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Date: Mon, 23 Feb 2009 10:43:20 -0500 (EST)
From: Christoph Lameter <cl@linux-foundation.org>
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To: Nick Piggin <nickpiggin@yahoo.com.au>
cc: Mel Gorman <mel@csn.ul.ie>,
       Linux Memory Management List <linux-mm@kvack.org>,
       Pekka Enberg <penberg@cs.helsinki.fi>, Rik van Riel <riel@redhat.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Johannes Weiner <hannes@cmpxchg.org>, Nick Piggin <npiggin@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Lin Ming <ming.m.lin@intel.com>,
       Zhang Yanmin <yanmin_zhang@linux.intel.com>
Subject: Re: [PATCH 04/20] Convert gfp_zone() to use a table of precalculated
 value
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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?