Thanks a lot. On Thu, Oct 21, 2010 at 9:21 AM, Jeff Squyres <jsquy...@cisco.com> wrote:
> Ah. The original code snipit you sent was: > > MPI::COMM_WORLD.Barrier(); > if if(rank == master) t1 = clock(); > "code A"; > MPI::COMM_WORLD.Barrier(); > if if(rank == master) t2 = clock(); > "code B"; > > Remember that the time that individual processes exit barrier is not > guaranteed to be uniform (indeed, it most likely *won't* be the same). MPI > only guarantees that a process will not exit until after all processes have > entered. So taking t2 after the barrier might be a bit misleading, and may > cause unexpected skew. > Yes, it makes sense but I have no idea how big the time difference for running barrier function. I don't expect it big either since all our compute nodes have same configuration. > Code B *probably* has no effect on time spent between t1 and t2. But > extraneous effects might cause it to do so -- e.g., are you running in an > oversubscribed scenario? And so on. > No. We have 1024 nodes available and I'm using 500. > > On Oct 21, 2010, at 9:24 AM, Storm Zhang wrote: > > > > > Thanks a lot for your reply. By commenting code B, I mean if I remove the > code B part, then the time spent on code A seems to run faster. I do have a > lot of communications in code B too. It involves 500 procs. I had thought > code B should have no effect on the time spent on code A if I use > MPI_Barrier. > > > > Linbao > > On Thu, Oct 21, 2010 at 5:17 AM, Jeff Squyres <jsquy...@cisco.com> > wrote: > > On Oct 20, 2010, at 5:51 PM, Storm Zhang wrote: > > > > > I need to measure t2-t1 to see the time spent on the code A between > these two MPI_Barriers. I notice that if I comment code B, the time seems > much less the original time (almost half). How does it happen? What is a > possible reason for it? I have no idea. > > > > I'm not sure what you're asking here -- do you mean that if you put some > comments in code B, it takes much less time than if you don't put comments? > If so, then the comments have nothing to do with the execution run-time -- > something else is going on that is causing the delay. Some questions: > > > > - how long does it take to execute code B -- microseconds, or seconds, or > ...? > > - how many processes are involved? > > - what are you doing in code B; is it communication intensive and/or do > you synchronize with other processes? > > - are you doing your timings on otherwise-empty machines? > > > > -- > > Jeff Squyres > > jsquy...@cisco.com > > For corporate legal information go to: > > http://www.cisco.com/web/about/doing_business/legal/cri/ > > > > > > _______________________________________________ > > users mailing list > > us...@open-mpi.org > > http://www.open-mpi.org/mailman/listinfo.cgi/users > > > > _______________________________________________ > > users mailing list > > us...@open-mpi.org > > http://www.open-mpi.org/mailman/listinfo.cgi/users > > > -- > Jeff Squyres > jsquy...@cisco.com > For corporate legal information go to: > http://www.cisco.com/web/about/doing_business/legal/cri/ > > > _______________________________________________ > users mailing list > us...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/users >
