One comment - the clarification should not mention the progress engine. We have a general requirement in the standard that MPI make progress and there is debate about what that requires. (Must there be an asynchronous progress engine). In general we do not attach a progress semantic to specific MPI calls. Even for something like MPI_Wait, there is no statement that MPI_Wait drives a progress engine. Only that MPI_Wait returns when the request is complete. It is not a part of MPI_Wait semantic to say that the MPI_Wait call runs a progress engine or does not run one. I assume almost every MPI implementation does run its progress engine within a blocked MPI_Wait and probably no MPI gives the progress engine a time limited kick in a MPI_Comm_rank call. Either way it is an impementation decision, not part of the standard. (Of course something like MPI_Comm_rank could not block but if an MPI implementor wanted to run the progress engine for a max of 50 microseconds on every single MPI call, the standard would not forbid it.) Dick Dick Treumann - MPI Team/TCEM IBM Systems & Technology Group Dept 0lva / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 Tele (845) 433-7846 Fax (845) 433-8363 mpi-21-bounces_at_[hidden] wrote on 02/06/2008 12:37:20 PM:

Dries,

Sounds fine. Nobody raised problems with this clarification up to now.

Please can you write a full proposal of exactly what to change/add/...

I would put it to MPI 2.1 Ballot 4.

Best regards Rolf

On Mon, 4 Feb 2008 16:24:03 +0100 Dries Kimpe <Dries.Kimpe_at_[hidden]> wrote:

...

...

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

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Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner_at_[hidden] High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner Nobelstr. 19, D-70550 Stuttgart, Germany . (Office: Allmandring 30) _______________________________________________ mpi-21 mailing list mpi-21_at_[hidden] http://lists.cs.uiuc.edu/mailman/listinfo/mpi-21

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