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

David Rientjes
2009-04-22T20:24:43Z

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;

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