[mpich-discuss] Implementation of MPICH collectives

Pavan Balaji balaji at mcs.anl.gov
Thu Sep 12 14:29:06 CDT 2013

Within the node, it uses shared memory.  Outside the node, it depends on the netmod you configured with.  tcp is the default netmod.

 -- Pavan

On Sep 12, 2013, at 2:24 PM, Jiri Simsa wrote:

> 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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Pavan Balaji

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