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POInterpreting time command output on a multi threaded program
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  1. POInterpreting time command output on a multi threaded program
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    copied!<p>I have a multi threaded program and I am profiling time taken starting before all pthread_create's and after all pthread_join's.</p> <p>Now I find that this time, lets call it <strong>X</strong>, which is shown below in <em>"Done in xms"</em> is actually user + sys time of <strong>time</strong> output. In my app the number argument to a.out controls how many threads to spawn. <strong><em>./a.out 1</em></strong> spawn 1 pthread and <strong><em>./a.out 2</em></strong> spawns 2 threads where each thread does the same amount of work.</p> <p>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.</p> <pre><code>[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></pre> <p><strong>Code</strong></p> <pre><code>int main(int argc, char **argv) { //Read the words getWords(); //Set number of words to use int maxWords = words.size(); if(argc &gt; 1) { int numWords = atoi(argv[1]); if(numWords &gt; 0 &amp;&amp; numWords &lt; 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(&amp;attr); pthread_attr_setdetachstate(&amp;attr, PTHREAD_CREATE_JOINABLE); int rc; clock_t startTime = clock(); for(unsigned i=0; i&lt;maxWords; i++) { //create thread rc = pthread_create(&amp;threads[i], NULL, processWord, (void *)&amp;words[i] ); if (rc){ cout &lt;&lt; "Error:unable to create thread: " &lt;&lt; i &lt;&lt; "," &lt;&lt; rc &lt;&lt; endl; exit(-1); } } // free attribute and wait for the other threads pthread_attr_destroy(&amp;attr); for(unsigned i=0; i&lt;maxWords; i++) { rc = pthread_join(threads[i], &amp;status); if (rc){ cout &lt;&lt; "Error:unable to join thread: " &lt;&lt; i &lt;&lt; "," &lt;&lt; rc &lt;&lt; endl; exit(-1); } } clock_t endTime = clock(); float diff = (((float)endTime - (float)startTime) / 1000000.0F ) * 1000; cout&lt;&lt;"Done in "&lt;&lt; diff &lt;&lt; "ms\n"; delete[] threads; delete model; } </code></pre>
 

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