On Mon, 11 Aug 2008 17:02:06 +0300
Adrian Bunk <bunk@kernel.org> wrote:
yes you're missing quite a few things...
lets build this up
1) when the cpu is idle (as in "idle loop C states/hlt"; p4_clockmod
doesn't mean anything.. the clock is stopped not just skipped.
2) when the cpu is executing code (eg non-idle), it takes more power
for a unit of time than it takes when it's idle
3) when the cpu is in thermal throttle (p4_clockmod) and does a
throttle cycle, it'll take less power for that cycle than a full
execution (at least of the power intensive type of instruction), but
more than if it would have been in idle.
now lets take your "Light load" example (light could mean anything less
than 100% cpu use for all I care) and lets assume a 50% throttle (just
because it makes the explenation simpler, not fundamental)
in 50% thermal throttle, after every cycle a throttle cycle is
inserted, this means that code that gets executed will take 2x as long
basically.
take your lightly loaded case, and lets say you have 20% CPU
utilization in the unthrottled case (again, random number, works with
any number). Now with the thermal throttle, you suddenly take 40%.
so before you had for one second "20% expensive, 80% low power"
now you have for one second "20% expensive, 20% throttle power, 60% low
power"
since throttle power is higher than low/idle power.. you lose.
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