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Subject: Re: [PATCH] x86: Reduce the default HZ value
From: Peter Zijlstra <peterz@infradead.org>
To: Christoph Lameter <cl@linux.com>
Cc: Alok Kataria <akataria@vmware.com>, "H. Peter Anvin" <hpa@zytor.com>,
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Date: Thu, 07 May 2009 19:13:42 +0200
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On Thu, 2009-05-07 at 19:09 +0200, Peter Zijlstra wrote:
> On Thu, 2009-05-07 at 10:13 -0400, Christoph Lameter wrote:
> > I think we need to reduce the general tick frequency to be as low as
> > possible. With high resolution timers the tick frequency is just the
> > frequency with which the timer interrupt disturbs a running application.
> > 
> > Are there any benefits remaining from frequent timer interrupts? I would
> > think that 60 HZ would be sufficient.
> > 
> > It would be good if the kernel would be truly tickless. Scheduler events
> > would be driven by the scheduling intervals and not the invokations of the
> > scheduler softirq.
> 
> The only thing that's driven by the softirq is load-balancing, there's
> way more to the scheduler-tick than kicking that thing awake every so
> often.
> 
> The problem is that running the scheduler of off hrtimers is too
> expensive. We have the code, we tried it, people complained.

Therefore, decreasing the HZ value to say 50, we'd get a minimum
involuntary preemption granularity of 20ms, something on the high end of
barely usable.