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

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Message-ID: <87wqszb7r0.fsf@linaro.org>
Date:	Fri, 22 Mar 2013 11:37:55 -0700
From:	Kevin Hilman <khilman@...aro.org>
To:	paulmck@...ux.vnet.ibm.com
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

"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?

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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