[mpich-discuss] Implementation of MPICH collectives

[Antonio J. Peña]

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

  Antonio


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


Pavan,


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?


--Jiri


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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