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Subject: Re: [OMPI users] Shared Memory - Eager VS Rendezvous
From: Simone Pellegrini (spellegrini_at_[hidden])
Date: 2012-05-23 08:05:28


I think I found the answer to my question on Jeff Squyres blog:
http://blogs.cisco.com/performance/shared-memory-as-an-mpi-transport-part-2/

However now I have a new question, how do I know if my machine uses the
copyin/copyout mechanism or the direct mapping?

Assuming that I am running on OpenMPI 1.5.x installed on top of a linux
Kernel 2.6.32?

cheers, Simone

On 05/22/2012 05:29 PM, Simone Pellegrini wrote:
> Dear all,
> I would like to have a confirmation on the assumptions I have on how
> OpenMPI implements the rendezvous protocol for shared memory.
>
> If process A sends a message to process B and the eager protocol is
> used then I assume that the message is written into a shared memory
> area and picked up by the receiver when the receive operation is posted.
>
> When the rendezvous is utilized however the message still need to end
> up in the shared memory area somehow. I don't think any RDMA-like
> transfer exists for shared memory communications. Therefore you need
> to buffer this message somehow, however I assume that you don't buffer
> the whole thing but use some type of pipelined protocol so that you
> reduce the size of the buffer you need to keep in the shared memory.
>
> Is it completely wrong? It would be nice if someone could point me
> somewhere I can find more details about this. In the OpenMPI tuning
> page there are several details regarding the protocol utilized for IB
> but very little for SM.
>
> thanks in advance,
> Simone P.
>
>