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

[David Rientjes]

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@xxxxxxxxx>
> Reviewed-by: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> ---
>  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
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