linux-kernel - Re: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs it

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Date:	Tue, 27 Mar 2012 20:19:15 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linaro-sched-sig@...ts.linaro.org,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Ingo Molnar <mingo@...nel.org>,
	Max Krasnyansky <maxk@...lcomm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephen Hemminger <shemminger@...tta.com>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Zen Lin <zen@...nhuawei.org>
Subject: Re: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs
 it

On Tue, 27 Mar 2012, Steven Rostedt wrote:

> > Obviously compiling a kernel with preemptiong introduces additional
> > overhead to guarantee more deterministic behavior. Additional overhead
> > increases latencies generated by the OS in general. Compile a kernel
> > without preemption and it will run faster and thus have lower latencies.
>
> I call that "lower overhead".

Good marketing but it does not change the facts.

> "Latency is a measure of time delay experienced in a system, the precise
> definition of which depends on the system and the time being measured.
> Latencies may have different meaning in different contexts."
>
> That last sentence is key. So lets avoid the term "latency" as it
> obviously has a different meaning to the both of us.
>
> Instead, lets use "determinism" (what we call latency in the realtime
> world) and "overhead" (what you seem to see as latency caused by the
> kernel).

I sure wish you would be using the term determinism instead of "latency".

Overhead causes latency and the definition that you quoted is what I am
talking about. Latencies are the delays in processing experienced by the
application through the speed of system calls and by interruptions  of
a user space process by the kernel for various reasons.





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