[Beowulf] power usage, Intel 5160 vs. AMD 2216

[Robert G. Brown]

On Thu, 12 Jul 2007, Joe Landman wrote:

> Mike Davis wrote:
>> That's excellent information. I was surpised by how close to the documented 
>> power use my number were. Knowing that even high disk use adds little power 
>> usage is important to though of us fighting the power/cooling wars.
>
> Memory dimms are a major consumer.  If you have the choice to make between 
> smaller cheaper DIMMs using more slots, versus fewer larger (and more costly) 
> DIMMs using fewer slots, the latter will usually consume less power.  8 GB 
> can be 8x1GB dimms, 4x2GB dimms (or 2x 4GB dimms, though this is still not 
> cost competitive even factoring in the power).
>
> The 2GB dimms emit the same heat as the 1 GB dimms.  So if you have a 1000 
> node cluster, and you use the larger (slightly more expensive) 2GB dimms vs 
> the 1GB dimms, you will emit somewhat less heat.  I haven't done the 
> analysis, but I bet it would be close to a good tradeoff for TCO.

The analysis is easy using Unka Rob's Foolproof Power Cost Estimate
Rate: $1/watt/year.  This is just a ballpark number, deliberately
slightly highballed -- the actual number is probably between $0.60 and
$0.80 cents -- but it depends on the cost per KW, taxes, efficiency of
your AC and temperature of your server room, and so on.  It's been a
while since I computed it but IIRC it is based on $0.08/KW-hour and and
AC CoP between 2 and 3.  Then lessee 24*365*0.08 = $700/KW/year, or
$0.70/W/year, to which I'm arbitrarily adding $0.30 for AC costs even
though it is more likely ~$0.20 -- the extra dime is that slop.  If your
power costs only $0.06/KW-hour and your AC is super efficient and the
outdoor temperature is on average cold and has a CoP of 5, well, adjust
accordingly.

Anyway, if a DIMM draws and average power of (say) 10W and is expected
to be on for 3 years, that means it costs roughly $30 over its lifetime.
so if the marginal cost of a 2 GB vs 2 1 GB DIMMs is $30 or less, it is
break even to a win to buy 2 GB DIMMs from a TCO point of view.  Similar
considerations hold for high vs low power CPUs, LCD vs CRT monitors, and
so on.  Just assume $1/W/year x (expected number of years of service),
with a bit of kentucky windage depending on power cost and AC efficiency
in your area relative to the baseline assumptions of $0.08/KW-hour and
average CoP of 2.5 (given your average outdoor ambient temperature and
the temperature you keep the server room at).  If you have load
measurements and actual bills to use you can do better, but this will
get you within a factor of 1.5 (either way).

> That and few parts means lower absolute number of failures, but that is 
> another issue.

Which I do not address either.  And larger DIMMs gives you more slots
for future expansion should you want to scale up a calculation besides,
for that matter, which might be worth something even if the marginal
cost of bigger DIMMs is a slight TCO loss (as it might be in a region
where power/cooling are really cheap).

      rgb

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-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu