Subj : Re: Per thread cpu-usage on Linux
To   : comp.programming.threads
From : John G. Shaw
Date : Wed Feb 16 2005 09:36 am

Christian Panten wrote:
> Hi,
> 
> I have a problem to calculate the cpu time used by a thread...
> The problem exists on several linux- and unix-systems.
> 
> First I try to describe what I want to calculate:
> I work on a program which I have parallelized with pthreads. Now I want 
> to calculate some times.
> 1. The running time of the whole programm: no problem: I use the 
> real-timer.
> 2. The cpu running time of all threads: there is still a problem because 
> there are varieties on several systems. But this is not the problem I 
> want to discuss here.
> 
> 3. My problem mow: I want to calculate the cpu-time of one thread (the 
> thread is still running).
> This I know:
> Linux-kernel 2.4: I can calculate the cpu-time of a thread by using 
> times(), because each thread seems to have its own counter.
> 
> Linux-kernel 2.6 with NPTL: The function times() returns the total 
> cpu-time of the whole process. And I don't know how to get separate 
> times for each thread.
> 
> I have made some trials:
> - calculate the cpu-time with clock_gettime(CLOCK_THREAD_CPUTIME_ID, ..)
> - reading the cpu-time from /proc/self/task/*ID*/stat
> - calculate the cpu-time with clock_gettime(CLOCK_PROCESS_CPUTIME_ID, ..)
> 
> I wrote a test-program with 10 threads for a linux-server with 8 
> processors. In doing so the times calculated with 
> CLOCK_THREAD_CPUTIME_ID and CLOCK_PROCESS_CPUTIME_ID are nearly equal 
> and the time read from /proc/... is in parts lower about several seconds 
> as both cpu-timer, but not all the time.
> 
> Here one line from my test-program: (kernel 2.6)
> thread:   6.470 read from /proc/...
> cpu:     10.220 calculated with CLOCK_PROCESS_...
> times:   72.420 calculated with times()
> thread2: 10.219 calculated with CLOCK_THREAD_...
> 
> I hope you can help me...
> 1. Does anyone know how I can calculate the cpu-time of single thread? 
> Reading the cpu time from /proc/... is not a satisfactory solution for me.
> 2. Does anyone know how I can solve the same problem with AIX 5.1?
> 
> Thx for your help...
> 
> Best regards
> Christian

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.

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John G. Shaw
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