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Subject: Re: [OMPI users] Question about MPI_Barrier
From: Eugene Loh (eugene.loh_at_[hidden])
Date: 2010-10-21 10:58:19


Jeff Squyres wrote:

>Ah. The original code snipit you sent was:
>
>MPI::COMM_WORLD.Barrier();
>if(rank == master) t1 = clock();
>"code A";
>MPI::COMM_WORLD.Barrier();
>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.
>
>
The barrier exit times are not guaranteed to be uniform, but in practice
this style of timing is often the best (or only practical) tool one has
for measuring the collective performance of a group of processes.

>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.
>
>
Right. The bottom line here is that from a causal point of view it
would seem that B should not impact the timings. Presumably, some other
variable is actually responsible here.