[Beowulf] Opteron downrates DDR400 to DDR333 performance?

Mark Hahn hahn at physics.mcmaster.ca
Sat Feb 4 09:51:46 PST 2006

>> So you could get 4GB of RAM on a processor if you use
>> single ranked DIMMs.

perhaps this is TMI, but the answer depends on which cpu rev 
you're using (and whether the bios optimizes the memory controller.)
the AMD bios-writers doc cited earlier shows that 4 dimms (2xdouble,
2xsingle) can still be used at ddr400 on rev e.  no config uses >6 
CS's at DDR400, and some drop to DDR333 at <=5 CS's.

> Another generation of cpu/memory controllers will take us to FBdimms which 
> will for the most part solve the issues here...

well, FBdimms maintain a high clock, but stretch latency quite a bit as 
you add devices.  and that's some distance off, I think - rumor has it 
that AMD is close to shipping DDR2 support.  it'll be intresting to see
what kinds of latency/bandwidth/loading they achieve there.  my understanding
is that one of the main appeals of DDR2 is use of lower voltage - presumably
for lower dissipation by the memory controller, but also possibly for easier
drive of a fully-configured system at full speed.  10.7 GBps should be a 
nice bump up from 6.4 (peak theoretical never to be achieved in reality ;)

regards, mark hahn.

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