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 Subject: Re: [OMPI users] Problem with MPI_BARRIER From: Ghislain Lartigue (ghislain.lartigue_at_[hidden]) Date: 2011-09-08 10:25:14 Next message: Jai Dayal: "Re: [OMPI users] Problem with MPI_BARRIER" Previous message: Teng Ma: "Re: [OMPI users] Problem with MPI_BARRIER" In reply to: Teng Ma: "Re: [OMPI users] Problem with MPI_BARRIER" Next in thread: Jai Dayal: "Re: [OMPI users] Problem with MPI_BARRIER" Reply: Jai Dayal: "Re: [OMPI users] Problem with MPI_BARRIER" Reply: Teng Ma: "Re: [OMPI users] Problem with MPI_BARRIER" Thanks, I understand this but the delays that I measure are huge compared to a classical ack procedure... (1000x more) And this is repeatable: as far as I understand it, this shows that the network is not involved. Ghislain. Le 8 sept. 2011 à 16:16, Teng Ma a écrit : > I guess you forget to count the "leaving time"(fan-out). When everyone > hits the barrier, it still needs "ack" to leave. And remember in most > cases, leader process will send out "acks" in a sequence way. It's very > possible: > > P0 barrier time = 29 + send/recv ack 0 > P1 barrier time = 14 + send ack 0 + send/recv ack 1 > P2 barrier time = 0 + send ack 0 + send ack 1 + send/recv ack 2 > > That's your measure time. > > Teng >> This problem as nothing to do with stdout... >> >> Example with 3 processes: >> >> P0 hits barrier at t=12 >> P1 hits barrier at t=27 >> P2 hits barrier at t=41 >> >> In this situation: >> P0 waits 41-12 = 29 >> P1 waits 41-27 = 14 >> P2 waits 41-41 = 00 > > > >> So I should see something like (no ordering is expected): >> barrier_time = 14 >> barrier_time = 00 >> barrier_time = 29 >> >> But what I see is much more like >> barrier_time = 22 >> barrier_time = 29 >> barrier_time = 25 >> >> See? No process has a barrier_time equal to zero !!! >> >> >> >> Le 8 sept. 2011 à 14:55, Jeff Squyres a écrit : >> >>> The order in which you see stdout printed from mpirun is not necessarily >>> reflective of what order things were actually printers. Remember that >>> the stdout from each MPI process needs to flow through at least 3 >>> processes and potentially across the network before it is actually >>> displayed on mpirun's stdout. >>> >>> MPI process -> local Open MPI daemon -> mpirun -> printed to mpirun's >>> stdout >>> >>> Hence, the ordering of stdout can get transposed. >>> >>> >>> On Sep 8, 2011, at 8:49 AM, Ghislain Lartigue wrote: >>> >>>> Thank you for this explanation but indeed this confirms that the LAST >>>> process that hits the barrier should go through nearly instantaneously >>>> (except for the broadcast time for the acknowledgment signal). >>>> And this is not what happens in my code : EVERY process waits for a >>>> very long time before going through the barrier (thousands of times >>>> more than a broadcast)... >>>> >>>> >>>> Le 8 sept. 2011 à 14:26, Jeff Squyres a écrit : >>>> >>>>> Order in which processes hit the barrier is only one factor in the >>>>> time it takes for that process to finish the barrier. >>>>> >>>>> An easy way to think of a barrier implementation is a "fan in/fan out" >>>>> model. When each nonzero rank process calls MPI_BARRIER, it sends a >>>>> message saying "I have hit the barrier!" (it usually sends it to its >>>>> parent in a tree of all MPI processes in the communicator, but you can >>>>> simplify this model and consider that it sends it to rank 0). Rank 0 >>>>> collects all of these messages. When it has messages from all >>>>> processes in the communicator, it sends out "ok, you can leave the >>>>> barrier now" messages (again, it's usually via a tree distribution, >>>>> but you can pretend that it directly, linearly sends a message to each >>>>> peer process in the communicator). >>>>> >>>>> Hence, the time that any individual process spends in the communicator >>>>> is relative to when every other process enters the communicator. But >>>>> it's also dependent upon communication speed, congestion in the >>>>> network, etc. >>>>> >>>>> >>>>> On Sep 8, 2011, at 6:20 AM, Ghislain Lartigue wrote: >>>>> >>>>>> Hello, >>>>>> >>>>>> at a given point in my (Fortran90) program, I write: >>>>>> >>>>>> =================== >>>>>> start_time = MPI_Wtime() >>>>>> call MPI_BARRIER(...) >>>>>> new_time = MPI_Wtime() - start_time >>>>>> write(*,*) "barrier time =",new_time >>>>>> ================== >>>>>> >>>>>> and then I run my code... >>>>>> >>>>>> I expected that the values of "new_time" would range from 0 to Tmax >>>>>> (1700 in my case) >>>>>> As I understand it, the first process that hits the barrier should >>>>>> print Tmax and the last process that hits the barrier should print 0 >>>>>> (or a very low value). >>>>>> >>>>>> But this is not the case: all processes print values in the range >>>>>> 1400-1700! >>>>>> >>>>>> Any explanation? >>>>>> >>>>>> Thanks, >>>>>> Ghislain. >>>>>> >>>>>> PS: >>>>>> This small code behaves perfectly in other parts of my code... >>>>>> >>>>>> >>>>>> _______________________________________________ >>>>>> users mailing list >>>>>> users_at_[hidden] >>>>> >>>>> >>>>> -- >>>>> Jeff Squyres >>>>> jsquyres_at_[hidden] >>>>> For corporate legal information go to: >>>>> >>>>> >>>>> _______________________________________________ >>>>> users mailing list >>>>> users_at_[hidden] >>>>> >>>> >>>> >>>> _______________________________________________ >>>> users mailing list >>>> users_at_[hidden] >>> >>> >>> -- >>> Jeff Squyres >>> jsquyres_at_[hidden] >>> For corporate legal information go to: >>> >>> >>> _______________________________________________ >>> users mailing list >>> users_at_[hidden] >>> >> >> >> _______________________________________________ >> users mailing list >> users_at_[hidden] >> > > > | Teng Ma Univ. of Tennessee | > | tma_at_[hidden] Knoxville, TN | > > _______________________________________________ > users mailing list > users_at_[hidden] > Next message: Jai Dayal: "Re: [OMPI users] Problem with MPI_BARRIER" Previous message: Teng Ma: "Re: [OMPI users] Problem with MPI_BARRIER" In reply to: Teng Ma: "Re: [OMPI users] Problem with MPI_BARRIER" Next in thread: Jai Dayal: "Re: [OMPI users] Problem with MPI_BARRIER" Reply: Jai Dayal: "Re: [OMPI users] Problem with MPI_BARRIER" Reply: Teng Ma: "Re: [OMPI users] Problem with MPI_BARRIER"