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SubjectRe: [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@csn.ul.ie>
> > Reviewed-by: Christoph Lameter <cl@linux-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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