Date: Wed, 2 Sep 2009 22:01:26 +0200
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
Message-ID: <20090902200125.GA1851@ucw.cz>
References: <20090901154327.GA10024@feather>
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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

-- 
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