CODEWITHSUNDEEP
linuxmultithreadingtimepthreads
Jithin
Jithin

Reputation: 1108

Interpreting time command output on a multi threaded program

I have a multi threaded program and I am profiling time taken starting before all pthread_create's and after all pthread_join's.

Now I find that this time, lets call it X, which is shown below in "Done in xms" is actually user + sys time of time output. In my app the number argument to a.out controls how many threads to spawn. ./a.out 1 spawn 1 pthread and ./a.out 2 spawns 2 threads where each thread does the same amount of work.

I was expecting X to be the real time instead of user + sys time. Can someone please tell me why this is not so? Then this really means my app is indeed running parallel without any locking between threads.

[jithin@whatsoeverclever tests]$ time ./a.out 1
Done in 320ms

real    0m0.347s
user    0m0.300s
sys     0m0.046s
[jithin@whatsoeverclever tests]$ time ./a.out 2
Done in 450ms

real    0m0.266s
user    0m0.383s
sys     0m0.087s
[jithin@whatsoeverclever tests]$ time ./a.out 3
Done in 630ms

real    0m0.310s
user    0m0.532s
sys     0m0.105s

Code

int main(int argc, char **argv) {

  //Read the words
  getWords();

  //Set number of words to use
  int maxWords = words.size();
  if(argc > 1) {
     int numWords = atoi(argv[1]);
     if(numWords > 0 && numWords < maxWords) maxWords = numWords;
  }

  //Init model
  model = new Model(MODEL_PATH);
  pthread_t *threads = new pthread_t[maxWords];
  pthread_attr_t attr;
  void *status;

  // Initialize and set thread joinable
  pthread_attr_init(&attr);
  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

  int rc;
clock_t startTime = clock();

  for(unsigned i=0; i<maxWords; i++) {
     //create thread
     rc = pthread_create(&threads[i], NULL, processWord, (void *)&words[i] );
     if (rc){
         cout << "Error:unable to create thread: " << i << "," << rc << endl;
         exit(-1);
      }
  }

  // free attribute and wait for the other threads
  pthread_attr_destroy(&attr);
  for(unsigned i=0; i<maxWords; i++) {
     rc = pthread_join(threads[i], &status);
     if (rc){
         cout << "Error:unable to join thread: " << i << "," << rc << endl;
         exit(-1);
      }
  }

  clock_t endTime = clock();

  float diff = (((float)endTime - (float)startTime) / 1000000.0F ) * 1000;
  cout<<"Done in "<< diff << "ms\n";
  delete[] threads;
  delete model;
}

Upvotes: 0

Views: 1076

Answers (1)

David Schwartz
David Schwartz

Reputation: 182753

The clock function is specifically documented to return the processor time used by a process. If you want to measure wall time elapsed, it's not the right function.

Upvotes: 1

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