[mpich-discuss] Semantics of MPI_Send and MPI_Isend

[Halim Amer]

I think understanding section 3.7.4 in MPI 3.1 is required as well. The 
non-overtaking requirement of pt2pt communication is extended to 
nonblocking communication with the notion of order. Basically, in your 
code the Isend operation will not be matched if the previous Send 
operation is still pending (if same target, same tag, and same 
communicator).

So yes, you will lose the benefit of asynchronous communication if you 
introduce synchronous communication in the middle. Although a system 
might buffer standard sends, it is implementation dependent and the user 
should not rely on it.

--Halim
www.mcs.anl.gov/~aamer

On 10/20/15 10:17 AM, Jeff Hammond wrote:
> I recommend that you read "Semantics of Point-to-Point Communication" in
> the MPI standard (section 3.5 in MPI 3.1).  It has all of the
> information that you seek.
>
> Best,
>
> Jeff
>
>
> On Tue, Oct 20, 2015 at 7:58 AM, Dorier, Matthieu <mdorier at anl.gov
> <mailto:mdorier at anl.gov>> wrote:
>  >
>  > Hi,
>  >
>  > In my program I have a loop of this form:
>  >
>  > loop {
>  >   MPI_Send(small message)
>  >   MPI_Isend(big message)
>  > }
>  > do_some_computation()
>  > MPI_Waitall
>  >
>  > The sends and isends all have the same target rank.
>  >
>  > Is there a risk that an MPI_Send forces a previously posted MPI_Isend
> to complete before it can complete itself (in which case I would loose
> the benefit of using non-blocking sends)?
>  >
>  > Thanks,
>  >
>  > Matthieu
>  >
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> --
> Jeff Hammond
> jeff.science at gmail.com <mailto:jeff.science at gmail.com>
> http://jeffhammond.github.io/
>
>
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