Hmmm...I believe that T-states will still always save power, though,
while the CPU is executing instructions. If the slowdown is acceptable,
I'm not sure why this method would not be a useful way to consume
less power.
Here's an example, based on the data I sent earlier:
Let's say we have a task that takes 1 CPU minute of time (at full power,
no throttle), and we need to execute it once every 10 minutes. Look at
the second table -- the largest entry under 10 minutes is 8.44
minutes (P3/75% throttle).
So, if we ran that box at P3/75% for 8.44 minutes, and P3/100% (C1,
essentially) total power consumption would be about:
36.25*8.5 + 34*1.5 = 359 W-min.
That's a better choice (about 4% better) than running P0/0% for 1 minute
and P3/100%(C1) for 9 minutes, which would consume about:
67*1 + 34*9 = 373 W-min.
Based on these measurements, at least, it seems to me that for a given
P-state, throttling could always save *some* power on this machine (as long
as we're still OK with the additional slowdown, which has to be the case
for any power/frequency manipulation, as Dana implied).
Does this meet the definition of "same amount of work in the same amount
of time, while consuming less energy"? I think it does. :-)
Mike
Andrei Dorofeev wrote:
> On 10/30/07, Dana H. Myers <Dana.Myers at sun.com> wrote:
>>> I would agree with Aubrey that T-states don't save power if by "save" we
>>> mean that we can do the same amount of work at the same amount of time
>>> while consuming less energy.
>> I don't think P-states meet this definition of saving power, either, do
>> they?
>
> Actually, I think P-states can be (at least theoretically) used in such a way
> that this definition is met. In an ideal environment where we know precisely
> what are the requirements in terms of CPU cycles of a given workload, we
> can afford putting CPU into lower FID/VID without loosing performance and
> increasing run times while saving power. Attempting to do the same thing
> with T-states will not save any power because we're going to stay in C1 with
> or without T-states. Also, just spending C1 time in lowest P-state alone
> can save power on AMD CPUs which don't automatically drop
> frequencies/voltages upon entering C1 (which is what C1E on Intel CPUs is
> for).
>
> - Andrei
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