On Mon 23-01-17 13:16:40, 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: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/vmscan.c | 15 ++++++++++-----
> 1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 915fc658de41..df0fe0cc438e 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1153,13 +1153,18 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>
> if (PageDirty(page)) {
> /*
> - * Only kswapd can writeback filesystem pages to
> - * avoid risk of stack overflow but only writeback
> - * if many dirty pages have been encountered.
> + * Only kswapd can writeback filesystem pages
> + * to avoid risk of stack overflow. But avoid
> + * injecting inefficient single-page IO into
> + * flusher writeback as much as possible: only
> + * write pages when we've encountered many
> + * dirty pages, and when we've already scanned
> + * the rest of the LRU for clean pages and see
> + * the same dirty pages again (PageReclaim).
> */
> if (page_is_file_cache(page) &&
> - (!current_is_kswapd() ||
> - !test_bit(PGDAT_DIRTY, &pgdat->flags))) {
> + (!current_is_kswapd() || !PageReclaim(page) ||
> + !test_bit(PGDAT_DIRTY, &pgdat->flags))) {
> /*
> * Immediately reclaim when written back.
> * Similar in principal to deactivate_page()
> --
> 2.11.0
>
> --
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--
Michal Hocko
SUSE Labs
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