[Mpi3-ft] Clarification Ticket 323: MPI_ANY_SOURCE and MPI_Test(any)

[George Bosilca]

The expected behavior is straightforward: 

- MPI_Test is a single completion function, it will therefore return MPI_ERR_PENDING, without setting flag to true or affecting the status.

- If MPI_Test is replaced by MPI_Testany, then the operation will return MPI_ERR_IN_STATUS, the index will be set, the flag will not be set, and the status will be updated. All multiple completion function will behave similarly (setting the status->error to MPI_ERR_PENDING) and returning MPI_ERR_IN_STATUS.

You are right, as no operation is completed, the flag will remain set to false. However, we are in the case where the MPI_Testany is returning an error, case that is not well defined in the current standard.

  george.

On Mar 16, 2012, at 09:52 , Josh Hursey wrote:

> I believe my reasoning is correct below, but thought I would ask the
> group to confirm.
> 
> Consider the following code snippet:
> ---------------------
> MPI_Irecv(..., MPI_ANY_SOURCE, ..., &req);
> /* Some other process in the communicator fails */
> MPI_Test(&req, &flag, &status);
> ---------------------
> 
> The proposal in #323 says that the request should be marked as
> MPI_ERR_PENDING and not complete. So what should the value of 'flag'
> and 'status' be when returning from MPI_Test?
> 
> According to the standard, 'flag = true' indicates two things:
> 1) the operation is completed
> 2) The 'status' object is set
> 
> For the MPI_ANY_SOURCE case above, the operation is -not- completed,
> so (1) is violated; therefore I think MPI_Test should set 'flag' equal
> to 'false'. However, is the 'status' also not set? Should MPI_Test
> return MPI_SUCCESS or MPI_ERR_PENDING?
> 
> If MPI_Test is to return MPI_ERR_PENDING directly, then there is no
> needed to inspect 'status'. However if we replace MPI_Test with
> MPI_Testany(1, &req, &index, &flag, &status) then the operation would
> return MPI_ERR_IN_STATUS, and the user must inspect the 'status' field
> for the true error value. So we would still set 'flag = false', but
> would also need to set the 'status'. That is if we want MPI_Test*
> return an error code that indicates that the request as 'failed, but
> not completed'.
> 
> According to the standard, if no operation is completed then
> MPI_Testany "returns flag = false, returns a value of MPI_UNDEFINED in
> index and status is undefined." So according to the MPI_Testany logic,
> in this case 'flag = false', 'status is undefined', and the operation
> should return MPI_SUCCESS. Is that the expected behavior for the code
> snippet above?
> 
> I think so, but I thought I would double check with the group.
> 
> This means that the user can only 'see' the MPI_ERR_PENDING state of
> the request when they call an MPI_Wait* operation, which might not be
> what they would normally want to do (because they do not want to
> block).
> 
> -- Josh
> 
> -- 
> Joshua Hursey
> Postdoctoral Research Associate
> Oak Ridge National Laboratory
> http://users.nccs.gov/~jjhursey
> _______________________________________________
> mpi3-ft mailing list
> mpi3-ft at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft