[Beowulf] Shared memory

Reuti reuti at staff.uni-marburg.de
Thu Jun 23 02:25:00 PDT 2005

John Hearns wrote:
> On Wed, 2005-06-22 at 08:46 +0100, Mark Westwood wrote:
>>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 ...
> Mark, that is very well put.
> May I add that 8 or 16 CPUs per node has become a realistic possibility?
> Four and eight way(*) Opteron machines are available, up to 128Gbytes
> RAM.
> Expect to see more of these machines out there as engineering
> workstations or for visualisation and animation/rendering.

It depends on the amount money you want to spend (or have at all). The 
Opterons 8xx are still a "little" bit more expensive than the 2xx version.

But back to the original question: instead of using OpenMP and MPI at 
once (which I see in the same way as Mark), I'd suggest to compile MPICH 
with shared memory support and setup the machinefile accordingly. Then 
you can easy look for any speed improvement using shared memory.

CU - Reuti

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