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

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Date:	Mon, 23 Feb 2009 16:40:47 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	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 Mon, Feb 23, 2009 at 10:43:20AM -0500, Christoph Lameter wrote:
> 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.

Not in kernbench at least, but that is no surprise. It's a small
percentage of the overall cost. It'll appear in the noise for anything
other than micro-benchmarks.

> > 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.
> 

Yes, it's an index lookup of a shared read-only cache line versus a lot
of code with branches to mispredict. I wasn't happy with the cache line
consumption but it was the first obvious alternative.

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

Ideally, yes, but I didn't spot any obvious way of figuring it out at
compile time then or now. Suggestions?

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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