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

[Bland, Wesley B.]

I wrote the patch. I'll take a look. 

> On Jul 6, 2014, at 8:36 AM, "Balaji, Pavan" <balaji at anl.gov> wrote:
> 
> 
> 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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