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

[Michal Hocko]

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