[Beowulf] Optimal BIOS settings for Tyan K8SRE

[Mark Hahn]

> What chipkill buys you is the reasonable assurance that if you
> have a long uptime and you get soft memory errors something will
> look at and correct the data before multiple errors have time to
> accumulate.

true, but I think you mean scrubbing, not chipkill.  from AMD's 
bios-writers doc, chipkill actually just changes the ECC to 
incorporate all 128b; the 'chip' part comes from the fact that 
with 4x dram chips, the syndrome maps to a particular chip
(and all 4 bits from that chip can be corrected.)

this has been a useful discussion, because I hadn't previously
realized just what was the benefit (prevent accumulation of 
multi-bit errors, proactively fix some singles) and cost
(a tiny amount of bandwidth, O(100KB/s).

> For performance I suspect you will see much more variation depending
> on the speed of memory in your system.

the max scrub rate (64B in 40ns) is 1.6 GB/s.  that's a lot, but is 
still lower than the bandwidth from even a single-dimm config.
(NOT to imply it's a sensible setting in that case!)

for my 8GB machines, I suspect a daily scrub (50KB/s per node)
will have undetectable overhead.