linux-kernel - Re: [PATCH 24/27] Convert gfp_zone() to use a table of precalculated values

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Message-ID: <20090318135222.GA4629@csn.ul.ie>
Date:	Wed, 18 Mar 2009 13:52:23 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	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>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 24/27] Convert gfp_zone() to use a table of
	precalculated values

On Mon, Mar 16, 2009 at 03:12:50PM -0400, Christoph Lameter wrote:
> On Mon, 16 Mar 2009, Mel Gorman wrote:
> 
> > +int gfp_zone_table[GFP_ZONEMASK] __read_mostly;
> 
> The gfp_zone_table is compile time determinable. There is no need to
> calculate it.
> 

The cost of calculating it is negligible and the code is then freed later
in boot. Does having a const table make any difference?

> const int gfp_zone_table[GFP_ZONEMASK] = {
> 	ZONE_NORMAL,		/* 00 No flags set */
> 	ZONE_DMA,		/* 01 Only GFP_DMA set */
> 	ZONE_HIGHMEM,		/* 02 Only GFP_HIGHMEM set */
> 	ZONE_DMA,		/* 03 GFP_HIGHMEM and GFP_DMA set */
> 	ZONE_DMA32,		/* 04 Only GFP_DMA32 set */
> 	ZONE_DMA,		/* 05 GFP_DMA and GFP_DMA32 set */
> 	ZONE_DMA32,		/* 06 GFP_DMA32 and GFP_HIGHMEM set */
> 	ZONE_DMA,		/* 07 GFP_DMA, GFP_DMA32 and GFP_DMA32 set */
> 	ZONE_MOVABLE,		/* 08 Only ZONE_MOVABLE set */
> 	ZONE_DMA,		/* 09 MOVABLE + DMA */
> 	ZONE_MOVABLE,		/* 0A MOVABLE + HIGHMEM */
> 	ZONE_DMA,		/* 0B MOVABLE + DMA + HIGHMEM */
> 	ZONE_DMA32,		/* 0C MOVABLE + DMA32 */
> 	ZONE_DMA,		/* 0D MOVABLE + DMA + DMA32 */
> 	ZONE_DMA32,		/* 0E MOVABLE + DMA32 + HIGHMEM */
> 	ZONE_DMA		/* 0F MOVABLE + DMA32 + HIGHMEM + DMA
> };
> 
> Hmmmm... Guess one would need to add some #ifdeffery here to setup
> ZONE_NORMAL in cases there is no DMA, DMA32 and HIGHMEM.
> 

Indeed, as I said, this is somewhat error prone which is why the patch
calculates the table at run-time instead of compile-time trickery.


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