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@xxxxxxxxxxxxxxxxxxx> 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); >
