Date: Wed, 12 Sep 2007 09:32:04 +0100
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Lee.Schermerhorn@hp.com, akpm@linux-foundation.org, ak@suse.de,
       clameter@sgi.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 4/6] Embed zone_id information within the zonelist->zones pointer
Message-ID: <20070912083203.GA5005@skynet.ie>
References: <20070911213006.23507.19569.sendpatchset@skynet.skynet.ie> <20070911213127.23507.34058.sendpatchset@skynet.skynet.ie> <20070912165138.5deb4db4.kamezawa.hiroyu@jp.fujitsu.com>
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From: mel@skynet.ie (Mel Gorman)
Sender: linux-kernel-owner@vger.kernel.org
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On (12/09/07 16:51), KAMEZAWA Hiroyuki didst pronounce:
> On Tue, 11 Sep 2007 22:31:27 +0100 (IST)
> Mel Gorman <mel@csn.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)
> ==
> 

That is an interesting idea. I'll try it out and see what happens.
Thanks

> 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.
> 

While this is true, it might not affect the size of struct zone because
of padding. One way to find out

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