How can I benchmark C code easily?

Is there a simple library to benchmark the time it takes to execute a portion of C code? What I want is something like:

int main(){
    benchmarkBegin(0);
    //Do work
    double elapsedMS = benchmarkEnd(0);

    benchmarkBegin(1)
    //Do some more work
    double elapsedMS2 = benchmarkEnd(1);

    double speedup = benchmarkSpeedup(elapsedMS, elapsedMS2); //Calculates relative speedup
}

It would also be great if the library let you do many runs, averaging them and calculating the variance in timing!

Use the function clock() defined in time.h:

startTime = (float)clock()/CLOCKS_PER_SEC;

/* Do work */

endTime = (float)clock()/CLOCKS_PER_SEC;

timeElapsed = endTime - startTime;

Basically, all you want is a high resolution timer. The elapsed time is of course just a difference in times and the speedup is calculated by dividing the times for each task. I have included the code for a high resolution timer that should work on at least windows and unix.

#ifdef WIN32

#include <windows.h>
double get_time()
{
	LARGE_INTEGER t, f;
	QueryPerformanceCounter(&t);
	QueryPerformanceFrequency(&f);
	return (double)t.QuadPart/(double)f.QuadPart;
}

#else

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

double get_time()
{
	struct timeval t;
	struct timezone tzp;
	gettimeofday(&t, &tzp);
	return t.tv_sec + t.tv_usec*1e-6;
}

#endif

Benchmark C code easily

#include <time.h>

int main(void) {
  clock_t start_time = clock();

  // code or function to benchmark

  double elapsed_time = (double)(clock() - start_time) / CLOCKS_PER_SEC;
  printf("Done in %f seconds\n", elapsed_time);
}

---

Easy benchmark of multi-threaded C code

If you want to benchmark multithreaded program you first need to take a closer look at clock:

Description

> The clock() function returns an approximation of processor time used by the program.

Return value

> The value returned is the CPU time used so far as a clock_t; to get the number of seconds used, divide by CLOCKS_PER_SEC. If the processor time used is not available or its value cannot be represented, the function returns the value (clock_t)(-1)

Hence it is very important to divide your elapsed_time by the number of threads in order to get the execution time of your function:

#include <time.h>
#include <omp.h>

#define THREADS_NB omp_get_max_threads()

#pragma omp parallel for private(i) num_threads(THREADS_NB)
clock_t start_time = clock();

// code or function to benchmark

double elapsed_time = (double)(clock() - start_time) / CLOCKS_PER_SEC;
printf("Done in %f seconds\n", elapsed_time / THREADS_NB); // divide by THREADS_NB!

---

Example

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <time.h>
#include <omp.h>

#define N 20000
#define THREADS_NB omp_get_max_threads()

void init_arrays(double *a, double *b) {
  memset(a, 0, sizeof(a));
  memset(b, 0, sizeof(b));
  for (int i = 0; i < N; i++) {
    a[i] += 1.0;
    b[i] += 1.0;
  }
}

double func2(double i, double j) {
  double res = 0.0;

  while (i / j > 0.0) {
    res += i / j;
    i -= 0.1;
    j -= 0.000003;
  }
  return res;
}

double single_thread(double *a, double *b) {
  double res = 0;
  int i, j;
  for (i = 0; i < N; i++) {
    for (j = 0; j < N; j++) {
      if (i == j) continue;
      res += func2(a[i], b[j]);
    }
  }
  return res;
}

double multi_threads(double *a, double *b) {
  double res = 0;
  int i, j;
  #pragma omp parallel for private(j) num_threads(THREADS_NB) reduction(+:res)
  for (i = 0; i < N; i++) {
    for (j = 0; j < N; j++) {
      if (i == j) continue;
      res += func2(a[i], b[j]);
    }
  }
  return res;
}

int main(void) {
  double *a, *b;
  a = (double *)calloc(N, sizeof(double));
  b = (double *)calloc(N, sizeof(double));
  init_arrays(a, b);

  clock_t start_time = clock();
  double res = single_thread(a, b);
  double elapsed_time = (double)(clock() - start_time) / CLOCKS_PER_SEC;
  printf("Default:  Done with %f in %f sd\n", res, elapsed_time);

  start_time = clock();
  res = multi_threads(a, b);
  elapsed_time = (double)(clock() - start_time) / CLOCKS_PER_SEC;
  printf("With OMP: Done with %f in %f sd\n", res, elapsed_time / THREADS_NB);
}

---

Compile with:

gcc -O3 multithread_benchmark.c -fopenmp && time ./a.out

---

Output:

Default:  Done with 2199909813.614555 in 4.909633 sd
With OMP: Done with 2199909799.377532 in 1.708831 sd

real    0m6.703s (from time function)

In POSIX, try getrusage. The relevant argument is RUSAGE_SELF and the relevant fields are ru_utime.tv_sec and ru_utime.tv_usec.

There may be existing utilities that help with this, but I suspect most will use some kind of sampling or possibly injection. But to get specific sections of code timed, you will probably have to add in calls to a timer like you show in your example. If you are using Windows, then the high performance timer works. I answered a similar question and showed example code that will do that. There are similar methods for Linux.