[Beowulf] Shared memory

[Vincent Diepeveen]

At 08:44 AM 6/23/2005 +0100, 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.

the 4 way quadboards are decently priced and all you need is 16 dimms (of
course you want maximum speed out of the quad and fill it entirely up). If
i remember well prices are far less than 2000 euro for such a quad board. 

the 8 way according to rumours costs somewhere around 80000 euro.

>Expect to see more of these machines out there as engineering
>workstations or for visualisation and animation/rendering.
>
>Add dual-core CPUs to the mix and you get a very attractive platform.

A big problem is that it's interesting to run at a quad, but please find me
a cluster that consists out of quad dual cores.

Usually clusters are slow intel dual xeons or something ugly bad from the
past. Most organisations need also like 1 year between ordering and
delivery of a cluster, by then such a processor is just so so so outdated
that it's factor 2 times slower or so than the latest cpu on the market. 

A small cluster consisting of those slow nodes just can't compete with a
quad for algorithms that profit a lot from fast communication speeds that
within a single mainboard happens.

Example now is the many myri clusters there are at uni's of like 3Ghz P4
single cpu @ 64 nodes.

I could get such clusters for world champs 2005 which happen from 13-20
august 2005.

A single quad opteron dual core just outpowers such clusters *dramatically*
for my chess software.

This apart from the fact that you can easily test your software at such a
quad without problems and without other users bugging you.

The difference is too huge in processing power.



>
>(*) two quad motherboards, so I guess realistically and 8-way OpenMP is
>the limit



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