866
Subj : Re: Per thread cpu-usage on Linux
To   : comp.programming.threads
From : Christoph Bartosch
Date : Wed Feb 16 2005 04:49 pm

John G. Shaw wrote:

> 
> Try something like the following using pthread_getcpuclockid()
> 
> 
> 
> static double timespec_to_double(struct timespec &ts)
> {
>    const double ns_to_sec= 1.0e-9;
>    double t= double(ts.tv_sec) + double(ts.tv_nsec)*ns_to_sec;
>    return t;
> }
> 
> // beginning of section to time
>    clockid_t clock_id;
>    pthread_getcpuclockid(pthread_self(),&clock_id);
>    struct timespec ts;
>    clock_gettime(clock_id,&ts);
>    double cpuTime0= timespec_to_double(ts);
> 
> // do it again at end of section to time (in the thread)
> // then subtract the two cputimes as usual.
> 

Hi, this seems not to work because the function seems to measure realtime.
The following programm using only the main thread should illustrate it:

extern "C" {
   #include <time.h>
   #include <sys/time.h>
   #include <sys/times.h>
   #include <sys/resource.h>
   #include <errno.h>
   #include <unistd.h>
   #include <pthread.h>
}

#include <iostream>

static double timespec_to_double(struct timespec &ts)
{
   const double ns_to_sec= 1.0e-9;
   double t= double(ts.tv_sec) + double(ts.tv_nsec)*ns_to_sec;
   return t;
}

int main()
{
   clockid_t clock_id;
   pthread_getcpuclockid(pthread_self(),&clock_id);
   struct timespec ts;
   clock_gettime(clock_id,&ts);
   double cpuTime0= timespec_to_double(ts);

   for (int i = 0;i != 5; ++i)
   {
      sleep(1);
      clock_gettime(clock_id,&ts);
      double cpuTime1= timespec_to_double(ts);
      std::cout << "Time: " << cpuTime1-cpuTime0 << std::endl;
   }
}

bartosch@preston:~/uhren> g++ *.C -pthread -lrt
bartosch@preston:~/uhren> time ./a.out
Time: 1.00131
Time: 2.00327
Time: 3.00523
Time: 4.00719
Time: 5.00915

real    0m5.016s
user    0m0.001s
sys     0m0.003s


The measured time is 5 seconds while the used cpu time is below 0.01
seconds. The original poster wants to measure the cpu time used by a single
thread.


Greets
Christoph Bartoschek

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