linux-kernel - Re: [PATCH] nohz1: Documentation

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Message-ID: <20130322192515.GU3637@linux.vnet.ibm.com>
Date:	Fri, 22 Mar 2013 12:25:15 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Kevin Hilman <khilman@...aro.org>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rob Landley <rob@...dley.net>, linux-kernel@...r.kernel.org,
	josh@...htriplett.org, zhong@...ux.vnet.ibm.com,
	geoff@...radead.org, tglx@...utronix.de
Subject: Re: [PATCH] nohz1: Documentation

On Fri, Mar 22, 2013 at 11:37:55AM -0700, Kevin Hilman wrote:
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes:
> 
> > On Thu, Mar 21, 2013 at 10:41:30AM -0700, Arjan van de Ven wrote:
> >> On 3/21/2013 10:18 AM, Paul E. McKenney wrote:
> >> >	o	Use the "idle=poll" boot parameter.  However, please note
> >> >		that use of this parameter can cause your CPU to overheat,
> >> >		which may cause thermal throttling to degrade your
> >> >		latencies --and that this degradation can be even worse
> >> >		than that of dyntick-idle.
> >> 
> >> it also disables (effectively) Turbo Mode on Intel cpus... which can
> >> cost you a serious percentage of performance
> >
> > Thank you, added!  Please see below for the updated list.
> >
> > 							Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > o	Dyntick-idle slows transitions to and from idle slightly.
> > 	In practice, this has not been a problem except for the most
> > 	aggressive real-time workloads, which have the option of disabling
> > 	dyntick-idle mode, an option that most of them take.  However,
> > 	some workloads will no doubt want to use adaptive ticks to
> > 	eliminate scheduling-clock-tick latencies.  Here are some
> > 	options for these workloads:
> >
> > 	a.	Use PMQOS from userspace to inform the kernel of your
> > 		latency requirements (preferred).
> 
> This is not only the preferred approach, but the *only* approach
> available on non-x86 systems.  Perhaps the others should be marked as
> x86-only?

Good point, added that.

							Thanx, Paul

> Kevin
> 
> > 	b.	Use the "idle=mwait" boot parameter.
> >
> > 	c.	Use the "intel_idle.max_cstate=" to limit the maximum
> > 		depth C-state depth.
> >
> > 	d.	Use the "idle=poll" boot parameter.  However, please note
> > 		that use of this parameter can cause your CPU to overheat,
> > 		which may cause thermal throttling to degrade your
> > 		latencies -- and that this degradation can be even worse
> > 		than that of dyntick-idle.  Furthermore, this parameter
> > 		effectively disables Turbo Mode on Intel CPUs, which
> > 		can significantly reduce maximum performance.
> 

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