[Mpi-forum] MPI_Request_free restrictions

Thu Aug 13 08:43:33 CDT 2020

The two cases you mentioned would have the same behavior at an application
level. However, there may be important differences in the implementation of
each operation. For example, an MPI_Put operation may be configured to not
generate a completion event, whereas an MPI_Rput would. The library may be
relying on the user to make a call on the request to process the event and
clean up resources. The implementation can take over this responsibility if
the user cancels the request, but why should we ask implementers to take on
this complexity and overhead?

My $0.02 is that MPI_Cancel is subtle and complicated, and we should be
very careful about where we allow it. I don't see the benefit to the
programming model outweighing the complexity and overhead in the MPI
runtime for the case of MPI_Rput. I also don't know that we were careful
enough in specifying the RMA memory model that a canceled request-based RMA
operation will still have well-defined behavior. My understanding is that
MPI_Cancel is required primarily for canceling receive requests to meet
MPI's quiescent shutdown requirement.

 ~Jim.

On Thu, Aug 13, 2020 at 8:11 AM HOLMES Daniel via mpi-forum <
mpi-forum at lists.mpi-forum.org> wrote:

> Hi all,
>
> To increase my own understanding of RMA, what is the difference (if any)
> between a request-based RMA operation where the request is freed without
> being completed and before the epoch is closed and a “normal” RMA operation?
>
> MPI_LOCK() ! or any other "open epoch at origin" procedure call
> doUserWorkBefore()
> MPI_RPUT(&req)
> MPI_REQUEST_FREE(&req)
> doUserWorkAfter()
> MPI_UNLOCK() ! or the matching “close epoch at origin" procedure call
>
> vs:
>
> MPI_LOCK() ! or any other "open epoch at origin" procedure call
> doUserWorkBefore()
> MPI_PUT()
> doUserWorkAfter()
> MPI_UNLOCK() ! or the matching “close epoch at origin" procedure call
>
> Is this a source-to-source translation that is always safe in either
> direction?
>
> In RMA, in contrast to the rest of MPI, there are two opportunities for
> MPI to “block” and do non-local work to complete an RMA operation: 1)
> during MPI_WAIT for the request (if any - the user may not be given a
> request or the user may choose to free the request without calling MPI_WAIT
> or the user might call nonblocking MPI_TEST) and 2) during the close epoch
> procedure, which is always permitted to be sufficiently non-local to
> guarantee that the RMA operation is complete and its freeing stage has been
> done. It seems that a request-based RMA operation becomes identical to a
> “normal” RMA operation if the user calls MPI_REQUEST_FREE on the request.
> This is like “freeing" the request from a nonblocking point-to-point
> operation but without the guarantee of a later synchronisation procedure
> that can actually complete the operation and actually do the freeing stage
> of the operation.
>
> In collectives, there is no “ensure all operations so far are now done”
> procedure call because there is no concept of epoch for collectives.
> In point-to-point, there is no “ensure all operations so far are now done”
> procedure call because there is no concept of epoch for point-to-point.
> In file operations, there is no “ensure all operations so far are now
> done” procedure call because there is no concept of epoch for file
> operations. (There is MPI_FILE_SYNC but it is optional so MPI cannot rely
> on it being called.)
> In these cases, the only non-local procedure that is guaranteed to happen
> is MPI_FINALIZE, hence all outstanding non-local work needed by the “freed”
> operation might be delayed until that procedure is called.
>
> The issue with copying parameters is also moot because all of them are
> passed-by-value (implicitly copied) or are data-buffers and covered by
> “conflicting accesses” RMA rules.
>
> Thus, to me it seems to me that RMA is a very special case - it could
> support different semantics, but that does not provide a good basis for
> claiming that the rest of the MPI Standard can support those different
> semantics - unless we introduce an epoch concept into the rest of the MPI
