linux-kernel - Re: [PATCH 4/4] nohz: Set isolcpus when nohz

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Message-ID: <20150521121246.GA4723@afzalpc>
Date:	Thu, 21 May 2015 17:42:46 +0530
From:	Afzal Mohammed <afzal.mohd.ma@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 4/4] nohz: Set isolcpus when nohz_full is set

Hi,

On Wed, May 20, 2015 at 02:00:26PM -0700, Paul E. McKenney wrote:

> > > Given that kernel initiated association to isolcpus, a user turning
> > > NO_HZ_FULL_ALL on had better not have much generic load to manage.  If
> > 
> > On a quad-core desktop system with NO_HZ_FULL_ALL, hackbench took 3x
> > time as compared to w/o this patch, except boot cpu every one else
> > jobless. Though NO_HZ_FULL_ALL (afaik) is not meant for generic load,
> > it was working fine, but not after this - it is now like a single core
> > system.
> 
> I have to ask...  What is your use case?  What are you wanting NO_HZ_FULL
> to do for you?

I was just playing NO_HZ_FULL with tip-[sched,timers]-* changes.

Thought that shutting down ticks as much as possible would be
beneficial to normal loads too, though it has been mentioned to be used
for specialized loads. Seems like drawbacks due to it weigh against
normal loads, but haven't so far observed any (on a laptop with normal
activities) before this change.

Regards
Afzal
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