[mpich-discuss] Implementation of MPICH collectives

[Pavan Balaji]

Depends on what the memcpy is doing.  It might be some internal data manipulation.

 -- Pavan

On Sep 13, 2013, at 4:34 PM, Jiri Simsa wrote:

> Hm, I have set that variable and then I have stepped through a program that calls MPI_Bcast (using mpiexec -n 2 <program> on a single node). The MPI_Bcast still seems to use memcpy() while I would expect it to use the sockets interface. Is the memcpy() to be expected?
> 
> --Jiri 
> 
> 
> On Fri, Sep 13, 2013 at 10:25 AM, Pavan Balaji <balaji at mcs.anl.gov> wrote:
> 
> Yes, you can set the environment variable MPIR_PARAM_CH3_NOLOCAL=1.
> 
>  -- Pavan
> 
> On Sep 13, 2013, at 7:53 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> 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
> > http://www.mcs.anl.gov/~balaji
> 
> --
> Pavan Balaji
> http://www.mcs.anl.gov/~balaji
> 
> 

--
Pavan Balaji
http://www.mcs.anl.gov/~balaji