and subtract res1 from res2, keeping in mind that these are signed values and can wrap around/overflow/underflow. The CPU and User time values are NOT relevant to the realtime values as they do not reflect time the process was waiting to be scheduled.

[CSMG Sarma]

I got it working. "ftime" saved the day!



void function()
{
struct timeb start;;
struct timeb stop;
unsigned short res;
ftime(&start);
/*
*my application code here
*/
ftime(&stop);
res = stop.millitm - start.millitm;
printf("\n Time elapsed is:%u milli seconds\n", res);
}


Hi Rubberman,
Do you mean that "times" is not appropriate?

[Lakshmipathi]

I got it working

Glad you got it working. Double check your results with something like gprof

[Rubberman]

I got it working. "ftime" saved the day!



void function()
{
struct timeb start;;
struct timeb stop;
unsigned short res;
ftime(&start);
/*
*my application code here
*/
ftime(&stop);
res = stop.millitm - start.millitm;
printf("\n Time elapsed is:%u milli seconds\n", res);
}


Hi Rubberman,
Do you mean that "times" is not appropriate?

Sorry for the confusion. I just meant that your use of times() was incorrect. Here is an example from the man page:

              #include <sys/times.h>
              #include <stdio.h>
              ...
              void start_clock(void);
              void end_clock(char *msg);
              ...
              static clock_t st_time;
              static clock_t en_time;
              static struct tms st_cpu;
              static struct tms en_cpu;
              ...
              void
              start_clock()
              {
                  st_time = times(&st_cpu);
              }

              /* This example assumes that the result of each subtraction
                 is within the range of values that can be represented in
                 an integer type. */
              void
              end_clock(char *msg)
              {
                  en_time = times(&en_cpu);

                  fputs(msg,stdout);
                  printf("Real Time: %jd, User Time %jd, System Time %jd\n",
                      (intmax_t)(en_time - st_time),
                      (intmax_t)(en_cpu.tms_utime - st_cpu.tms_utime),
                      (intmax_t)(en_cpu.tms_stime - st_cpu.tms_stime));
              }

The times() function returns the real time (clock time from some arbitrary point), so to use it to get the elapsed time, you need to call it at the start of your program, saving that value to a variable. Then at the end of your program you would call it again, and subtract the first value from the new one. In your case, you were overwriting the variable. In addition, the tms structure contains cpu and userspace time values, not taking into account, as I mentioned, time the process was in a wait state.