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

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Message-ID: <20090507173608.GC6693@linux.vnet.ibm.com>
Date:	Thu, 7 May 2009 10:36:09 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Christoph Lameter <cl@...ux.com>,
	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>
Subject: Re: [PATCH] x86: Reduce the default HZ value

On Thu, May 07, 2009 at 07:18:38PM +0200, Peter Zijlstra wrote:
> 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.

One approach would be to enter nohz mode when running a CPU-bound
application on a CPU that had nothing else (other than the idle task)
on its runqueue and for which rcu_needs_cpu() returns zero.  In this
mode, RCU would need to be informed on each system call, perhaps with an
rcu_kernel_enter() and rcu_kernel_exit() that work like rcu_irq_enter()
and rcu_irq_exit() -- and that perhaps replace rcu_irq_enter() and
rcu_irq_exit().

Then RCU would ignore any CPU that was executing a CPU-bound application,
allowing the HZ to be dialed down as low as you like, or perhaps really
entering something like nohz mode.

							Thanx, Paul
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