On Fri, Feb 10, 2017 at 06:23:41PM +0100, Vlastimil Babka wrote: > The main goal of direct compaction is to form a high-order page for > allocation, > but it should also help against long-term fragmentation when possible. Most > lower-than-pageblock-order compactions are for non-movable allocations, which > means that if we compact in a movable pageblock and terminate as soon as we > create the high-order page, it's unlikely that the fallback heuristics will > claim the whole block. Instead there might be a single unmovable page in a > pageblock full of movable pages, and the next unmovable allocation might pick > another pageblock and increase long-term fragmentation. > > To help against such scenarios, this patch changes the termination criteria > for > compaction so that the current pageblock is finished even though the > high-order > page already exists. Note that it might be possible that the high-order page > formed elsewhere in the zone due to parallel activity, but this patch doesn't > try to detect that. > > This is only done with sync compaction, because async compaction is limited to > pageblock of the same migratetype, where it cannot result in a migratetype > fallback. (Async compaction also eagerly skips order-aligned blocks where > isolation fails, which is against the goal of migrating away as much of the > pageblock as possible.) > > As a result of this patch, long-term memory fragmentation should be reduced. > > Signed-off-by: Vlastimil Babka <vba...@suse.cz>
Acked-by: Johannes Weiner <han...@cmpxchg.org>
