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To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Why HZ on i386 is 100 ?
In-Reply-To: <3CC4861C.F21859A6@mvista.com.suse.lists.linux.kernel> <E16zuPf-0007yD-00@the-village.bc.nu.suse.lists.linux.kernel>
From: Andi Kleen <ak@suse.de>
Date: 23 Apr 2002 09:17:25 +0200
Message-ID: <p73g01m994q.fsf@oldwotan.suse.de>
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Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

> > The problem is in accounting (or time slicing if you prefer) where we
> > need to start a timer each time a task is context switched to, and stop
> > it when the task is switched away.  The overhead is purely in the set up
> > and tear down.  MOST of these never expire.
> 
> Done properly on many platforms a variable tick is very very easy and also
> very efficient to handle. X86 is a paticular problem case because the timer
> is so expensive to fiddle with

Depends. On modern x86 you can either use the local APIC timer or
the mmtimers (ftp://download.intel.com/ial/home/sp/mmts097.pdf - 
should be in newer x86 chipsets). Both should be better than the 
8254 timer and are also not expensive to work with. 

-Andi
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