[Beowulf] Advice on 4 CPU node configurations

[Craig Tierney]

On 2/10/11 11:52 AM, Stuart Barkley wrote:
> On Wed, 9 Feb 2011 at 09:38 -0000, Prentice Bisbal wrote:
>
>> About 6 months ago, I purchased a couple of Dell PowerEdge R815s
>> with 128 GB of RAM and 32 cores. We looked at similar RAM
>> configurations a couple of years ago. and the cost premium for that
>> much RAM was prohibitive (the price for additional RAM seemed to go
>> up exponentially with the amount of RAM) so we stayed at 32 GB.
>> This time around, the premium for the additional RAM seemed
>> marginal. Not sure how large you can go and keep that "marginal"
>> relationship, but it's much larger than it was a couple of years
>> ago.
>
> How much does the RAM speed matter to people?
>
> We have a lot of 2 CPU (8 core) systems and have been considering
> memory upgrades (or just purchasing new systems with more memory since
> we are still growing).
>
> Currently we are running 1 4GB DIMM per channel at 1333 MHz.  If we
> add a second 4GB DIMM per channel then the memory will run at 1066 MHz
> (and to 800 MHz with 3 DIMMs per channel).  Alternatively, we could
> replace the 4GB modules with 8GB ones (keeping the old ones for spares
> or to side-grade some other machines to 48GB/1066 MHz).
>
> Have people been worrying about the lower memory bus rate with
> multiple DIMMs on each channel?  Do "ordinary" applications notice the
> difference?
>

Our weather/hurricane codes are memory bound, so yes we worry about
it.  We aren't installing so much memory that we are paying a price
premium to ensure only one channel is filled, so it isn't a big deal.

Have we tested it?  No.  But we haven't had to.

Craig



> I know it depends upon the application and I have no idea if this will
> matter to our applications.  Mostly, I just happy things are running
> at all rather than fighting every ounce of performance.
>
> Stuart Barkley