linux-kernel - Re: [PATCH 3/7] mm: page_alloc: Use zone node IDs to approximate locality

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Date:	Tue, 17 Dec 2013 11:13:52 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/7] mm: page_alloc: Use zone node IDs to approximate
 locality

On Mon, Dec 16, 2013 at 03:25:07PM -0500, Johannes Weiner wrote:
> On Fri, Dec 13, 2013 at 02:10:03PM +0000, Mel Gorman wrote:
> > zone_local is using node_distance which is a more expensive call than
> > necessary. On x86, it's another function call in the allocator fast path
> > and increases cache footprint. This patch makes the assumption zones on a
> > local node will share the same node ID. The necessary information should
> > already be cache hot.
> > 
> > Signed-off-by: Mel Gorman <mgorman@...e.de>
> > ---
> >  mm/page_alloc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 64020eb..fd9677e 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1816,7 +1816,7 @@ static void zlc_clear_zones_full(struct zonelist *zonelist)
> >  
> >  static bool zone_local(struct zone *local_zone, struct zone *zone)
> >  {
> > -	return node_distance(local_zone->node, zone->node) == LOCAL_DISTANCE;
> > +	return zone_to_nid(zone) == numa_node_id();
> 
> Why numa_node_id()?  We pass in the preferred zone as @local_zone:
> 

Initially because I was thinking "local node" and numa_node_id() is a
per-cpu variable that should be cheap to access and in some cases
cache-hot as the top-level gfp API calls numa_node_id().

Thinking about it more though it still makes sense because the preferred
zone is not necessarily local. If the allocation request requires ZONE_DMA32
and the local node does not have that zone then preferred zone is on a
remote node.

-- 
Mel Gorman
SUSE Labs
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