timing the C

Hi,

Just a quick one by way of a short story

I started off by finding out the execution time of my c programs using the the time_t variables in time.h.

These only seemed to record a minimum of seconds which was OK when my programs took many minutes, and even hours sometimes :-)

Anyhow, the resolution is much too coarse so when I started MPI programs, well I was much happier with the very precise timings you could get off that library.

So, if with sequential programs I took to using MPI, only for the timer. I've done worse things ...

However I found sys/time.h in my include directory and it doesn't have milliseconds res. but it has microseconds. That's pretty good. I get the time of day in secs and milliseconds and then I subtract a new reading of the time day when all my operations are finished.

So question is, does that all sound OK? I mean, I can't be accused of a cheap hack anymore?

Here's what I do. Watch your calcs at the end.

Code:

---

        int rc ;
       
        struct timeval tv_start, tv_end ;
        struct timezone tz ;

        rc = gettimeofday(&tv_start, &tz) ;
        if (rc) {
                printf("gettimeofday() failed, rc = %d\n", rc) ;
                return rc ;
        }

// do something...

        rc = gettimeofday(&tv_end, &tz) ;
        if (rc) {
                printf("gettimeofday() failed, rc = %d\n", rc) ;
                return rc ;
        }

        printf("tv_start = %ld.%ld, tv_end = %ld.%ld\n", tv_start.tv_sec, tv_start.tv_usec, tv_end.tv_sec, tv_end.tv_usec) ;
        if ( tv_end.tv_usec < tv_start.tv_usec) {
        //        printf("tv_start_usec - tv_end_usec = %d\n", tv_start.tv_usec - tv_end.tv_usec) ;
                tv_end.tv_sec-- ;            // borrow from seconds
                tv_end.tv_usec += 1000000 ;  // a usec is 1 milllionth of a second.
        }
        printf("Elapsed Time (seconds) was %ld.%06ld\n", (tv_end.tv_sec- tv_start.tv_sec), (tv_end.tv_usec - tv_start.tv_usec))  ;

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thanks Dino.

I do the "calcs" as you put it like this

Code:

---

gettimeofday(&tv, &tz); secs=tv.tv_sec; misecs=tv.tv_usec;

---

which is perfectly readable

Code:

---

gettimeofday(&tv, &tz); secs=(tv.tv_sec - secs)*1000000+(tv.tv_usec - misecs);
        printf("Op time: %8.6f secs.\n",(double)secs/1000000);

---

Quote:

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Originally Posted by stabu

thanks Dino.

I do the "calcs" as you put it like this

Code:

---

gettimeofday(&tv, &tz); secs=tv.tv_sec; misecs=tv.tv_usec;

---

which is perfectly readable

Code:

---

gettimeofday(&tv, &tz); secs=(tv.tv_sec - secs)*1000000+(tv.tv_usec - misecs);
        printf("Op time: %8.6f secs.\n",(double)secs/1000000);

---

---

My head hurts. Explain, with your calculations, how a start time of 1.999999 and an end time of 2.000001 will yield the right time. In your case, won't tv.tv_usec - misecs yield a negative value?

I would expect (2-1)*1000000 + (1-999999) to evaluate to 2, assuming variables are accurately named.

Oh. Yeah. I am splitting the calcs and keeping them split, and the OP is merging them into a final value at the end.

Thanks tabstop.

Sorry for giving you head pain! Least of all to someone who has offered advice.

Quote:

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which is perfectly readable

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I was being ironic, I left out the similies.
I'll correct my etiquette in future.