[Mpi-comments] Non blocking receive of unknown size

[Alain Miniussi]

Gilles,

Not so sure, that argument could be made for MPI_Irecv (used in the 
original code), which is not allowed to deadlook. There is a special 
send mode called "ready" that explicitly assume that the recv call has 
been made, but that's just an optimization.
 From 3.4: "Thus, a send in standard mode can be started whether or not 
a matching receive has been posted."

I kept the MPI_Ssend because it was in the original example, but the 
question still hold if replaced with MPI_Send which leaves more choice 
to the implementation.

If MPI_Iprobe initiate a reception (which is not explicitly stated, and 
which is basically my question) the first MPI_Send could just wait for 
that event and send the data (maybe using the buffered mode).

An argument could be made that since the receiver buffer is not 
provided, and the implementation could choose a communication mode 
requiring that a user provided buffer to be ready ready. But then the 
same argument could be made for MPI_PROBE, and state 3.8 that:
"The MPI_PROBE, MPI_IPROBE, MPI_MPROBE, and MPI_IMPROBE operations allow 
incoming messages to be checked for,  without actually receiving them."
and later on (p 65):
"it is not necessary to receive a message immediately after it has been 
probed for, and the same message may be probed for several times before 
it is received."

So it seems that the implementation is expected to send the size first, 
for MPI_[IM]probe to look it up, but that the message itself can be kept 
pending and that the matching send can be kept blocked until the message 
is actually copied. Put another way, the message size must be made 
available to the receiver no matter what communication protocol is chosen.

Cheers

Alain

On 13/09/2018 15:24, gilles--- via mpi-comments wrote:
>
>  Alain,
>
> Regardless of what the standard says or how it is interpreted, your 
> example can simply deadlock if rank 1 returns from MPI_Iprobe()
>
> before rank 0 calls MPI_Ssend().
>
> Cheers,
>
> Gilles
>
> ----- Original Message -----
>
>     Hi,
>
>     (text location provided w.r.t MPI 3.1
>     https://www.mpi-forum.org/docs/mpi-3.1/mpi31-report.pdf)
>
>      a) In 3.2.4, page 29, there is a discussion regarding the size of
>     the buffer and size of the actual message with an "advice to
>     users" section stating that :
>     "The MPI_PROBE function described in Section 3.8 can be used to
>     receive messages of unknown length."
>
>      b) There is no such discussion in 3.7.2 so one could assume
>     (maybe too optimistically) that the same advice applies w.r.t.
>     MPI_Iprobe.
>
>      c) In 3.8.1. page 66 line 9 "The MPI implementation of MPI_PROBE
>     and MPI_IPROBE needs to guarantee progress: [calling those
>     functions, msg should arrive eventually if send and not intercepted].
>
>      d) The "Progress" note in 3.7.4, illustrated by example 3.14,
>     seems to indicate that a synchronous send can not be blocked if a
>     recv has been posted regardless of the completion of that receive.
>
>     But I could not find anything explicitly stating that MPI_Iprobe
>     would initiate a receive the way MPI_Irecv does.
>
>     That is, a slightly modified 3.14 example:
>
>     CALL MPI_COMM_RANK(comm, rank, ierr)
>     IF (RANK.EQ.0) THEN
>         CALL MPI_SSEND(a, n, MPI_REAL, 1, 0, comm, ierr)
>         CALL MPI_SEND(b, 1, MPI_REAL, 1, 1, comm, ierr)
>     ELSE IF (rank.EQ.1) THEN
>         CALL MPI_IPROBE(0, 0, comm, flag, status1, ierr)
>         CALL MPI_RECV(b, 1, MPI_REAL, 0, 1, comm, status2, ierr)
>         ... extract size info from status1  once available ....
>         CALL MPI_IRECV(a, n, MPI_REAL, 0, 0, comm, r, ierr)
>         CALL MPI_WAIT(r, status, ierr)
>     END IF
>
>     could deadlock in a correct MPI implementation.
>
>     If I am correct in assuming that the MPI standard ask for the
>     MPI_Iprobe to progress, it is not required to initiate. If true,
>     it means the the point a) above does not extend to the non
>     blocking communications and that the MPI does not provide a way to
>     receive messages of unknown size whithout adding more
>     opportunities for deadlocks.
>
>     Am I correct ? and if yes, does that indicate that the discussion
>     and advice of point a could be adapted and explicitly added for
>     the non blocking case (point b).
>
>     Thanks
>
>     Alain
>
>
>
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