linux-kernel - Re: [PATCH] x86: Reduce the default HZ value

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Message-Id: <1241716718.6311.1531.camel@laptop>
Date:	Thu, 07 May 2009 19:18:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Alok Kataria <akataria@...are.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"alan@...rguk.ukuu.org.uk" <alan@...rguk.ukuu.org.uk>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] x86: Reduce the default HZ value

On Thu, 2009-05-07 at 19:13 +0200, Peter Zijlstra wrote:
> 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.

Another user is RCU, the grace period is tick driven, growing these
ticks by a factor 50 or so might require some tinkering with forced
grace periods when we notice our batch queues getting too long.

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