[mpich-discuss] Implementation of MPICH collectives

Antonio J. Peña apenya at mcs.anl.gov
Fri Sep 13 07:56:36 CDT 2013

You can set the MPIR_PARAM_CH3_NO_LOCAL environment variable to 1. 
Refer to the README.envvar file.


On Friday, September 13, 2013 08:53:36 AM Jiri Simsa wrote:


Thank you for your answer. That's precisely what I was looking for. Any 
chance there is a way to force the intranode communication to use tcp?


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[1]> 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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