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

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Message-ID: <20130321171806.GX3637@linux.vnet.ibm.com>
Date:	Thu, 21 Mar 2013 10:18:06 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	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,
	khilman@...aro.org, geoff@...radead.org, tglx@...utronix.de
Subject: Re: [PATCH] nohz1: Documentation

On Thu, Mar 21, 2013 at 08:45:07AM -0700, Arjan van de Ven wrote:
> On 3/20/2013 5:27 PM, Steven Rostedt wrote:
> >I'm not sure I would recommend idle=poll either. It would certainly
> >work, but it goes to the other extreme. You think NO_HZ=n drains a
> >battery? Try idle=poll.
> 
> 
> do not ever use idle=poll on anything production.. really bad idea.
> 
> if you temporary cannot cope with the latency, you can use the PMQOS system
> to limit (including going all the way to idle=poll).
> but using idle=poll completely is very nasty for the hardware.
> 
> In addition we should document that idle=poll will cost you peak performance,
> possibly quite a bit.
> 
> the same is true for the kernel paramter to some extend; it's there to work around
> broken bioses/hardware/etc; if you have a latency/runtime requirement, it's much better
> to use PMQOS for this from userspace.

Thank you for the info, Arjan!  Does the following capture the tradeoffs?

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:

	o	Use PMQOS from userspace to inform the kernel of your
		latency requirements (preferred).

	o	Use the "idle=mwait" boot parameter.

	o	Use the "intel_idle.max_cstate=" to limit the maximum
		depth C-state depth.

	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.

							Thanx, Paul

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