Date: Wed, 2 Sep 2009 14:30:43 -0700
From: Josh Triplett <josh@joshtriplett.org>
To: Pavel Machek <pavel@ucw.cz>
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: <20090902213000.GA7137@josh-work.beaverton.ibm.com>
References: <20090901154327.GA10024@feather>
 <20090902200125.GA1851@ucw.cz>
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On Wed, Sep 02, 2009 at 10:01:26PM +0200, Pavel Machek wrote:
> 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?

32-bit x86, ThinkPad T60p (work laptop).  I've observed similar
latencies on x86-64, and others have observed them on 64-bit powerpc.

On top of that, almost all of that 80us consists of variations on "Do I
have any work to do?  No?  OK then.".

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

Possibly, yes.  But I want good responsiveness when the system *does*
have work to do.

- Josh Triplett
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