linux-kernel - Re: [PATCH 04/20] Convert gfp_zone() to use a table of precalculated value

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Message-ID: <alpine.DEB.1.10.0902231042440.7790@qirst.com>
Date:	Mon, 23 Feb 2009 10:43:20 -0500 (EST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Mel Gorman <mel@....ul.ie>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH 04/20] Convert gfp_zone() to use a table of precalculated
 value

On Tue, 24 Feb 2009, Nick Piggin wrote:

> > Are you sure that this is a benefit? Jumps are forward and pretty short
> > and the compiler is optimizing a branch away in the current code.
>
> Pretty easy to mispredict there, though, especially as you can tend
> to get allocations interleaved between kernel and movable (or simply
> if the branch predictor is cold there are a lot of branches on x86-64).
>
> I would be interested to know if there is a measured improvement. It
> adds an extra dcache line to the footprint, but OTOH the instructions
> you quote is more than one icache line, and presumably Mel's code will
> be a lot shorter.

Maybe we can come up with a version of gfp_zone that has no branches and
no lookup?

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