linux-kernel - Re: [PATCH 3/3] PM: Introduce Intel PowerClamp Driver

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Message-ID: <21157900.tI2QCFTxxq@vostro.rjw.lan>
Date:	Wed, 14 Nov 2012 00:02 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	paulmck@...ux.vnet.ibm.com,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Linux PM <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	Len Brown <len.brown@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Zhang Rui <rui.zhang@...el.com>, Rob Landley <rob@...dley.net>
Subject: Re: [PATCH 3/3] PM: Introduce Intel PowerClamp Driver

On Tuesday, November 13, 2012 02:45:11 PM Arjan van de Ven wrote:
> On 11/13/2012 2:23 PM, Paul E. McKenney wrote:
> > On Tue, Nov 13, 2012 at 01:39:22PM -0800, Jacob Pan wrote:
> >> On Tue, 13 Nov 2012 13:16:02 -0800
> >> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> >>
> >>>> Please refer to Documentation/thermal/intel_powerclamp.txt for more
> >>>> details.  
> >>>
> >>> If I read this correctly, this forces a group of CPUs into idle for
> >>> about 600 milliseconds at a time.  This would indeed delay grace
> >>> periods, which could easily result in user complaints.  Also, given
> >>> the default RCU_BOOST_DELAY of 500 milliseconds in kernels enabling
> >>> RCU_BOOST, you would see needless RCU priority boosting.
> >>>
> >> the default idle injection duration is 6ms. we adjust the sleep
> >> interval to ensure idle ratio. So the idle duration stays the same once
> >> set. So would it be safe to delay grace period for this small amount in
> >> exchange for less over head in each injection period?
> > 
> > Ah, 6ms of delay is much better than 600ms.  Should be OK (famous last
> > words!).
> 
> well... power clamping is not "free".
> You're going to lose performance as a trade off for dropping instantaneous power consumption....

Yes.  It is good to realize that when the clamping triggers, we already
have some more to worry about than losing some performance. :-)

The problem here is to find a way to lose as little performance as we possibly
can and prevent the system from overheating at the same time.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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