[mpich-discuss] MPICH v1.2.7p1 and SMP clusters

[Gustavo Miranda Teixeira]

Hello Marcus,

I can't see what your point is. Do you want to know in which core is the
processes allocated? Like if it's on the same processor or different ones?


On Tue, Jan 13, 2009 at 3:06 PM, Marcus Vinicius Brandão Soares <
mvbsoares at gmail.com> wrote:

> Hello Gustavo and all,
>
> You described that you are using two machines with a dual processor in each
> one. If I can model it in a simple graph, we have two vertices and two
> unidirectional edges.
>
> Each machine has a dual processor, each one with dual core, so there are 8
> processor. But lets think again in the graph model: now we have two
> vertices, each one with two more vertices; these last two vertices have two
> more vertices too, and so this is the end.
>
> Do you know the structure of the communication lines of the core processors
> ?
>
> 2009/1/13 Gustavo Miranda Teixeira <magusbr at gmail.com>
>
> Hello everyone!
>>
>> I've been experiencing some issues when using MPICH v1.2.7p1 and a SMP
>> cluster and thought maybe some one can help me here.
>>
>> I have a small cluster with two dual processor machines with gigabit
>> ethernet communication. Each processor is a dual core which sums up to 8
>> cores of processors. When I run an application spreading 4 processes in both
>> the machines (like distributing 2 processes in one machine and 2 processes
>> in another) I get a significantly better performance than when I run the
>> same application using 4 processes in only one machine. Isn`t it a bit
>> curious? I know some people who also noticed that, but no one can explain me
>> why this happens. Googling it didn't helped either. I originally thought it
>> was a problem from my kind of application (a heart simulator which using
>> PETSc to solve some differential equations) but some simple experimentations
>> showed a simple MPI_Send inside a huge loop causes the same issue. Measuring
>> cache hits and misses showed it`s not a memory contention problem. I also
>> know that a in-node communication in MPICH uses the loopback interface, but
>> as far as I know a message that uses loopback interface simply takes a
>> shortcut to the input queue instead of being sent to the device, so there is
>> no reason for the message to take longer to get to the other processes. So,
>> I have no idea why it`s taking longer to use MPICH in the same machine. Does
>> anyone else have noticed that too? Is there some logical explanation for
>> this to happen?
>>
>> Thanks,
>> Gustavo Miranda Teixeira
>>
>
>
>
> --
> Marcus Vinicius
> --
> "Havendo suficientes colaboradores,
> Qualquer problema é passível de solução"
> Eric S. Raymond
> A Catedral e o Bazar
>
> "O passado é apenas um recurso para o presente"
> Clave de Clau
>
> "Ninguém é tão pobre que não possa dar um abraço; e
> Ninguém é tão rico que não necessite de um abraço.
> Anônimo
>
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