[Beowulf] Opteron downrates DDR400 to DDR333 performance?

[Nate Faerber]

On Thu, 2006-02-02 at 14:05 -0600, NazarioJose at JohnDeere.com wrote:
> Hello, computer gurus -
> do any of you know if the AMD Opteron downrates the PC3200 400 MHz DIMMS 
> (DDR400) to 333 MHz (DDR333) performance  when the system has more that 2 
> Gbytes of memory installed?
> 
> i was told that when an opteron socke has more that 2GBytes of memory, then 
> the cpu will automatically downrate the memory rate to an equivalence of 
> DDR333 memory.

It doesn't necessarily depend on the amount of memory installed.  It
depends on the total number of ranks in your memory DIMMs which is
likely determined by the number of DIMMs installed.  You get downclocked
to PC2700 once you hit 8 ranks total on a memory controller (one per
Opteron CPU.)  So you could get 4GB of RAM on a processor if you use
single ranked DIMMs.

See section 4.1.3.4 here:
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/26094.PDF

Find previous coverage of this subject here:
http://beowulf.org/archive/2005-July/013377.html

-- 
Nate Faerber, Senior Engineer - Clusters
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