[mvapich-discuss] Allreduce time when using MPI+OpenMP is too large comparing to when using MPI alone

[Sarunya Pumma]

Hi Mamzi,

Thank you very much for your response.

I used MPI_Init(&argc, &argv) in my code. There are the OpenMP threads
running in the background for the OMP+MPI implementation. Here is my code:

#pragma omp parallel
{
  int num = omp_get _num_threads()
  printf("Number of threads %d\n", num);
}

for (int i = 0; i < iter; i++) {
  MPI_Allreduce(msg_s, msg_r, count, MPI_FLOAT, MPI_SUM, MPI_COMM_WORLD);
}

Note that if I comment the #pragma omp parallel out and compile the code
with the -openmp flag, I will observe the similar performance for MPI with
1 proc and OMP+MPI with 1 proc.

Please let me know if you need more information

Thank you very much

Best,
Sarunya


On Mon, Mar 6, 2017 at 3:29 PM, Bayatpour, Mamzi <
bayatpour.1 at buckeyemail.osu.edu> wrote:

>
> Hello Sarunya,
>
> We've been looking at the issue you reported internally. However, we are
> not able to reproduce the performance trends you reported. We observed
> similar performance for MPI with 1 process per node and MPI+OMP with 1
> process per node.
>
> Could you please provide more details about the application that you are
> testing? Are you using MPI_SINGLE_THREAD or MPI_MULTI_THREAD for your
> MPI_Init function? Are any OpenMP threads running in the background during
> the MPI_Allreduce call? A small reproducer could help us a lot.
>
> Thanks,
> Mamzi
>
> ------------------------------
> *From:* Bayatpour, Mamzi
> *Sent:* Friday, March 3, 2017 10:12:24 PM
> *To:* mvapich-discuss at cse.ohio-state.edu
> *Cc:* sarunya at vt.edu
> *Subject:* Re: [mvapich-discuss] Allreduce time when using MPI+OpenMP is
> too large comparing to when using MPI alone
>
>
> Hello Sarunya,
>
> Thanks for reporting the issue to us. We are taking a look at it and will
> get back to you soon.
>
> Thanks,
> Mamzi
>
>
>
>
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