Subj : Re: Per thread cpu-usage on Linux
To   : comp.programming.threads
From : John G. Shaw
Date : Wed Feb 16 2005 06:26 pm

Christoph Bartosch wrote:
> 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


The intent was to use the  pthread_getcpuclockid() function within a 
running thread (i.e., a function invoked via pthread_create()). I'm a 
bit surprised it even runs under the main program (I thought 
pthread_self() would not be valid or even crash). I have used this 
approach successfully on a parallel computer (SGI Altix series), and it 
seems to return the thread's CPU time as opposed to "real time" in the 
thread. It's possible that pthread_self() does not return a 
thread-dependent clock_id if it is not invoked under a running pthread. 
It's also possible this may be a system-dependent behaviour. SGI's linux 
has been significantly modified from most others. (???)

-- 
*****************************************************
John G. Shaw
To reply, remove "noSpam" from the reply-to field.
*****************************************************

.