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Date: Tue, 27 Mar 2012 19:42:42 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
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To: Steven Rostedt <rostedt@goodmis.org>
cc: Frederic Weisbecker <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, linaro-sched-sig@lists.linaro.org,
        Alessio Igor Bogani <abogani@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, Avi Kivity <avi@redhat.com>,
        Chris Metcalf <cmetcalf@tilera.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
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Subject: Re: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs
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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.