Don't forget matching. The model depends on a relation between send and receive. This is the fundamental reason for potential difference in overlap. If you talk implementation, which we technically shouldn't for this argument, eventually the fact that a matching receiver is required for a send does impact over multiple sends.  

the one-sided model

a->b 

is independent of b. 

the two sided model

a<->b

because of its dependance implies validity -- you can hide the cost of validity but can't eliminate it.  

Vinod.

> From: keith.d.underwood@intel.com
> To: mpi-forum@lists.mpi-forum.org
> Date: Mon, 27 Apr 2009 06:48:57 -0600
> Subject: Re: [Mpi-forum] MPI One-Sided Communication
>
>
>>On the Earth Simulator, there are/were several application codes which are
>>using one-sided communication (instead of 2-sided). They used one-sided
>>communication especially to overlap communication and computation.
>>When I remember correctly, at least one of this applications won a Gordon
>>Bell Award of SC.
>
> The ambiguity of the progress rule notwithstanding, there is no particular reason that one-sided should give you better overlap than two-sided. If this is the reason that people use one-sided, maybe we should revisit the progress rule ;-)
>
> Keith
>
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