Clock_gettime () Vs. gettimeofday () for measuring OpenMP runtime

Question

I am working on some C code that implements a triple nested loop to calculate matrix matrix multiplication when parallelizing it using OpenMP. I am trying to accurately measure the amount of time it takes when a for loop starts when it ends. I have used gettimeofday () so far, but I noticed that sometimes he didn’t want him to accurately record the amount of time it took for the for loop. He seemed to say that it took longer than it actually was.

Here is the source code:

struct timeval start end;
double elapsed;

gettimeofday(&start, NULL);
#pragma omp parallel for num_threads(threads) private(i, j, k)
for(...)
{
 ...
 for(...)
 {
  ...
  for(...)
  {
   ...
  }
 }
}

gettimeofday(&end, NULL);
elapsed = (end.tv_sec+1E-6*end.tv_usec) - (start.tv_sec+1E-6*start.tv_usec)

And here is the same code using clock_gettime ():

 struct timespec start1, finish1;
 double elapsed1;

clock_gettime(CLOCK_MONOTONIC, &start1);

  #pragma omp parallel for num_threads(threads) private(i, j, k)
    for(...)
    {
     ...
     for(...)
     {
      ...
      for(...)
      {
       ...
      }
     }
    }

clock_gettime(CLOCK_MONOTONIC, &finish1);
elapsed1 = (finish1.tv_sec - start1.tv_sec);
elapsed1 += (finish1.tv_nsec - start1.tv_nsec)/1000000000.0;

3-4 , , gettimeofday() clock_gettime(), , clock_gettime():

struct timespec start1, finish1;
double elapsed1;

struct timeval start end;
double elapsed;

clock_gettime(CLOCK_MONOTONIC, &start1);
gettimeofday(&start, NULL);

  #pragma omp parallel for num_threads(threads) private(i, j, k)
    for(...)
    {
     ...
     for(...)
     {
      ...
      for(...)
      {
       ...
      }
     }
    }

gettimeofday(&end, NULL);
clock_gettime(CLOCK_MONOTONIC, &finish1);

elapsed = (end.tv_sec+1E-6*end.tv_usec) - (start.tv_sec+1E-6*start.tv_usec)

elapsed1 = (finish1.tv_sec - start1.tv_sec);
elapsed1 += (finish1.tv_nsec - start1.tv_nsec)/1000000000.0;

? ? .

Answer 1

elapsed = (end.tv_sec+1E-6*end.tv_usec) - (start.tv_sec+1E-6*start.tv_usec) , .

• elapsed = (end.tv_sec - start.tv_sec) - (start.tv_usec- end.tv_usec)/1E6. OP 2- 3- , .

• , , .

  clock_gettime(CLOCK_MONOTONIC, &start1);
  gettimeofday(&start, NULL);
  
  ...
  
  // gettimeofday(&end, NULL);
  // clock_gettime(CLOCK_MONOTONIC, &finish1);
  clock_gettime(CLOCK_MONOTONIC, &finish1);
  gettimeofday(&end, NULL);
  

• : 3-, , (0,5 ), , . @Dietrich Epp .

  gettimeofday(&t, NULL);
  do { 
    gettimeofday(&start, NULL);
  } while (start == t);
  

,

long long elapsed_ns = (1LL*finish1.tv_sec - start1.tv_sec)*1000000000LL + 
    finish1.tv_nsec - start1.tv_nsec;

Answer 2

. , , . , - , . gettimeofday(), , :

elapsed = end.tv_sec + 1E-6 * end.tv_usec - start.tv_sec + 1E-6 * start.tv_usec

:

elapsed = (end.tv_sec + 1E-6 * end.tv_usec) - (start.tv_sec + 1E-6 * start.tv_usec)

- , gettimeofday(), # define:

#define TIME_GET(time) (time).tv_sec+1E-6*(time).tv_usec
#define TIME_GET_RESULT(start,end) TIME_GET(end)-TIME_GET(start)

#define, :

#define TIME_GET(time) ((time).tv_sec+1E-6*(time).tv_usec)

clock_gettime(), , #define, gettimeofday(), .