linux-kernel - Re: [PATCH 3/6] sched: high-res preemption tick

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Message-Id: <1193911946.27652.252.camel@twins>
Date:	Thu, 01 Nov 2007 11:12:26 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Mike Galbraith <efault@....de>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/6] sched: high-res preemption tick

On Wed, 2007-10-31 at 22:53 +0100, Andi Kleen wrote:
> Peter Zijlstra <a.p.zijlstra@...llo.nl> writes:
> 
> > Use HR-timers (when available) to deliver an accurate preemption tick.
> >
> > The regular scheduler tick that runs at 1/HZ can be too coarse when nice
> > level are used. The fairness system will still keep the cpu utilisation 'fair'
> > by then delaying the task that got an excessive amount of CPU time but try to
> > minimize this by delivering preemption points spot-on.
> 
> This might be costly when hrtimers happen to use an more expensive
> to reprogram time source. Even an APIC timer access is fairly slow. 
> And you'll potentially add the to lots of context switces.
> 
> Not sure that is a good idea for performance in general.

Right, now I remember. 

The idea was to run the rest of the kernel at HZ=50 or so, nothing but
scheduling needs it anymore, and with this patch the scheduler doesn't
need it anymore either. Should be good for power. This new hrtick thing
only does a lot of ticks when there are a lot of runnable tasks, it
starts at 2 with a tick per latency/2.

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