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SubjectRe: [PATCH 1/2] mm: Disable zone_reclaim_mode by default
On 04/08/2014 06:34 AM, Mel Gorman wrote:
> zone_reclaim_mode causes processes to prefer reclaiming memory from local
> node instead of spilling over to other nodes. This made sense initially when
> NUMA machines were almost exclusively HPC and the workload was partitioned
> into nodes. The NUMA penalties were sufficiently high to justify reclaiming
> the memory. On current machines and workloads it is often the case that
> zone_reclaim_mode destroys performance but not all users know how to detect
> this. Favour the common case and disable it by default. Users that are
> sophisticated enough to know they need zone_reclaim_mode will detect it.
>
> Signed-off-by: Mel Gorman <mgorman@suse.de>

Reviewed-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>

> ---
> Documentation/sysctl/vm.txt | 17 +++++++++--------
> mm/page_alloc.c | 2 --
> 2 files changed, 9 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> index d614a9b..ff5da70 100644
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -751,16 +751,17 @@ This is value ORed together of
> 2 = Zone reclaim writes dirty pages out
> 4 = Zone reclaim swaps pages
>
> -zone_reclaim_mode is set during bootup to 1 if it is determined that pages
> -from remote zones will cause a measurable performance reduction. The
> -page allocator will then reclaim easily reusable pages (those page
> -cache pages that are currently not used) before allocating off node pages.
> -
> -It may be beneficial to switch off zone reclaim if the system is
> -used for a file server and all of memory should be used for caching files
> -from disk. In that case the caching effect is more important than
> +zone_reclaim_mode is disabled by default. For file servers or workloads
> +that benefit from having their data cached, zone_reclaim_mode should be
> +left disabled as the caching effect is likely to be more important than
> data locality.
>
> +zone_reclaim may be enabled if it's known that the workload is partitioned
> +such that each partition fits within a NUMA node and that accessing remote
> +memory would cause a measurable performance reduction. The page allocator
> +will then reclaim easily reusable pages (those page cache pages that are
> +currently not used) before allocating off node pages.
> +
> Allowing zone reclaim to write out pages stops processes that are
> writing large amounts of data from dirtying pages on other nodes. Zone
> reclaim will write out dirty pages if a zone fills up and so effectively
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3bac76a..a256f85 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1873,8 +1873,6 @@ static void __paginginit init_zone_allows_reclaim(int nid)
> for_each_online_node(i)
> if (node_distance(nid, i) <= RECLAIM_DISTANCE)
> node_set(i, NODE_DATA(nid)->reclaim_nodes);
> - else
> - zone_reclaim_mode = 1;
> }
>
> #else /* CONFIG_NUMA */
>


--
Thanks.
Zhang Yanfei


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