From: Nick Piggin <npiggin@suse.de>
To: Alan <alan@lxorguk.ukuu.org.uk>
Cc: Andrew Morton <akpm@osdl.org>, Ingo Molnar <mingo@elte.hu>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [patch] sched: avoid div in rebalance_tick
Date: Fri, 12 Jan 2007 11:27:40 +0100 [thread overview]
Message-ID: <20070112102740.GC28611@wotan.suse.de> (raw)
In-Reply-To: <20070112095940.0795a998@localhost.localdomain>
On Fri, Jan 12, 2007 at 09:59:40AM +0000, Alan wrote:
> On Fri, 12 Jan 2007 07:02:13 +0100
> Nick Piggin <npiggin@suse.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.
next prev parent reply other threads:[~2007-01-12 10:27 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-01-12 6:02 [patch] sched: avoid div in rebalance_tick Nick Piggin
2007-01-12 9:59 ` Alan
2007-01-12 10:27 ` Nick Piggin [this message]
2007-01-13 6:46 ` Nick Piggin
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