[Beowulf] Opteron memory rank limits with DDR-400
josip at lanl.gov
Wed Jul 27 12:26:26 EDT 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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