[mpiwg-persistence] Completion call before all partitions marked ready

[Joachim Protze]

Hello wg-persistence,

Looking at the MPI 4.0 document, it is not clear to us, whether it is 
allowed to call a completion call for a partitioned communication 
request before all partitions are marked ready. A simple single-threaded 
example would be:

```C
MPI_Psend_init(message, partitions, COUNT, MPI_DOUBLE, dest, tag,
                MPI_COMM_WORLD, MPI_INFO_NULL, &request);
MPI_Start(&request);
for(i = 0; i < partitions-1; ++i)
{
     MPI_Pready(i, request);
}
MPI_Test(&request, &flag, MPI_STATUS_IGNORE); // flag will always be 0
MPI_Pready(partitions-1, request);
MPI_Wait(&request, MPI_STATUS_IGNORE);
MPI_Request_free(&request);
```

The question becomes more relevant in a multi-threaded context. One 
thread could finish the work early and call MPI_Wait to detect when all 
partitions were sent.

 From my understanding, the only requirement is that all partitions must 
be marked ready with explicit ready calls before the operation can 
complete. Replacing the test in above example with a wait call would 
result in deadlock.

Best
Joachim

-- 
Dr. rer. nat. Joachim Protze

IT Center
Group: High Performance Computing
Division: Computational Science and Engineering
RWTH Aachen University
Seffenter Weg 23
D 52074  Aachen (Germany)
Tel: +49 241 80- 24765
Fax: +49 241 80-624765
protze at itc.rwth-aachen.de
www.itc.rwth-aachen.de
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