[mpich-devel] MPI_Reduce_scatter_block() with zero-size, NULL buffers.

[Balaji, Pavan]

Yup, you are right.  The check is overly aggressive.  We’ll fix it.

For others following along, here’s the ticket tracking this issue.

http://trac.mpich.org/projects/mpich/ticket/2124

  — Pavan

On Jul 6, 2014, at 7:20 AM, Lisandro Dalcin <dalcinl at gmail.com> wrote:

> On 6 July 2014 14:34, Balaji, Pavan <balaji at anl.gov> wrote:
>> 
>> Yes, this is intentional.  It’s an additional error check and is showing a bug in your user code.
>> 
> 
> 
> 
> Do you really think this buffer specification is aliased?: sbuf=(nil),
> rbuf=(nil), recvcount=0, MPI_INT
> 
> I mean, the message count are zero, and the buffers are both NULL. In
> case of count=0, all reductions should be basically a non-op, there is
> nothing to reduce. IMHO, using NULL pointers to represent zero-sized
> buffers is common practice. Perhaps this check should be relaxed for
> sbuf=rbuf=NULL ?
> 
> Please look at this test in mpi4py where I explicitly test for
> zero-size messages:
> https://bitbucket.org/mpi4py/mpi4py/src/master/test/test_msgzero.py
> 
> While this is in Python, the outcome is that the various p2p,
> collectives and reductions are called with buf=NULL, count=0,
> datatype=MPI_INT. From the tests there, all the block and vector
> collectives pass, but the reductions do not.
> 
> 
> 
> -- 
> Lisandro Dalcin
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