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Date:   Thu, 26 Jan 2017 10:08:04 +0000
From:   Mel Gorman <>
To:     Johannes Weiner <>
Cc:     Andrew Morton <>,,,
Subject: Re: [PATCH 4/5] mm: vmscan: only write dirty pages that the scanner
 has seen twice

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 <>

Acked-by: Mel Gorman <>

Mel Gorman

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