Subject: Re: high power consumption in recent kernels
From: Mike Galbraith <efault@gmx.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Norbert Preining <preining@logic.at>,
       Arjan van de Ven <arjan@infradead.org>, linux-kernel@vger.kernel.org,
       Ingo Molnar <mingo@elte.hu>, akpm <akpm@linux-foundation.org>,
       tglx <tglx@linutronix.de>
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Date: Thu, 08 Jul 2010 21:40:40 +0200
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On Thu, 2010-07-08 at 17:59 +0200, Peter Zijlstra wrote:
> On Thu, 2010-07-08 at 15:23 +0200, Peter Zijlstra wrote:
> > On Thu, 2010-07-08 at 21:46 +0900, Norbert Preining wrote:
> > > Looks promising, reverting the old patch, adding that one, building,
> > > running, unplugging ppower, powertop runs now since some time,
> > > it seems that we are back to better situation: 
> > 
> > Hrmm, Mike seems you wrecked power usage.. 
> > 
> > So nohz_ratelimit() prevents us from entering NOHZ when the last attempt
> > was less than 1/2 a jiffy ago (fwiw: NSEC_PER_SEC/HZ == TICK_NSEC).
> > 
> > Its either entering idle or irq_exit trying to enter nohz state, if we
> > keep skipping it it means that we get enough interrupt activity to
> > render nohz useless anyway.. so not quite sure how this wrecks things..
> 
> OK, so Arjan said the gain could come from tricking the idle governor
> into not using deeper C states. He also said he significantly cured said
> governor in .35.
> 
> Mike could you re-run your netperf tests that showed the 10% throughput
> gain? Hopefully the fixed governor will yield the same result and we can
> kill off this ratelimit thing.

The gain is (well was last time I checked), but as noted, I'd just call
it a misguided optimization and be done with it.

	-Mike

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