[mvapich-discuss] (no subject)

[Hari Subramoni]

Hello Ivan,

Your understanding of the semantics is correct. MVAPICH2 also follows these
semantics. However there could be some interactions happening with the
underlying algorithm being used for the Scatter operation in MVAPICH2 on
your system.

To further debug this, could you please provide us with the following
details

1. The size of the job you're running
2. The message size being used for the Scatter operation
3. Processor type
4. Interconnect type
5. OFED version

Thx,
Hari.
On Jan 28, 2016 11:40 PM, "Ivan Raikov" <ivan.g.raikov at gmail.com> wrote:

> Hi all,
>
>     I recently tried to use some of my OpenMPI code in an MVAPICH2
> environment, and I noticed a discrepancy in the semantics of
> MPI_Scatter and other collective operations, so I thought I would ask
> about this here.
>
> I have been following the pattern below when calling collective
> operations in OpenMPI:
>
> if (myrank == root) {
>     MPI_Scatter(vector, N, MPI_INTEGER, recvbuf, N, MPI_INTEGER, root,
> MPI_COMM_WORLD);
>   }
>   else {
>     MPI_Scatter(NULL, 0, MPI_DATATYPE_NULL, recvbuf, N, MPI_INTEGER,
> root, MPI_COMM_WORLD);
>   }
>
> My understanding of the MPI_Scatter documentation was that the
> sendbuf, sendcount, and sendtype arguments are "significant only at
> root" meaning that their values should be ignored by all non-root
> processes. This certainly works in OpenMPI. However, the same code
> results in no data being sent to the non-root processes with MVAPICH2
> 2.1. Have I completely misunderstood the specification, or does
> MVAPICH2 have different collective operation semantics for a reason?
> Thanks in advance for any clarification and best regards,
>
>
>   -Ivan Raikov
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