linux-kernel - Re: [patch] sched: avoid div in rebalance

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Date:	Fri, 12 Jan 2007 11:27:40 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Alan <alan@...rguk.ukuu.org.uk>
Cc:	Andrew Morton <akpm@...l.org>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch] sched: avoid div in rebalance_tick

On Fri, Jan 12, 2007 at 09:59:40AM +0000, Alan wrote:
> On Fri, 12 Jan 2007 07:02:13 +0100
> Nick Piggin <npiggin@...e.de> wrote:
> 
> > Just noticed this while looking at a bug.
> > Avoid an expensive integer divide 3 times per CPU per tick.
> 
> Integer divide is cheap on some modern processors, and multibit shift
> isn't on all embedded ones.

Well integer divide unit is non-pipelined on P4 K8 Core2 and probably
most processors, AFAIK. So the 3 divs would take 240 cycles on a P4,
perhaps.

> How about putting back scale = 1 and using
> 
> scale += scale;
> 
> instead of the shift and getting what ought to be even better results

Yes I gues we ccan do this as well, good idea. I'll make a
quick userspace benchmark and post some numbers with my next
submission.
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