[mpich-discuss] MPI_Rsend advantage

[Jeff Hammond]

I skimmed Ch3 and there is no difference for rendezvous.  For eager, the
Rsend doesn't bother with the unnecessary MPI_THREAD_MULTIPLE deadlock
check and the short eager optimization (payload in header).  Thus, I would
expect no benefit due to Rsend and potentially a performance loss for small
messages, if the short eager path is worthwhile.

In Ch4, the code paths for Send and Rsend are identical in
src/mpid/ch4/src/ch4_send.h.

Because most applications and benchmarks use the normal Send-Recv
(including nonblocking) variants, this is where the optimization work
happens.  Unless you use an implementation that documents an optimized
implementation of Rsend, I would not bother using it.

Jeff

On Wed, Sep 6, 2017 at 12:58 PM, Dorier, Matthieu <mdorier at anl.gov> wrote:
>
> Let's say process A MPI_Sends a message to process B, as a result process
B posts an MPI_Irecv then MPI_Sends a message to A to notify it that the
receive was posted, following which A calls MPI_Rsend.
>
>
> It's a dumb example, but just to say that in some scenarios it's possible
to know in advance that an MPI_Recv or an MPI_Irecv was posted.
>
>
> No my question is, what does Mpich optimizes within MPI_Rsend; what is
done differently from MPI_Send?
>
>
> Thanks,
>
>
> Matthieu
>
> ________________________________
> From: Atchley, Scott <atchleyes at ornl.gov>
> Sent: Wednesday, September 6, 2017 7:41:54 PM
> To: discuss at mpich.org
> Subject: Re: [mpich-discuss] MPI_Rsend advantage
>
> If you can guarantee that it is posted, then you can guarantee that it
will match. If you cannot guarantee that it is posted and will match, then
it may be placed in the unexpected buffer and will need to be copied into
the final location once the receive is posted.
>
> My question is, “How do you know it has been posted?”
>
>
> > On Sep 6, 2017, at 6:21 AM, Dorier, Matthieu <mdorier at anl.gov> wrote:
> >
> > Hi,
> >
> > Out of curiosity, what gain is there to using MPI_Rsend instead of
MPI_Send when I know that the receiver has posted a receive? Do I avoid a
rendez-vous protocol? Do I avoid a buffer copy somewhere?
> >
> > Thanks,
> >
> > Matthieu
> > _______________________________________________
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--
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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