[Beowulf] network transfer issue to disk, old versus new hardware
hahn at mcmaster.ca
Tue Jun 5 12:40:22 EDT 2007
>> Athcool does cut the idle temperatures of the nodes considerably, but
>> apparently also prevents them from performing this sort of transfer at full
>> speed, whether or not buffer is used.
> Well, near the top of the athcool website there is a warning and one the
> listed items is 'a slowdown in harddisk performance' - so nothing new here
athcool works by putting the cpu-northbridge interface into a low-power mode.
the difficulties people had with it was that this sort of down-clocking
was new at the time, and not well-handled by all chips, probably on
both the chipset and cpu sides. erata centered on how long it took to
stabilize the PLL's involved.
things are quite different nowadays - AMD put the northbridge entirely
on-cpu, so it has fully control, and can modulate clocks extensively
and differentially. I don't know how common (or effective) it is to modulate
HT power, but such features show up prominently in recent HT revs. it's
interesting to speculate about Intel - mostly it solved this by dominating
the chipset market for its own CPUs. I'm guessing Intel will fall somewhat
behind AMD in system-wide power savings, at least until CSI. even then,
I'm a little unclear how good Intel's initial implementation will be -
the fact that they've chosen to not simply adopt HT indicates to me that
Intel will be re-learning AMD's lessons.
>> Which is interesting because it didn't have any measurable effect on CPU
>> bound processes. I had thought it would shut itself off and get out of the
I'd expect athcool to not affect a cache-friendly cpu-bound process,
but to hurt pretty badly if you have cache misses. networking (using the
normal network stack) count as memory-bound, I think, rather than kinds
of IO which might be more DMA-intensive. that is, if a disk is streaming
many MB into memory, the CPU's northbridge interface should be able to
go low-power (though most disk transfers are only in the 64K range...)
regards, mark hahn.
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