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Date: Tue, 18 Sep 2012 14:03:13 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mgorman@suse.de>, Minchan Kim <minchan@kernel.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [patch] mm, numa: reclaim from all nodes within reclaim
 distance
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On Tue, 18 Sep 2012 00:03:57 -0700 (PDT)
David Rientjes <rientjes@google.com> wrote:

> RECLAIM_DISTANCE represents the distance between nodes at which it is
> deemed too costly to allocate from; it's preferred to try to reclaim from
> a local zone before falling back to allocating on a remote node with such
> a distance.
> 
> To do this, zone_reclaim_mode is set if the distance between any two
> nodes on the system is greather than this distance.  This, however, ends
> up causing the page allocator to reclaim from every zone regardless of
> its affinity.
> 
> What we really want is to reclaim only from zones that are closer than 
> RECLAIM_DISTANCE.  This patch adds a nodemask to each node that
> represents the set of nodes that are within this distance.  During the
> zone iteration, if the bit for a zone's node is set for the local node,
> then reclaim is attempted; otherwise, the zone is skipped.

Is this a theoretical thing, or does the patch have real observable
effects?

This change makes it more important that the arch code implements
node_distance() accurately (wrt RECLAIM_DISTANCE), yes?  I wonder how
much code screwed that up, and what the effects of such a screwup would
be, and how arch maintainers would go about detecting then fixing such
an error?