[mpi-forum] MPI_Startall / MPI_Waitall / MPI_Testall clarification?

[Dries Kimpe]

>From the standard: (same for MPI_Startall, MPI_Testall)

The error-free execution of MPI_WAITALL(count, array_of_requests,
array_of_statuses) has the same effect as the execution of
MPI_WAIT(&array_of_request[i], &array_of_statuses[i]), for i=0 ,...,
count-1, in some arbitrary order.

What about count==0? 

1) Is it allowed?  

2) If it is allowed, should a valid pointer be provided as the
array_of_requests argument?

Looking at mpich-1.0.6p1, I see the following:


Function     Allows count==0    Allows bufptr=0 when count==0
MPI_Testall     Y                       Y
MPI_Testsome    Y                       Y
MPI_Testany     Y                       Y
MPI_Waitsome    Y                       Y
MPI_Waitall     Y                       Y
MPI_Waitany     Y                       Y
MPI_Startall    Y                       N

For MPI_Startall:

Fatal error in MPI_Startall: Invalid argument, error stack:
MPI_Startall(147): MPI_Startall(count=0, req_array=(nil)) failed
MPI_Startall(80).: Null pointer in parameter array_of_requests[0]0:Return code = 0, signaled with Interrupt

Considering the other ...all functions allow count==0 and ignore the array
in this case, the behaviour of MPI_Startall  is probably just an oversight
in mpich.

For some reason, OpenMPI has exactly the same behavior;
MPI_Startall doesn't allow req_array==0 even if count==0
(MPI_ERR_REQUEST is raised/returned)

There is also an issue: progress.  For example, in OpenMPI, calling
MPI_Testall with count==0 doesn't do anything at all. More specific, it
doesn't call the progress engine. 

Mpich however, does call the progress engine, even if count==0.
(On the other hand, MPI_Waitall does NOT try to make progress if count==0)

I propose to clarify where needed, and explicitly allow count==0;
Also, if count==0, then 0 should be accepted as pointer for the request
array, and no guarantee of progress should be made.

If count==0, the returned error code should be MPI_SUCCESS (unless of
course, an asynchronous error is detected).

It would suffice to add "When count is zero, this call has no effect."
(For MPI_Startall, MPI_Testall, MPI_Waitall this can be right after the
text quoted on top of this message; For MPI_Waitany, MPI_Testany,
MPI_Waitsome, MPI_Testsome, the addition should be at the end of the
paragraph.)


  Greetings,
  Dries