I think you start going down a slippery slope when you start to guess about the state of a resource that has been put under the control on another entity, without that entity explicitly declaring something.
Rich
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-------- Original message --------From: Jeff Hammond via mpi-forum <mpi-forum@lists.mpi-forum.org>Date: 8/15/20 12:51 (GMT-05:00)To: "Skjellum, Anthony" <Tony-Skjellum@utc.edu>Cc: Jeff Hammond <jeff.science@gmail.com>, Main MPI Forum mailing list <mpi-forum@lists.mpi-forum.org>Subject: Re: [Mpi-forum] MPI_Request_free restrictions
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Yes, but do we think the use case of never detecting completion is likely? I am not arguing for that, but rather that users might free the request but then detect completion another way, such as by waiting on the request on a subset of processes.
Jeff
On Wed, Aug 12, 2020 at 5:40 PM Skjellum, Anthony <Tony-Skjellum@utc.edu> 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 typesposed (e.g., Alltoallv/w)... It will not be knowable to the application if the vectors are in use by MPI still afterthe 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@utc.edu [or skjellum@gmail.com]
cell: 205-807-4968
From: mpi-forum <mpi-forum-bounces@lists.mpi-forum.org> on behalf of Jeff Hammond via mpi-forum <mpi-forum@lists.mpi-forum.org>
Sent: Saturday, August 8, 2020 12:07 PM
To: Main MPI Forum mailing list <mpi-forum@lists.mpi-forum.org>
Cc: Jeff Hammond <jeff.science@gmail.com>
Subject: Re: [Mpi-forum] MPI_Request_free restrictionsWe 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@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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