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SubjectRe: [PATCH 25/25] mm, compaction: Do not direct compact remote memory
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On 1/4/19 1:50 PM, Mel Gorman wrote:
> Remote compaction is expensive and possibly counter-productive. Locality
> is expected to often have better performance characteristics than remote
> high-order pages. For small allocations, it's expected that locality is
> generally required or fallbacks are possible. For larger allocations such
> as THP, they are forbidden at the time of writing but if __GFP_THISNODE
> is ever removed, then it would still be preferable to fallback to small
> local base pages over remote THP in the general case. kcompactd is still
> woken via kswapd so compaction happens eventually.
>
> While this patch potentially has both positive and negative effects,
> it is best to avoid the possibility of remote compaction given the cost
> relative to any potential benefit.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Generally agree with the intent, but what if there's e.g. high-order (but not
costly) kernel allocation on behalf of user process on cpu belonging to a
movable node, where the only non-movable node is node 0. It will have to keep
reclaiming until a large enough page is formed, or wait for kcompactd?
So maybe do this only for costly orders?

Also I think compaction_zonelist_suitable() should be also updated, or we might
be promising the reclaim-compact loop e.g. that we will compact after enough
reclaim, but then we won't.

> ---
> mm/compaction.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index ae70be023b21..cc17f0c01811 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2348,6 +2348,16 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
> continue;
> }
>
> + /*
> + * Do not compact remote memory. It's expensive and high-order
> + * small allocations are expected to prefer or require local
> + * memory. Similarly, larger requests such as THP can fallback
> + * to base pages in preference to remote huge pages if
> + * __GFP_THISNODE is not specified
> + */
> + if (zone_to_nid(zone) != zone_to_nid(ac->preferred_zoneref->zone))
> + continue;
> +
> status = compact_zone_order(zone, order, gfp_mask, prio,
> alloc_flags, ac_classzone_idx(ac), capture);
> rc = max(status, rc);
>

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