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Subject: Re: [OMPI users] nonblock alternative to MPI_Win_complete
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2011-02-24 08:36:58


I personally find the entire MPI one-sided chapter to be incredibly confusing and subject to arbitrary interpretation. I have consistently advised people to not use it since the late '90s.

That being said, the MPI one-sided chapter is being overhauled in the MPI-3 forum; the standardization process for that chapter is getting pretty close to consensus. The new chapter is promised to be much better.

My $0.02 is that you might be better severed staying away from the MPI-2 one-sided stuff because of exactly the surprises and limitations that you've run in to, and wait for MPI-3 implementations for real one-sided support.

On Feb 24, 2011, at 8:21 AM, Toon Knapen wrote:

> But that is what surprises me. Indeed the scenario I described can be implemented using two-sided communication, but it seems not to be possible when using one sided communication.
>
> Additionally the MPI 2.2. standard describes on page 356 the matching rules for post and start, complete and wait and there it says : "MPI_WIN_COMPLETE(win) initiate a nonblocking send with tag tag1 to each process in the group of the preceding start call. No need to wait for the completion of these sends."
> The wording 'nonblocking send' startles me somehow !?
>
> toon
>
>
> On Thu, Feb 24, 2011 at 2:05 PM, James Dinan <dinan_at_[hidden]> wrote:
> Hi Toon,
>
> Can you use non-blocking send/recv? It sounds like this will give you the completion semantics you want.
>
> Best,
> ~Jim.
>
>
> On 2/24/11 6:07 AM, Toon Knapen wrote:
> In that case, I have a small question concerning design:
> Suppose task-based parallellism where one node (master) distributes
> work/tasks to 2 other nodes (slaves) by means of an MPI_Put. The master
> allocates 2 buffers locally in which it will store all necessary data
> that is needed by the slave to perform the task. So I do an MPI_Put on
> each of my 2 buffers to send each buffer to a specific slave. Now I need
> to know when I can reuse one of my buffers to already store the next
> task (that I will MPI_Put later on). The only way to know this is call
> MPI_Complete. But since this is blocking and if this buffer is not ready
> to be reused yet, I can neither verify if the other buffer is already
> available to me again (in the same thread).
> I would very much appreciate input on how to solve such issue !
> thanks in advance,
> toon
> On Tue, Feb 22, 2011 at 7:21 PM, Barrett, Brian W <bwbarre_at_[hidden]
> <mailto:bwbarre_at_[hidden]>> wrote:
>
> On Feb 18, 2011, at 8:59 AM, Toon Knapen wrote:
>
> > (Probably this issue has been discussed at length before but
> unfortunately I did not find any threads (on this site or anywhere
> else) on this topic, if you are able to provide me with links to
> earlier discussions on this topic, please do not hesitate)
> >
> > Is there an alternative to MPI_Win_complete that does not
> 'enforce completion of preceding RMS calls at the origin' (as said
> on pag 353 of the mpi-2.2 standard) ?
> >
> > I would like to know if I can reuse the buffer I gave to MPI_Put
> but without blocking on it, if the MPI lib is still using it, I want
> to be able to continue (and use another buffer).
>
>
> There is not. MPI_Win_complete is the only way to finish a
> MPI_Win_start epoch, and is always blocking until local completion
> of all messages started during the epoch.
>
> Brian
>
> --
> Brian W. Barrett
> Dept. 1423: Scalable System Software
> Sandia National Laboratories
>
>
>
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-- 
Jeff Squyres
jsquyres_at_[hidden]
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