linux-kernel - Re: [PATCH v3 5/6] timerfd: Add support for deferrable timers

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Message-ID: <20140221041134.GA20564@teo>
Date:	Thu, 20 Feb 2014 20:11:34 -0800
From:	Anton Vorontsov <anton@...msg.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Alexey Perevalov <a.perevalov@...sung.com>,
	linux-kernel@...r.kernel.org, john.stultz@...aro.org,
	anton@...sg.org, kyungmin.park@...sung.com, cw00.choi@...sung.com,
	akpm@...ux-foundation.org
Subject: Re: [PATCH v3 5/6] timerfd: Add support for deferrable timers

On Thu, Feb 20, 2014 at 12:09:43PM +0100, Thomas Gleixner wrote:
> On Thu, 20 Feb 2014, Alexey Perevalov wrote:
> > From: Anton Vorontsov <anton@...msg.org>
> > 
> > This patch implements a userland-side API for generic deferrable timers,
> > per linux/timer.h:
> > 
> >  * A deferrable timer will work normally when the system is busy, but
> >  * will not cause a CPU to come out of idle just to service it; instead,
> >  * the timer will be serviced when the CPU eventually wakes up with a
> >  * subsequent non-deferrable timer.
> > 
> > These timers are crucial for power saving, i.e. periodic tasks that want
> > to work in background when the system is under use, but don't want to
> > cause wakeups themselves.
> > 
> > The deferred timers are somewhat orthogonal to high-res external timers,
> > since the deferred timer is tied to the system load, not just to some
> > external decrementer source.
> 
> Again this changelog makes no sense. What's orthogonal to high-res
> timers and why are they external?

Not trying to defend the current series, just felt the need clarify this
one.

By orthogonal I meant that comparing to high resolution timers' use cases,
deferred timers can be super-low resolution, super inaccurate. We don't
know exactly when they will fire, all we know is something like "every 0.2
seconds, iff the system/user is doing something, otherwise don't bother."

As for external (my bad, shouldn't invent personal terminology): the
hrtimers are tied to some clock source (which is "external" to me), but
deferred timers are mostly tied to the system's activity.

Thanks,

Anton
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