linux-kernel - Re: [PATCH 4/6] Embed zone_id information within the zonelist->zones pointer

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Message-Id: <20070912165138.5deb4db4.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 12 Sep 2007 16:51:38 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Lee.Schermerhorn@...com, akpm@...ux-foundation.org, ak@...e.de,
	clameter@....com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 4/6] Embed zone_id information within the
 zonelist->zones pointer

On Tue, 11 Sep 2007 22:31:27 +0100 (IST)
Mel Gorman <mel@....ul.ie> wrote:

> Using two zonelists per node requires very frequent use of zone_idx(). This
> is costly as it involves a lookup of another structure and a substraction
> operation. As struct zone is always word aligned and normally cache-line
> aligned, the pointer values have a number of 0's at the least significant
> bits of the address.
> 
> This patch embeds the zone_id of a zone in the zonelist->zones pointers.
> The real zone pointer is retrieved using the zonelist_zone() helper function.
> The ID of the zone is found using zonelist_zone_idx().  To avoid accidental
> references, the zones field is renamed to _zones and the type changed to
> unsigned long.
> 
At first, I welcome this patch. thanks!.


A comment after reading all is, how about defining zonelist as following
instead of encoding in pointer ?
==
struct zone_pointer {
	struct zone *zone;
	int	     node_id;  
	int	     zone_idx;
};

struct zonelist {
	struct zone_pointer	_zones[MAX_ZONES_PER_ZONELIST + 1];
};

#define zonelist_zone(zl)      (zl)->zone
#define zonelist_zone_idx(zl)  (zl)->zone_idx
#ifdef CONFIG_NUMA
#define zonelist_zone_nid(zl)  (zl)->node_id
#else
#define zonelist_zone_nid(zl, i)  (0)
==

If we really want to avoid unnecessary access to "zone" while walking zonelist,
above may do something good.  Cons is this makes sizeof zonlist bigger.

Thanks,
-Kame

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