linux-kernel - Re: [PATCH 07/32] cpuset: Set up interface for nohz flag

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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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