How to measure the time in multi-core machines with pthreads?
After spending about an hour trying to figure out what was the problem that my threaded version of a program took more time than the single-core one, I think I rediscovered the wheel. Simply put, I can not use the clock () function to determine the "real" time I spent working in parallel. What do I mean? Example:
A program creates some threads and each one of them will make argv[1] calls to the rand () function (just to do something so that time goes by :)). Then in the end I compute the time recorded by clock () differences and differences with time(). Obviously the ratio below is almost two because my machine is dual core. But ...
Question: How can I measure the exact time elapsed and not the approximation in seconds with time()?
Code:
$ g++ try_to_time.cpp -pthread -o try
$ ./try 50000000
Just started counting!
All threads joined!
Duration 1: 11.8488 secs.
Duration 2:
* started : 1206834449 seconds after 1/1/1970.
* finished: 1206834455 seconds after 1/1/1970.
Duration 3: 6 secs.
$
And the program
Code:
$ cat try_to_time.cpp
#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <ctime>
#include <pthread.h>
using namespace std;
#define NUM_THREADS 10
#define DIGITS 4
void * thread_function (void *);
int main (int argc, char * argv [])
{
time_t start1, start2, start3;
time_t end1, end2, end3;
int iterations, i, rc;
void * status;
double duration;
if (argc != 2) {
cout << "Usage error. I am expecting one positive integer." << endl;
exit (1);
}
iterations = atoi ( argv [1] );
// create opaque objects.
pthread_t * threads = new pthread_t [NUM_THREADS];
// Make threads Joinable for sure.
pthread_attr_t attr;
pthread_attr_init (&attr);
pthread_attr_setdetachstate (&attr, PTHREAD_CREATE_JOINABLE);
start1 = clock ();
start2 = time (NULL);
time (&start3);
cout << "Just started counting!" << std::endl;
for (i = 0; i < NUM_THREADS; i++) {
rc = pthread_create(&(threads[i]), &attr, thread_function, (void *) &iterations);
if (rc) {
cerr << "ERROR; return code from pthread_create() is " << rc << std::endl;
exit (2);
}
}
for (i = 0; i < NUM_THREADS; i++)
pthread_join ( threads [i], &status );
cout << "All threads joined!" << std::endl;
end1 = clock ();
end2 = time (NULL);
time (&end3);
duration = static_cast<double> (end1 - start1);
if (duration < static_cast<double> (CLOCKS_PER_SEC)) {
cout << "Few iterations. Try again with a bigger number." << endl;
exit (3);
}
cout << "Duration 1: " << fixed << setprecision (DIGITS)
<< static_cast<double> (duration / CLOCKS_PER_SEC) << " secs." << endl;
cout << "Duration 2: " << endl;
cout << "* started : " << start2 << " seconds after 1/1/1970." << endl;
cout << "* finished: " << end2 << " seconds after 1/1/1970." << endl;
cout << "Duration 3: " << static_cast<int> (difftime ( end3, start3 ) ) << " secs." << endl;
return 0;
}
void * thread_function (void * input)
{
int loops = *((int *) input);
int i;
for (i = 0; i < loops; i++)
rand () % RAND_MAX; // Just a dummy operation.
pthread_exit (NULL);
}
