Re: [PATCH] mm: memcg: Use larger chunks for proactive reclaim

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Jan 31, 2024 at 04:24:41PM +0000, T.J. Mercier wrote:
> Before 388536ac291 ("mm:vmscan: fix inaccurate reclaim during proactive
> reclaim") we passed the number of pages for the reclaim request directly
> to try_to_free_mem_cgroup_pages, which could lead to significant
> overreclaim in order to achieve fairness. After 0388536ac291 the number
> of pages was limited to a maxmimum of 32 (SWAP_CLUSTER_MAX) to reduce
> the amount of overreclaim. However such a small chunk size caused a
> regression in reclaim performance due to many more reclaim start/stop
> cycles inside memory_reclaim.
> 
> Instead of limiting reclaim chunk size to the SWAP_CLUSTER_MAX constant,
> adjust the chunk size proportionally with number of pages left to
> reclaim. This allows for higher reclaim efficiency with large chunk
> sizes during the beginning of memory_reclaim, and reduces the amount of
> potential overreclaim by using small chunk sizes as the total reclaim
> amount is approached. Using 1/4 of the amount left to reclaim as the
> chunk size gives a good compromise between reclaim performance and
> overreclaim:
> 
> root - full reclaim       pages/sec   time (sec)
> pre-0388536ac291      :    68047        10.46
> post-0388536ac291     :    13742        inf
> (reclaim-reclaimed)/4 :    67352        10.51
> 
> /uid_0 - 1G reclaim       pages/sec   time (sec)  overreclaim (MiB)
> pre-0388536ac291      :    258822       1.12            107.8
> post-0388536ac291     :    105174       2.49            3.5
> (reclaim-reclaimed)/4 :    233396       1.12            -7.4
> 
> /uid_0 - full reclaim     pages/sec   time (sec)
> pre-0388536ac291      :    72334        7.09
> post-0388536ac291     :    38105        14.45
> (reclaim-reclaimed)/4 :    72914        6.96
> 
> Fixes: 0388536ac291 ("mm:vmscan: fix inaccurate reclaim during proactive reclaim")
> Signed-off-by: T.J. Mercier <tjmercier@google.com>
> ---
>  mm/memcontrol.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 46d8d02114cf..d68fb89eadd2 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6977,7 +6977,8 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
>  			lru_add_drain_all();
>  
>  		reclaimed = try_to_free_mem_cgroup_pages(memcg,
> -					min(nr_to_reclaim - nr_reclaimed, SWAP_CLUSTER_MAX),
> +					max((nr_to_reclaim - nr_reclaimed) / 4,
> +					    (nr_to_reclaim - nr_reclaimed) % 4),

I don't see why the % 4 is needed. It only kicks in when the delta
drops below 4, but try_to_free_mem_cgroup_pages() already has

		.nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),

so it looks like dead code.