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SubjectRe: SMP cpu deep sleep
Wes Felter wrote:

> I worked on this last year (I call it CPU packing, because the idea is to
> pack the load onto the fewest number of CPUs).
> The CPU hotplug patch is the way to go, but the hardware is the problem. I
> talked to an Intel CPU architect at MICRO last year and he confirmed that
> SMP Intel systems don't support any low-power modes besides HLT. AMD's
> documentation says that Opterons support voltage/frequency scaling (aka
> Cool 'n' Quiet), but AFAICT the documentation is wrong. In summary, you
> are doomed.

For power saving, HLT is hard to beat ;-) You note HLT as if there was
some good reason not to use it... Mask everything except some BACK2WORK
int from the night watchman CPU. I would really like this on some
machines which seem to leave all CPUs generating heat even when booted
with a uni kernel.

Whilst thinking about this, *if* using HLT is practical in therms of
power saving, perhaps all but the last CPU could HLT if the run queue
was empty, and only be awakened by the "last" CPU, in some case where
the run queue length was longer than {some_value}.

-bill davidsen (
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me
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