Date: Thu, 04 May 2006 10:18:35 -0400 (EDT)
From: Nicolas Pitre <nico@cam.org>
Subject: Re: sched_clock() uses are broken
In-reply-to: <44597A19.7050107@wildturkeyranch.net>
To: George Anzinger <george@wildturkeyranch.net>
Cc: Russell King <rmk+lkml@arm.linux.org.uk>, Andi Kleen <ak@suse.de>,
       lkml <linux-kernel@vger.kernel.org>
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On Wed, 3 May 2006, George Anzinger wrote:

> Nicolas Pitre wrote:
> > Yet that counter isn't necessarily nanosecond based.  So rescaling the
> > returned value to nanosecs requires expensive divisions which could be done
> > only once within sched_clock_diff() instead of twice as often in each
> > sched_clock() calls.
> 
> Oh phooey!!  Scaling can be done with a mpy and a shift.  See the new clock
> code where the TSC (or what ever) is scaled to ns.

I know.

And if you want to preserve more than 32 bits of precision you need 4 
mpy_and_add insns with a shift on ARM at least.

But the point remains that it is more efficient to do it once rather 
than twice or more.


Nicolas
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