Le 15 sept. 2011 à 16:37, Eugene Loh a écrit : > On 9/15/2011 5:51 AM, Ghislain Lartigue wrote: >> start_0 = MPI_Wtime() >> >> start_1 = MPI_Wtime() >> call foo() >> end_1 = MPI_Wtime() >> write(*,*) "timer1 = ",end1-start1 >> >> start_2 = MPI_Wtime() >> call bar() >> end_2 = MPI_Wtime() >> write(*,*) "timer2 = ",end2-start2 >> >> end_0 = MPI_Wtime() >> write(*,*) "timer0 = ",end0-start0 >> >> ================================================== >> >> When I run my code on a "small" number of processors, I find that >> timer0=timer1+timer2 with a very good precision (less than 1%). >> However, as I increase the number of processors, this is not true any more: >> I can have 10%, 20% or even more discrepancy! >> The more processor I use, the bigger errors are observed. >> >> Obviously, my code is much bigger than the simple example above, but the >> principle is exactly the same. > In the simple example, if timer0 is much bigger than timer1+timer2, we'd be > inclined to attribute extra time to the timer calls or the write > statements... in any case, to time spent between end_1 and start_2 or between > end_2 and end_0.
=> no this can not be, the time spent in the timers and the write operation is very small compared to the overall code (as indicated by the result on 1proc) > Are you sure in the actual code there are no substantial operations in those > sections? => I am sure of that (same reason as above) > Also, is it possible your processes are not running during some of those > times? => No idea... What do you have in mind precisely? > Are you oversubscribing? => No way... > Also, instead of printing out endX-startX, how about writing out endX and > startX individually so you get all six timestamps and can see in greater > detail where the discrepancy is arising. => This is a good idea: I will try that... > _______________________________________________ > users mailing list > us...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/users >
