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

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Date:	Mon, 11 Feb 2013 10:59:09 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	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


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> > I'm worried about the proliferation of not easily separable 
> > config options. We already have way too many timer and 
> > scheduler options to begin with.
> 
> Like Steve said, this is for overhead reasons. The syscall 
> uses the slow path so that's ok. But we add a callback to 
> every exception, irq entry/exit, scheduler sched switch, 
> signal handling, user and kernel preemption point. This all 
> could be lowered using static keys but even that doesn't make 
> me feel comfortable with this idea.
> 
> Moreover, for now this is going to be used only on extreme 
> usecases such as real time and HPC. If we really have to merge 
> this into an all-in-one nohz kconfig, I suggest we wait for 
> the feature to mature a bit and prove that it can be useful 
> further those specialized workloads, and also that we can 
> ensure it's off-case overhead is not significant.

I have no problems with making it an option initially - as long 
as the options are logically named and interconnected.

In terms of overhead, a big plus is the reduction in user-space 
execution overhead. At HZ=1000 we easily have 0.5%-1.0% overhead 
currently. That is a *lot* of overhead if the box does mostly 
user-space execution - which most boxes do, both servers and 
desktops - not HPC systems.

Thanks,

	Ingo
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