mm: vmscan: only write dirty pages that the scanner has seen twice
Dirty pages can easily reach the end of the LRU while there are still clean pages to reclaim around. Don't let kswapd write them back just because there are a lot of them. It costs more CPU to find the clean pages, but that's almost certainly better than to disrupt writeback from the flushers with LRU-order single-page writes from reclaim. And the flushers have been woken up by that point, so we spend IO capacity on flushing and CPU capacity on finding the clean cache. Only start writing dirty pages if they have cycled around the LRU twice now and STILL haven't been queued on the IO device. It's possible that the dirty pages are so sparsely distributed across different bdis, inodes, memory cgroups, that the flushers take forever to get to the ones we want reclaimed. Once we see them twice on the LRU, we know that's the quicker way to find them, so do LRU writeback. Link: http://lkml.kernel.org/r/20170123181641.23938-5-hannes@cmpxchg.org Signed-off-by:Johannes Weiner <hannes@cmpxchg.org> Acked-by:
Minchan Kim <minchan@kernel.org> Acked-by:
Michal Hocko <mhocko@suse.com> Acked-by:
Mel Gorman <mgorman@suse.de> Acked-by:
Hillf Danton <hillf.zj@alibaba-inc.com> Cc: Rik van Riel <riel@redhat.com> Signed-off-by:
Andrew Morton <akpm@linux-foundation.org> Signed-off-by:
Linus Torvalds <torvalds@linux-foundation.org>
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