[Beowulf] Woodcrest Memory bandwidth

Mark Hahn hahn at physics.mcmaster.ca
Tue Aug 15 10:55:14 EDT 2006


>> Good point which makes perfect sense to me.
>> Given that the theoretical maximum is actually 21.3 GB/s
>> the real maximum Triad number must be 21.3/3 = 7.1 GB/s.

I don't get this - triad does two reads and one write.
if you don't use store-through ('nt' versions of mov),
then the write also implies a read for write-allocate (filling 
the cache line).

without store-through, the peak theoretical number reported by 
stream should be 3*peak/4.  the 4 is because there are 3r+1w,
and the 3 because stream doesn't give credit for write-allocate.

> Then how do you explain a dual opteron with two 6.4GB/sec (peak)
> memory system, 12.8GB/sec total per node managing 9-10GB/sec?
>
> 12.8/3=4.26GB/sec.  People are seeing well over twice that.

since pathscale does write-through, the peak really should be 12.8,
so achieving 9-10 is decent but not paradoxical.  (the peak would 
correspond to 1.07 Gflops, significantly below the peak theoretical
pipeline rate of 2*clock flops...)
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