linux-kernel - Re: [ANNOUNCE] 3.8-rc6-nohz4

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Message-ID: <1360340288.2621.112.camel@gandalf.local.home>
Date:	Fri, 08 Feb 2013 11:18:08 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [ANNOUNCE] 3.8-rc6-nohz4

On Fri, 2013-02-08 at 16:53 +0100, Frederic Weisbecker wrote:
> 2013/2/7 Christoph Lameter <cl@...ux.com>:
> > On Thu, 7 Feb 2013, Frederic Weisbecker wrote:
> >
> >> Not with hrtick.
> >
> > hrtick? Did we not already try that a couple of years back and it turned
> > out that the overhead of constantly reprogramming a timer via the PCI bus
> > was causing too much of a performance regression?
> 
> Yeah Peter said that especially reprogramming the clock everytime we
> call schedule() was killing the performances. Now may be on some
> workloads, with the tick stopped, we can find some new results.

I could imagine this being dynamic. If the system isn't very loaded, and
the scheduler is giving lots of time slices to tasks, then perhaps it
could switch to a reprogramming the clock based scheduling. Or maybe, we
could switch to a "skip ticks" method. That is, instead of completely
disabling the tick, make the tick go off every other time or less, and
use the NO_HZ code to calculate the missed ticks.

-- Steve


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