[Beowulf] DC Power Dist. Yields 20%

Geoff Jacobs gdjacobs at gmail.com
Fri Aug 11 08:41:43 PDT 2006

Mark Hahn wrote:
>> http://www.eweek.com/article2/0,1759,2000867,00.asp
>> 20% is a lot, both in terms of consumption and cooling capacity. I'm
> the article says "up to 15%" at the facility level - did you get 20%
> by figuring a savings at the rack level, too?
I got 20% by misquoting, sorry. Still, 15% is nothing to sneeze at.

> afaikt, the article is based on an assumption that everything is powered
> by an online UPS, and probably that node PSU's are low-performance (say,
> 65%).  sometimes studies like this ignore fact that incoming power is
> inherently AC (that is, only start looking at efficiency given DC supply.)
> I'd love to see DC gain more traction - PSU's are certainly one of the
> flakier components in our systems, though per node (HP DL145G2), they
> only contain 2 of 14 fans (or of 18 moving parts).  I don't know whether
> there's a reason to think many small AC-DC PSU's would be less efficient
> than a couple really big ones (factoring in the cost and inefficiency of
> DC power distribution).
Do your PDUs receive 220 VAC or ~400 VAC?

They're talking about a 380 VDC distribution grid. On it's face, this
infrastructure would be at least as efficient as a supply of 270 VAC.
Unfortunately, the article doesn't lean too heavily on hard facts wrt
their claims.

I really just have a physicist's understanding of the forces at work
here. At this point, perhaps we should get Jim Lux to educate us?

> I'd certainly be interested in a distribution system (whether AC or DC)
> that avoided so damn many plugs and sockets and breakers and PDUs.
> I guess I'm more enthused about servers becoming lower-powered, and also
> quite interested in better ways to dissipate the heat than raised floors
> and traditional chillers...
Heh heh. Water cooled racks?

>> curious how long it would take before DC supplied racks become cost
>> effective.
> well, there's already a standard DC supply - to the motherboard.
> one impediment might be that it's got +12, -12, 5, 3.3 and probably
> a couple others.  if it were just a matter of providing lightly
> regulated 12V, life would probably be a lot simpler.  and I'm not sure
> MB's would be much more complicated, either, since the current
> main consumer, the CPU, already has a fairly flexible and high-power
> onboard dc-dc converter.  (I wonder how efficient it is, typically...)

Well, a DC-DC power converter is pretty straight forward. I expect part
of the trouble with switching power supplies is those high frequency
mosfets, which you wouldn't have (I expect) with a DC-DC converter.

Geoffrey D. Jacobs

Go to the Chinese Restaurant,
Order the Special

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