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Message-ID: <1332444030.24328.66.camel@marge.simpson.net>
Date: Thu, 22 Mar 2012 20:20:30 +0100
From: Mike Galbraith <efault@....de>
To: Christoph Lameter <cl@...ux.com>
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>,
Steven Rostedt <rostedt@...dmis.org>,
Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
Zen Lin <zen@...nhuawei.org>
Subject: Re: [PATCH 07/32] cpuset: Set up interface for nohz flag
On Thu, 2012-03-22 at 11:26 -0500, Christoph Lameter wrote:
> On Thu, 22 Mar 2012, Mike Galbraith wrote:
>
> > > > We use here a per cpu refcounter. As long as a CPU
> > > > is contained into at least one cpuset that has the
> > > > nohz flag set, it is part of the set of CPUs that
> > > > run into adaptive nohz mode.
> > >
> > > What are the drawbacks for nohz?
> >
> > For nohz in general, latency. To make it at all usable for rt loads, I
>
> Well nohz while a process is running on a dedicated cpu means the cpu is
> running full power and no disruptions occur. This is a tremendous benefit.
In the context of single task burning in userspace, you bet.
> Less than 10us jitter can alrady be accomplished by building a kernel with
> certain options off (like for example preemption...) and ensuring that
> stuff stays off certain processors. Lets not confuse realtime with low
> latency. Real time in the sense of deterministic execution is bad for
> latency because overhead is added to ensure the determinism which
> increases latency.
Yeah, I know RT pays heavily for determinism. It loses on best case.
> > of the current box, triple digit for simple synchronized frame timers +
> > compute worker-bees load on 64 cores. Patch 4 probably helps that, but
> > don't _think_ it'll fix it. If you (currently) ever become balancer,
> > you're latency target is smoking wreckage.
>
> Yes so we need something to tell the system which cpu is the sacrificial
> lamb that will not run low latency applications.
Definitely a lamb is required.
(This set is targeted at HPC, so I'll shut up now.. but RT is HPC too)
-Mike
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