[Beowulf] XEON power variations

Tiago Marques a28427 at ua.pt
Sun Oct 4 19:07:52 PDT 2009

Hi Tom,

On Tue, Sep 15, 2009 at 8:45 PM, Tom Rockwell <rockwell at pa.msu.edu> wrote:
> Hi,
> Intel assigns the same power consumption to different clockspeeds of L, E, X
> series XEON.  All L series have the same rating, all E series etc.  So,
> taking their numbers, the fastest of each type will always have the best
> performance per watt.  And there is no power consumption penalty for buying
> the fastest clockspeed of each type.  Vendor's power calculators reflect
> this (dell.com/calc for example).  To me this seems like marketing
> simplification...  Anybody know any different, e.g. have you seen other
> numbers from vendors or tested systems yourself?
> Is the power consumption of a system with an E5502 CPU really the same as
> one with an E5540?

No but the consumption and the E5520 may be higher than the E5540,
although I wouldn't expect variations of more than 20% on a 80W CPU.
Don't take anything as granted.
When most processors had a fixed voltage input per generation, you
almost certainly could expect them to have higher power consumption
based on the clockspeed since voltage was the same for all models and
it only varied slightly due to current leakage differences between
batches of CPUs. With current process shrinks, current leakage became
a bigger problem then back then and it now varies much more depending
on CPU batches.
After the power wall became a problem, apparently manufacturers
started binning processors by leakage which affects TDP. Intel
currently employs a voltage range(for the higher p-state voltage, not
just between p-states) for processor lines, which they use to adjust
the processors TDP depending on requirements and not just the
clockspeed target. They can lower voltage other than the higher
default value in the range to help make up for leakage but it needs
extra testing to ensure stability then, testing that takes more time
and money.
On the consumer market it's common to find CPUs that are capable of
3GHz but are sold at 2.5GHz and have sometimes higher power
consumption than a 3GHz part if you overclock it just by clock
multiplier means, effectively making it an equal sample by all
measures. The part was obviously capable of a bigger clock but had
it's voltage and clock reduced to fit the TDP, often even barely. This
results in strange situations where the higher clocked, sometimes even
higher voltage part, consumes less power than a lower clocked one with
the same power grade.
Intel's priority is to make money, so they don't care how much power
the processor needs as long as it fits the TDP goal.
I expect the situation in servers is the same, although with a
(slightly) higher degree of thoroughness, hence the 60-80-95-130
grades and not just 65-95-130 grades as in desktops.

Best regards,
Tiago Marques

> Thanks,
> Tom Rockwell
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