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From: Pavel Machek <pavel@ucw.cz>
To: Josh Triplett <josh@joshtriplett.org>
Cc: linux-kernel@vger.kernel.org, Anton Blanchard <anton@samba.org>,
	Tim Pepper <lnxninja@linux.vnet.ibm.com>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	John Stultz <johnstul@us.ibm.com>,
	Christoph Lameter <cl@linux-foundation.org>,
	Jamey Sharp <jamey@minilop.net>
Subject: Re: [RFC PATCH] Turn off the tick even when not idle
Date: Wed, 2 Sep 2009 22:01:26 +0200	[thread overview]
Message-ID: <20090902200125.GA1851@ucw.cz> (raw)
In-Reply-To: <20090901154327.GA10024@feather>

Hi!

> When a process does some number crunching for a while, without involving
> the kernel, the kernel still interrupts it HZ times per second to figure
> out if it has any work to do.  With a system dedicated to doing such
> number crunching, the answer will almost always come up "no"; however,
> the kernel takes a while figuring out all the "no"s from various
> subsystems, every timer tick.  On my system, the timer tick takes about
> 80us, every 1/HZ seconds; that represents a significant overhead.  80us
> out of every 1ms, for instance, means 8% overhead.  Furthermore, the
> time taken varies, and the timer interrupts lead to jitter in the
> performance of the number crunching.

8% overhead on hz=1000 is quite high --- what hw is that?

You should be able to get similar results with HZ=1, right?
									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

  parent reply	other threads:[~2009-09-02 20:01 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2009-09-01 15:43 [RFC PATCH] Turn off the tick even when not idle Josh Triplett
2009-09-01 19:56 ` Christoph Lameter
2009-09-01 16:46   ` jim owens
2009-09-01 18:08   ` Josh Triplett
2009-09-01 22:35     ` Christoph Lameter
2009-09-01 20:26       ` Josh Triplett
2009-09-02 20:01 ` Pavel Machek [this message]
2009-09-02 21:30   ` Josh Triplett
2009-09-03  5:15 ` Anton Blanchard

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