[mpich-discuss] problem with collective on sub-communicator

[Miguel Oliveira]

Hi all,

I wrote a very simple master/slave code in MPI and I'm having problems with MPI_Reduce, or even, MPI_Barrier, inside a subset of the world communicator.
These operations don't seem to be waiting for all the processes in the subgroup.

The code is a straightforward master/slave case where the master generates random numbers when requested and then retrieves a reduction of the sum of these
done on the slaves.

When run on more than three processes sometimes it happens that the message after the reduction, done from one of the slaves to inform the master of the final
result gets to the master before some of the requests for random numbers... This ought to be impossible with a blocking reduction...

Am I missing something?

Code is attached.

Help is appreciated.

Cheers,

MAO

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