linux-kernel - Re: [PATCH 07/41] cpuset: Set up interface for nohz flag

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Date:	Wed, 9 May 2012 09:22:21 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Mike Galbraith <efault@....de>
cc:	Peter Zijlstra <peterz@...radead.org>,
	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>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>, Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 07/41] cpuset: Set up interface for nohz flag

On Wed, 9 May 2012, Mike Galbraith wrote:

> > Its not a good match for this. You would want a per cpu attribute for
> > nohz.
>
> Or per cpuset, which can be the same thing as per cpu if you want.

But now we start to manage per cpu characteristics with cpusets whereas
before cpusets was used mainly to manage groups of processors assigned to
applications! This means there will be a requirement to use cpusets in
environments that have not used them before.

> > isolcpus are not part of load balancer domains.
>
> Yup, so if you have an application with an RT component, somewhat
> sensitive, needs isolation from rest of a big box, but app also has
> SCHED_OTHER components.  isolcpus is a pain, everything has to be static
> and nailed to the floor.  Load just works when plugged into a cpuset.

Well you have low latency requirements. If you code for lowest latency
then you have to consider cache sizes, cache sharing etc etc. This means
you will have to nail down everything anyways. Cpusets would just be
another thing that one has to worry about.

The loads definitely wont work right if just "plugged into a cpuset".
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