[mpich-discuss] Implementation of MPICH collectives

Jiri Simsa jsimsa at cs.cmu.edu
Thu Sep 12 14:24:27 CDT 2013

The high-order bit of my question is: What OS interface(s) does MPICH use
to transfer data from one MPI process to another?

On Thu, Sep 12, 2013 at 1:36 PM, Jiri Simsa <jsimsa at cs.cmu.edu> wrote:

> Hello,
> I have been trying to understand how MPICH implements collective
> operations. To do so, I have been reading the MPICH source code and
> stepping through mpiexec executions.
> For the sake of this discussion, let's assume that all MPI processes are
> executed on the same computer using: mpiexec -n <n> <mpi_program>
> This is my current abstract understanding of MPICH:
> - mpiexec spawns a hydra_pmi_proxy process, which in turn spawns <n>
> instances of <mpi_program>
> - hydra_pmi_proxy process uses socket pairs to communicate with the
> instances of <mpi_program>
> I am not quite sure though what happens under the hoods when a collective
> operation, such as MPI_Allreduce, is executed. I have noticed that
> instances of <mpi_program> create and listen on a socket in the course of
> executing MPI_Allreduce but I am not sure who connects to these sockets.
> Any chance someone could describe the data flow inside of MPICH when a
> collective operation, such as MPI_Allreduce, is executed? Thanks!
> Best,
> --Jiri Simsa
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