[Beowulf] network transfer issue to disk, old versus new hardware

[Mark Hahn]

>> 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.