linux-kernel - Re: [PATCH documentation 1/2] nohz1: Add documentation.

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Message-ID: <20130412175419.GG29861@linux.vnet.ibm.com>
Date:	Fri, 12 Apr 2013 10:54:19 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, Arjan van de Ven <arjan@...ux.intel.com>,
	Kevin Hilman <khilman@...aro.org>,
	Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH documentation 1/2] nohz1: Add documentation.

On Fri, Apr 12, 2013 at 10:05:04AM +0200, Peter Zijlstra wrote:
> On Thu, 2013-04-11 at 12:13 -0700, Paul E. McKenney wrote:
> > > > +2. Many architectures will place dyntick-idle CPUs into deep sleep
> > > > +   states, which further degrades from-idle transition latencies.
> > > 
> > > Above you say "to and from the idle loop", now it is from-idle. Simply say:
> > > 
> > > "... which further degrades idle transision latencies" which means both :).
> > 
> > If people speak for this item, I will update it.  Arjan suggested removing
> > it entirely.
> 
> So I haven't yet read the entire document, but:
> 
> +2.     Many architectures will place dyntick-idle CPUs into deep sleep
> +       states, which further degrades from-idle transition latencies.
> +
> +Therefore, systems with aggressive real-time response constraints
> +often run CONFIG_NO_HZ=n kernels in order to avoid degrading from-idle
> +transition latencies.
> 
> I'm not sure that's the reason.. We can (and do) limit C states to curb
> the idle-exit times. The reason we often turn off NOHZ all together is
> to further reduce the cost of the idle paths.
> 
> All the mucking about with clock states and such is a rather expensive
> thing
> to do all the time.

Ah, thank you!  This might help me address Arjan's concerns as well.
How about the following for the disadvantages of CONFIG_NO_HZ=y?

							Thanx, Paul

------------------------------------------------------------------------

1.	It increases the number of instructions executed on the path
	to and from the idle loop.

2.	On many architectures, dyntick-idle mode also increases the
	number of times that clocks must be reprogrammed, and this
	reprogramming can be quite expensive.

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