Re: A few questions and issues with dynticks, NOHZ and powertop

From: Paul E. McKenney
Date: Mon Apr 05 2010 - 12:40:36 EST

On Mon, Apr 05, 2010 at 09:23:16AM -0700, Arjan van de Ven wrote:
> On 4/5/2010 9:22, Paul E. McKenney wrote:
> >On Mon, Apr 05, 2010 at 09:07:33AM -0700, Arjan van de Ven wrote:
> >>On 4/5/2010 8:14, Paul E. McKenney wrote:
> >>>So the main issue is that for many workloads, it is best to run full bore
> >>>and get done quickly, thus allowing the entire machine to be powered down?
> >>
> >>yep
> >>
> >>>If so, it seems likely that there would be some workloads that were sometimes
> >>>unable to use all the CPUs, in which case shutting down (idling, offlining,
> >>>dyntick-idling, whatever) the excess CPUs might nevertheless be the right
> >>>thing to do.
> >>
> >>but the point is that the normal scheduler + idle behavior gives you exactly that
> >>in a natural way !
> >>If you don't have enough work (tasks) to keep all cores busy, the others are and stay idle.
> >
> >So your earlier objection was not to dyntick-idle as such, but rather
> >to artificially constraining the scheduler to induce dyntick-idle?
> my objection was against the notion that offlining cpus helps power/energy ;-)

Fair enough, at least in general. I should hasten to add that Lai's
patch also helps in the case where NR_CPUS is greater than the number
of CPUs on the system.

Thanx, Paul
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