On Mon, Jan 23, 2017 at 01:16:40PM -0500, Johannes Weiner wrote:

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.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Mel Gorman <mgorman@suse.de>

-- 
Mel Gorman
SUSE Labs

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