[Beowulf] Opteron memory rank limits with DDR-400

Josip Loncaric josip at lanl.gov
Wed Jul 27 09:26:26 PDT 2005


Can anyone confirm that Opteron processors Rev. E and later can operate 
four dual-rank 2GB memory modules (8 ranks total) at full DDR-400 speed?

AMD used to recommend no more than 4 ranks of DDR-400 memory.  See 
http://forums.amd.com/lofiversion/index.php/t39745.html where the 
relevant quote from AMD technical service reads:

"AMD does recommend to downclock the memory of the system to 333MHz,
if more than 4 ranks is used in the DIMM slots. What this means is
that only 2 sticks of 2 rank memory is recommended to run at the full
400MHz or 4 sticks of 1 rank memory. There is a memory timing issue
with more than 4 ranks of memory, which is a limitation of the memory
controller on the Opteron chips."

In the past, this downclocking was automatically enforced by some 
BIOSes, but supposedly there is no need to do so with currently shipping 
Opteron Rev. E and later, provided that the motherboard also allows full 
8 ranks at DDR-400.

I'd just like to be sure...  Also, has anyone observed increased memory 
latency with dual-rank modules?


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