[mpich-discuss] determinism in collectives

Jeff Hammond jhammond at alcf.anl.gov
Mon Nov 12 21:47:29 CST 2012

Yes, what I was saying before is that we know that MPI on Blue Gene/Q
drops into the MPICH2 algorithm for MPI_Reduce on for
MPI_DOUBLE_COMPLEX with MPI_SUM on irregular communicators and that
MPIC_Send and MPIC_Recv do not change their behavior on this machine,
so whatever is true of stock MPICH2 1.4 is true for MPI on Blue

This confirms what we suspected, which is that BGQ-MPI has
deterministic reductions in this case because MPICH2 does.


On Mon, Nov 12, 2012 at 9:43 PM, Pavan Balaji <balaji at mcs.anl.gov> wrote:
> The default machine-independent implementation of MPICH is
> deterministic.  However, each device can (and most times does) implement
> its own version of MPI_Reduce.  We have no control on them.
>  -- Pavan
> On 11/12/2012 09:40 PM US Central Time, Jeff Hammond wrote:
>> We are primarily interested in MPICH2, as the motivation for this
>> question relates to a particular supercomputer that uses MPICH2 and,
>> in particular, the MPICH2 implementation of MPI_Reduce for
>> (comm,type,op) of interest to us.
>> Jeff
>> On Mon, Nov 12, 2012 at 9:35 PM, Pavan Balaji <balaji at mcs.anl.gov> wrote:
>>> The MPI standard does not guarantee this.  For example, an
>>> implementation could use topology-aware reductions or an MPI_ANY_SOURCE
>>> while waiting for messages from peers making the order in which the
>>> reduction operations are applied different for each run.
>>>  -- Pavan
>>> On 11/12/2012 09:00 PM US Central Time, Nichols A. Romero wrote:
>>>> Hi,
>>>> I apologize if this has been asked a trillion times, if its documented somewhere just point it out to me online.
>>>> Are collective operations, such as sums, using MPI_Reduce deterministic? In other words, if I run MPI_Reduce N times on P tasks, will the sums always occur in the same order. Thus it would be reproducible to the expected precision.
>>> --
>>> Pavan Balaji
>>> http://www.mcs.anl.gov/~balaji
>>> _______________________________________________
>>> discuss mailing list     discuss at mpich.org
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> --
> Pavan Balaji
> http://www.mcs.anl.gov/~balaji

Jeff Hammond
Argonne Leadership Computing Facility
University of Chicago Computation Institute
jhammond at alcf.anl.gov / (630) 252-5381

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