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 19:42:42 -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:

> On Tue, 2012-03-27 at 11:13 -0500, Christoph Lameter wrote:
>
> > Please do not use "realtime" since I am not sure what you mean by that.
> > Its for a low latency applications that cannot use "realtime" because that
> > implies high latencies.
>
> This statement totally confuses me, as the whole point of the -rt
> (realtime) patch, is for lower latencies. Where do you get "realtime"
> implies high latencies from?

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.

Realtime avoids high latency spikes but in general increases average OS
latencies.



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