Re: [PATCH 4/5] mm: vmscan: only write dirty pages that the scanner has seen twice

[Minchan Kim]

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@xxxxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>