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Subject: Re: [OMPI users] [EXTERNAL] Re: Question regarding osu-benchamarks 3.1.1
From: Venkateswara Rao Dokku (dvrao.584_at_[hidden])
Date: 2012-03-01 00:13:28


Hi,
      I tried executing those tests with the other devices like tcp instead
of ib with the same open-mpi 1.4.3.. It went fine but it took time to
execute, when i tried to execute the same test on the customized OFED
,tests are hanging at the same message size..

Can u please tel me, what could me the possible issue over there, so that
you can narrow down the issue..
i.e.. Do i have to move to open-mpi 1.5 tree or there is a issue with the
customized OFED ( in RDMA scenario's or anything (if u can specify)).

On Thu, Mar 1, 2012 at 1:45 AM, Jeffrey Squyres <jsquyres_at_[hidden]> wrote:

> On Feb 29, 2012, at 2:57 PM, Jingcha Joba wrote:
>
> > So if I understand correctly, if a message size is smaller than it will
> use the MPI way (non-RDMA, 2 way communication), if its larger, then it
> would use the Open Fabrics, by using the ibverbs (and ofed stack) instead
> of using the MPI's stack?
>
> Er... no.
>
> So let's talk MPI-over-OpenFabrics-verbs specifically.
>
> All MPI communication calls will use verbs under the covers. They may use
> verbs send/receive semantics in some cases, and RDMA semantics in other
> cases. "It depends" -- on a lot of things, actually. It's hard to come up
> with a good rule of thumb for when it uses one or the other; this is one of
> the reasons that the openib BTL code is so complex. :-)
>
> The main points here are:
>
> 1. you can trust the openib BTL to do the Best thing possible to get the
> message to the other side. Regardless of whether that message is an
> MPI_SEND or an MPI_PUT (for example).
>
> 2. MPI_PUT does not necessarily == verbs RDMA write (and likewise, MPI_GET
> does not necessarily == verbs RDMA read).
>
> > If so, could that be the reason why the MPI_Put "hangs" when sending a
> message more than 512KB (or may be 1MB)?
>
> No. I'm guessing that there's some kind of bug in the MPI_PUT
> implementation.
>
> > Also is there a way to know if for a particular MPI call, OF uses
> send/recv or RDMA exchange?
>
> Not really.
>
> More specifically: all things being equal, you don't care which is used.
> You just want your message to get to the receiver/target as fast as
> possible. One of the main ideas of MPI is to hide those kinds of details
> from the user. I.e., you call MPI_SEND. A miracle occurs. The message is
> received on the other side.
>
> :-)
>
> --
> Jeff Squyres
> jsquyres_at_[hidden]
> For corporate legal information go to:
> http://www.cisco.com/web/about/doing_business/legal/cri/
>
>
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>

-- 
Thanks & Regards,
D.Venkateswara Rao,
Software Engineer,One Convergence Devices Pvt Ltd.,
Jubille Hills,Hyderabad.