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Asked: 2026-06-13T16:24:55+00:00

I have a machine with 8 processors. I want to alternate using OpenMP and

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I have a machine with 8 processors. I want to alternate using OpenMP and MPI on my code like this:

OpenMP phase:

  • ranks 1-7 wait on a MPI_Barrier
  • rank 0 uses all 8 processors with OpenMP

MPI phase:

  • rank 0 reaches barrier and all ranks use one processor each

So far, I’ve done:

  • set I_MPI_WAIT_MODE 1 so that ranks 1-7 don’t use the CPU while on the barrier.
  • set omp_set_num_threads(8) on rank 0 so that it launches 8 OpenMP threads.

It all worked. Rank 0 did launch 8 threads, but all are confined to one processor. On the OpenMP phase I get 8 threads from rank 0 running on one processor and all other processors are idle.

How do I tell MPI to allow rank 0 to use the other processors? I am using Intel MPI, but could switch to OpenMPI or MPICH if needed.

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1 Answer

  1. Editorial Team

    Thanks all for the comments and answers. You are all right. It’s all about the “PIN” option.

    To solve my problem, I just had to:

    I_MPI_WAIT_MODE=1

    I_MPI_PIN_DOMAIN=omp

    Simple as that. Now all processors are available to all ranks.

    The option

    I_MPI_DEBUG=4

    shows which processors each rank gets.

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