[Beowulf] Shared memory
Mark Westwood
mark.westwood at ohmsurveys.com
Wed Jun 22 00:46:24 PDT 2005
Jeff
We too have a Beowulf cluster with nodes having 2 CPUs. The memory in
each node is shared between the 2 CPUs. We use Grid Engine to place
processes onto processors at run time. Some of our codes require a lot
of memory for each process, and will be placed one process on a node,
some of our codes require less memory and will be placed two processes
to a node. There are different queues defined for the two types of job.
Our MPI codes are 'processor / node blind' in the sense that there are
no special features of the code arising directly from the possibility
that, at run time, two processes executing the program might be sharing
a node. Users determine, prior to a job's execution, how much memory
each process in a parallel computation will require, and submit the job
to the right queue.
My thinking is that mixed-mode programming, in which a code uses MPI for
inter-node communication and shared-memory programming (eg OpenMP) for
intra-node communication, is not worth the effort when you only have 2
CPUs in each node. In fact, my thinking is that it's not even worth the
time experimenting to gather evidence. I guess that makes me prejudiced
against mixed-mode programming on a typical COTS cluster. Now, if you
were to offer me, say, 8 or 16 CPUs per node, I might think again. Or
indeed if I am shot down in flames by contributors to this list who have
done the experiments ...
One reference which might be of interest:
http://www.epcc.ed.ac.uk/DIRECT/newsletter9/mixed.html
Regards
Mark Westwood
Jeff zhang wrote:
> Each node of my Beowulf cluster has two CPUs. The memory is shared
> between the two CPUs. How is MPI handling the memory in this
> situation? What is the most efficient way to program under this
> situation?
> Thanks.
>
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--
Mark Westwood
Parallel Programmer
OHM Ltd
The Technology Centre
Offshore Technology Park
Claymore Drive
Aberdeen
AB23 8GD
United Kingdom
+44 (0)870 429 6586
www.ohmsurveys.com
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