Newsgroups: comp.parallel.mpi
From: A Gordon Smith <smith@epcc.ed.ac.uk>
Subject: Re: MPI-2 onesided and handshaking?
Organization: Edinburgh Parallel Computing Centre
Date: Tue, 11 Jun 1996 09:36:53 GMT
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Message-ID: <31BD3E35.41C67EA6@epcc.ed.ac.uk>

Axel Thimm wrote:
> 
> Hello,
> the EPCC/CRI implementation of MPI already has some draft (?)
> implemetations of one-sided communication to appear in MPI-2.


I should clarify that the CRI/EPCC implementation is a subset
of the MPI 2 One-sided Communications proposal presented at SC'95.
There are certainly significant differences between this and the
current proposal.


> Using onesided communication, the receiver cannot be sure the data has
> already arrived, unless
> 1) he/she checks the data (if the data itself contains a counter) or
> 2) uses request-semaphores with thresholds.

> But the MPI-2 specs for the T3D warn, that if using theese semaphores
> latency might increase too much.

Our user document warns that there is an overhead relative to the
cost of the transfer alone. The request counter increment adds
around about 5us to the 1-2us latency of a Put or Get on T3D, but
this is still considerably less than the cost of a Send-Recv pair.

> Is there another way of handshaking?

MPI_Barrier synchronises, and waits for all remote accesses to
complete, across a process group. MPI_Fence waits for all remote
access made by the caller to complete.

> Using twosided communication after the put operation has completed,
> could overtake the put transfer (the put might be buffered) and the
> handshake would be wrong.
> 
> Ragards,
>         Axel Thimm.
> --
> Axel Thimm <thimm@dirac.physik.fu-berlin.de>
> Fachbereich Physik, Freie Universitaet Berlin

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