Message-ID: <4249F1B0.6050800@nortel.com>
Date: Tue, 29 Mar 2005 18:24:16 -0600
From: Chris Friesen <cfriesen@nortel.com>
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To: Peter Chubb <peterc@gelato.unsw.edu.au>
CC: krishna <krishna.c@globaledgesoft.com>,
       Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: How to measure time accurately.
References: <424779F3.5000306@globaledgesoft.com>	<4248E282.1000105@nortel.com> <16969.58762.180127.283274@wombat.chubb.wattle.id.au>
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Peter Chubb wrote:
>>>>>>"Chris" == Chris Friesen <cfriesen@nortel.com> writes:

> Chris> Most cpus have some way of getting at a counter or decrementer
> Chris> of various frequencies.  Usually it requires low-level hardware
> Chris> knowledge and often it needs assembly code.
> 
> As a device driver is inside the linux kernel (unless you're writein a
> user-mode device driver :-)) you can use the getcycles() macro that's
> defined for most architectures.  It provides a snapshot of the
> cycle-counter.

For ppc this only gives 32-bit values, which overflow every 129 seconds 
on my G5.  Depending on how long you're trying to time, this could be a 
problem.

Chris
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