How to create a high resolution timer in Linux to measure program performance?

Question

I'm trying to compare GPU to CPU performance. For the NVIDIA GPU I've been using the cudaEvent_t types to get a very precise timing.

For the CPU I've been using the following code:

// Timers clock_t start, stop; float elapsedTime = 0;  // Capture the start time  start = clock();  // Do something here .......  // Capture the stop time stop = clock(); // Retrieve time elapsed in milliseconds elapsedTime = (float)(stop - start) / (float)CLOCKS_PER_SEC * 1000.0f; 

Apparently, that piece of code is only good if you're counting in seconds. Also, the results sometime come out quite strange.

Does anyone know of some way to create a high resolution timer in Linux?

Answer 1

Check out clock_gettime, which is a POSIX interface to high-resolution timers.

If, having read the manpage, you're left wondering about the difference between CLOCK_REALTIME and CLOCK_MONOTONIC, see Difference between CLOCK_REALTIME and CLOCK_MONOTONIC?

See the following page for a complete example: http://www.guyrutenberg.com/2007/09/22/profiling-code-using-clock_gettime/

#include <iostream> #include <time.h> using namespace std;  timespec diff(timespec start, timespec end);  int main() {     timespec time1, time2;     int temp;     clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time1);     for (int i = 0; i< 242000000; i++)         temp+=temp;     clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time2);     cout<<diff(time1,time2).tv_sec<<":"<<diff(time1,time2).tv_nsec<<endl;     return 0; }  timespec diff(timespec start, timespec end) {     timespec temp;     if ((end.tv_nsec-start.tv_nsec)<0) {         temp.tv_sec = end.tv_sec-start.tv_sec-1;         temp.tv_nsec = 1000000000+end.tv_nsec-start.tv_nsec;     } else {         temp.tv_sec = end.tv_sec-start.tv_sec;         temp.tv_nsec = end.tv_nsec-start.tv_nsec;     }     return temp; } 

Answer 2

To summarise information presented so far, these are the two functions required for typical applications.

#include <time.h>  // call this function to start a nanosecond-resolution timer struct timespec timer_start(){     struct timespec start_time;     clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start_time);     return start_time; }  // call this function to end a timer, returning nanoseconds elapsed as a long long timer_end(struct timespec start_time){     struct timespec end_time;     clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end_time);     long diffInNanos = (end_time.tv_sec - start_time.tv_sec) * (long)1e9 + (end_time.tv_nsec - start_time.tv_nsec);     return diffInNanos; } 

Here is an example of how to use them in timing how long it takes to calculate the variance of a list of input.

struct timespec vartime = timer_start();  // begin a timer called 'vartime' double variance = var(input, MAXLEN);  // perform the task we want to time long time_elapsed_nanos = timer_end(vartime); printf("Variance = %f, Time taken (nanoseconds): %ld\n", variance, time_elapsed_nanos);