Matthew Garrett on the race to idle [LWN.net]

Matthew Garrett on the race to idle

Posted May 11, 2008 10:25 UTC (Sun) by IkeTo (subscriber, #2122)
In reply to: Matthew Garrett on the race to idle by JoeBuck
Parent article: Matthew Garrett on the race to idle

> Leakage power is constant.

Should it be proportional to voltage?

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Matthew Garrett on the race to idle

Posted May 12, 2008 0:10 UTC (Mon) by bronson (subscriber, #4806) [Link]

It's been many years since I've done VLSI but, yes, we normally modeled leakage as a constant.
It's mostly affected by temperature and process characteristics, not voltage.

If applied voltage is near the thermal voltage (25.9mV) then I suppose you might have to
include voltage in the equation.  But we're nowhere near that today and I'm not sure anyone
would ever want to run that close to the noise floor (I know, I know, 640K should be enough
for anybody...  we'll see!)

Matthew Garrett on the race to idle

Posted May 12, 2008 8:31 UTC (Mon) by IkeTo (subscriber, #2122) [Link]

> we normally modeled leakage as a constant.

My understanding is that leakage *current* is nearly constant (unless the voltage difference
is very small), so leakage power is proportional to voltage, and leakage energy is
proportional to voltage multiplied by time, i.e., voltage divides by frequency.  Since voltage
divides by frequency is increases not very much when you decrease frequency, this is nearly
constant energy.  If leakage *power* is constant instead, the leakage energy will be
proportional to the inverse of frequency.  Then voltage scaling would be doing something very
bad to energy consumption!

Matthew Garrett on the race to idle

Posted May 13, 2008 8:20 UTC (Tue) by forthy (guest, #1525) [Link]

> Should it be proportional to voltage?

It actually goes exponentially with voltage and temperature. This means scaling up the voltage for higher frequency and in turn heating up the CPU increases leakage by factors, which makes the original power saving equation somehow dubious.

Matthew Garrett on the race to idle

Posted May 13, 2008 9:08 UTC (Tue) by IkeTo (subscriber, #2122) [Link]

Would you mind clarifying what do you mean by "original power saving equation" and "dubious"?