linux-kernel - Re: [RFC] Dynamic Tick and Deferrable Timer Support

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Message-Id: <1234206642.10457.45.camel@jstultz-laptop>
Date:	Mon, 09 Feb 2009 11:10:42 -0800
From:	John Stultz <johnstul@...ibm.com>
To:	Pavel Machek <pavel@...e.cz>
Cc:	Jon Hunter <jon-hunter@...com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] Dynamic Tick and Deferrable Timer Support

On Sat, 2009-02-07 at 10:20 +0100, Pavel Machek wrote:
> On Thu 2009-01-29 09:36:00, john stultz wrote:
> > On Thu, Jan 29, 2009 at 8:29 AM, Jon Hunter <jon-hunter@...com> wrote:
> > > Pallipadi, Venkatesh wrote:
> > > I have spent several weeks trying to suppress kernel timers using the
> > > deferred timers and lengthen the sleep time. I am now able to get the device
> > > to sleep for minutes but I found that max_delta_ns is a limiting factor. I
> > > will be surprised if you can sleep for longer than ~2.15 seconds with the
> > > current implementation.
> > 
> > As an aside, there are some further hardware limitations in the
> > timekeeping core that limit the amount of time the hardware can sleep.
> > For instance, the acpi_pm clocksource wraps every 2.5 seconds or so,
> > so we have to wake up periodically to sample it to avoid wrapping
> > issues.
> > 
> > Just to be able to deal with all the different hardware out there, the
> > timekeeping core expects to wake up twice a second to do this
> > sampling. It may be possible to push this out if you are using other
> 
> That's strange... I think I seen less than 2 wakeups per second on
> powertop...? (thinkpad x60, nothing exotic).

Yea, I don't think there is an interface that the timekeeping code
communicates that through. Probably a good idea to get that established
before folks try to push further then a second and end up with trouble
on hardware with short clocksources.

thanks
-john


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