linux-kernel - Re: high power consumption in recent kernels

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Message-Id: <1278618040.7498.21.camel@marge.simson.net>
Date:	Thu, 08 Jul 2010 21:40:40 +0200
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Norbert Preining <preining@...ic.at>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	akpm <akpm@...ux-foundation.org>, tglx <tglx@...utronix.de>
Subject: Re: high power consumption in recent kernels

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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