linux-kernel - Re: [PATCH 16/22] Do not setup zonelist cache when there is only one node

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Message-ID: <20090423001907.GB26643@csn.ul.ie>
Date:	Thu, 23 Apr 2009 01:19:08 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.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>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 16/22] Do not setup zonelist cache when there is only
	one node

On Wed, Apr 22, 2009 at 01:24:26PM -0700, David Rientjes wrote:
> On Wed, 22 Apr 2009, Mel Gorman wrote:
> 
> > There is a zonelist cache which is used to track zones that are not in
> > the allowed cpuset or found to be recently full. This is to reduce cache
> > footprint on large machines. On smaller machines, it just incurs cost
> > for no gain. This patch only uses the zonelist cache when there are NUMA
> > nodes.
> > 
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > Reviewed-by: Christoph Lameter <cl@...ux-foundation.org>
> > ---
> >  mm/page_alloc.c |    7 +++++--
> >  1 files changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 7f45de1..e59bb80 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1467,8 +1467,11 @@ this_zone_full:
> >  		if (NUMA_BUILD)
> >  			zlc_mark_zone_full(zonelist, z);
> 
> If zonelist caching is never used for UMA machines, why should they ever 
> call zlc_mark_zone_full()?  It will always dereference 
> zonelist->zlcache_ptr and immediately return without doing anything.
> 
> Wouldn't it better to just add
> 
> 	if (num_online_nodes() == 1)
> 		continue;
> 

num_online_nodes() is actually a really heavy function. It calls hweight
on a bitmap which is probably why it's not happening already. There is a
nr_online_nodes later in the patchset though. With nr_online_nodes, it's
a good idea to avoid a function call so I've taken note to do that patch
in pass 2.

Thanks

> right before this call to zlc_mark_zone_full()?  This should compile out 
> the remainder of the loop for !CONFIG_NUMA kernels anyway.
> 
> >  try_next_zone:
> > -		if (NUMA_BUILD && !did_zlc_setup) {
> > -			/* we do zlc_setup after the first zone is tried */
> > +		if (NUMA_BUILD && !did_zlc_setup && num_online_nodes() > 1) {
> > +			/*
> > +			 * we do zlc_setup after the first zone is tried but only
> > +			 * if there are multiple nodes make it worthwhile
> > +			 */
> >  			allowednodes = zlc_setup(zonelist, alloc_flags);
> >  			zlc_active = 1;
> >  			did_zlc_setup = 1;
> > -- 
> > 1.5.6.5
> 

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