You are right that I am assuming that if an NBC completes at one rank, I can reason about the state of that operation at another. I can certainly come up with scenarios where that is dangerous. Jeff On Sat, Aug 15, 2020 at 9:57 AM Richard Graham <[email protected]> wrote:

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

Sent via the Samsung Galaxy S9, an AT&T 5G Evolution capable smartphone

-------- Original message -------- From: Jeff Hammond via mpi-forum <[email protected]> Date: 8/15/20 12:51 (GMT-05:00) To: "Skjellum, Anthony" <[email protected]> Cc: Jeff Hammond <[email protected]>, Main MPI Forum mailing list < [email protected]> Subject: Re: [Mpi-forum] MPI_Request_free restrictions

*External email: Use caution opening links or attachments* 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 <[email protected]> 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)

[email protected] [or [email protected]]

cell: 205-807-4968

------------------------------ *From:* mpi-forum <[email protected]> on behalf of Jeff Hammond via mpi-forum <[email protected]> *Sent:* Saturday, August 8, 2020 12:07 PM *To:* Main MPI Forum mailing list <[email protected]> *Cc:* Jeff Hammond <[email protected]> *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 < [email protected]> 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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