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Subject: Re: [OMPI users] RE : MPI hangs on multiple nodes
From: Gus Correa (gus_at_[hidden])
Date: 2011-09-19 16:19:40

Hi Ole, Eugene

For what it is worth, I tried Ole's program here,
as Devendra Rai had done before.
I ran it across two nodes, with a total of 16 processes.
I tried mca parameters for openib Infiniband,
then for tcp on Gigabit Ethernet.
Both work.
I am using OpenMPI 1.4.3 compiled with GCC 4.1.2 on CentOS 5.2.

Gus Correa

Gus Correa wrote:
> Hi Eugene
> You're right, it is blocking send, buffers can be reused after MPI_Send
> returns.
> My bad, I only read your answer to Sebastien and Ole
> after I posted mine.
> Could MPI run out of [internal] buffers to hold the messages, perhaps?
> The messages aren't that big anyway [5000 doubles].
> Could MPI behave differently regarding internal
> buffering when communication is intra-node vs. across the network?
> [It works intra-node, according to Ole's posting.]
> I suppose Ole rebuilt OpenMPI on his newly installed Ubuntu.
> Gus Correa
> Eugene Loh wrote:
>> I'm missing the point on the buffer re-use. It seems to me that the
>> sample program passes some buffer around in a ring. Each process
>> receives the buffer with a blocking receive and then forwards it with
>> a blocking send. The blocking send does not return until the send
>> buffer is safe to reuse.
>> On 9/19/2011 7:37 AM, Gus Correa wrote:
>>> You could try the examples/connectivity.c program in the
>>> OpenMPI source tree, to test if everything is alright.
>>> It also hints how to solve the buffer re-use issue
>>> that Sebastien [rightfully] pointed out [i.e., declare separate
>>> buffers for MPI_Send and MPI_Recv].
>>> Sébastien Boisvert wrote:
>>>> Is it safe to re-use the same buffer (variable A) for MPI_Send and
>>>> MPI_Recv given that MPI_Send may be eager depending on
>>>> the MCA parameters ?
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