[mpich-discuss] determinism in collectives

Pavan Balaji balaji at mcs.anl.gov
Mon Nov 12 21:43:24 CST 2012

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
>> To manage subscription options or unsubscribe:
>> https://lists.mpich.org/mailman/listinfo/discuss

Pavan Balaji

More information about the discuss mailing list