> Standard. This is not unreasonable: the notifications in GASPI, for
> example, guarantee completion of not just the operation they are attached
> to but *all* operations issued in the “queue” they represent since the last
> notification. Their queue concept serves the purpose of an epoch. I’m sure
> there are other examples in other APIs. It seems to me likely that the
> proposal for MPI_PSYNC for partitioned communication operations is moving
> in the direction of an epoch, although limited to remote completion of all
> the partitions in a single operation, which accidentally guarantees that
> the operation can be freed locally using a local procedure.
>
> Cheers,
> Dan.
> —
> Dr Daniel Holmes PhD
> Architect (HPC Research)
> d.holmes at epcc.ed.ac.uk
> Phone: +44 (0) 131 651 3465
> Mobile: +44 (0) 7940 524 088
> Address: Room 2.09, Bayes Centre, 47 Potterrow, Central Area, Edinburgh,
> EH8 9BT
> —
> The University of Edinburgh is a charitable body, registered in Scotland,
> with registration number SC005336.
> —
>
> On 13 Aug 2020, at 01:40, Skjellum, Anthony via mpi-forum <
> mpi-forum at lists.mpi-forum.org> wrote:
>
> FYI, one argument (also used to force us to add restrictions on MPI
> persistent collective initialization to be blocking)... The
> MPI_Request_free on an NBC poses a problem for the cases where there are
> array types
> posed (e.g., Alltoallv/w)... It will not be knowable to the application if
> the vectors are in use by MPI still after
> the  free on an active request.  We do *not* mandate that the MPI
> implementation copy such arrays currently, so they are effectively "held as
> unfreeable" by the MPI implementation till MPI_Finalize.  The user cannot
> deallocate them in a correct program till after MPI_Finalize.
>
> Another effect for NBC of releasing an active request, IMHO,  is that you
> don't know when send buffers are free to be deallocated or receive buffers
> are free to be deallocated... since you don't know when the transfer is
> complete OR the buffers are no longer used by MPI (till after MPI_Finalize).
>
> Tony
>
>
>
>
> Anthony Skjellum, PhD
> Professor of Computer Science and Chair of Excellence
> Director, SimCenter
> University of Tennessee at Chattanooga (UTC)
> tony-skjellum at utc.edu  [or skjellum at gmail.com]
> cell: 205-807-4968
>
> ------------------------------
> *From:* mpi-forum <mpi-forum-bounces at lists.mpi-forum.org> on behalf of
> Jeff Hammond via mpi-forum <mpi-forum at lists.mpi-forum.org>
> *Sent:* Saturday, August 8, 2020 12:07 PM
> *To:* Main MPI Forum mailing list <mpi-forum at lists.mpi-forum.org>
> *Cc:* Jeff Hammond <jeff.science at gmail.com>
> *Subject:* Re: [Mpi-forum] MPI_Request_free restrictions
>
> We should fix the RMA chapter with an erratum. I care less about NBC but
> share your ignorance of why it was done that way.
>
> Sent from my iPhone
>
> On Aug 8, 2020, at 6:51 AM, Balaji, Pavan via mpi-forum <
> mpi-forum at lists.mpi-forum.org> wrote:
>
>  Folks,
>
> Does someone remember why we disallowed users from calling
> MPI_Request_free on nonblocking collective requests?  I remember the
> reasoning for not allowing cancel (i.e., the operation might have completed
> on some processes, but not all), but not for Request_free.  AFAICT,
> allowing the users to free the request doesn’t make any difference to the
> MPI library.  The MPI library would simply maintain its own refcount to the
> request and continue forward till the operation completes.  One of our
> users would like to free NBC requests so they don’t have to wait for the
> operation to complete in some situations.
>
> Unfortunately, when I added the Rput/Rget operations in the RMA chapter, I
> copy-pasted that text into RMA as well without thinking too hard about it.
> My bad!  Either the RMA committee missed it too, or they thought of a
> reason that I can’t think of now.
>
> Can someone clarify or remind me what the reason was?
>
> Regards,
>
>   — Pavan
>
> MPI-3.1 standard, page 197, lines 26-27:
>
> “It is erroneous to call MPI_REQUEST_FREE or MPI_CANCEL for a request
> associated with a nonblocking collective operation.”
>
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