Re: [PATCH v2] mm: memcg: Use larger batches for proactive reclaim

[Yosry Ahmed]

On Fri, Feb 2, 2024 at 2:10 PM T.J. Mercier <tjmercier@xxxxxxxxxx> 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. After 0388536ac291 the number of pages was limited to a
> maximum 32 (SWAP_CLUSTER_MAX) to reduce the amount of overreclaim.
> However such a small batch size caused a regression in reclaim
> performance due to many more reclaim start/stop cycles inside
> memory_reclaim.
>
> Reclaim tries to balance nr_to_reclaim fidelity with fairness across
> nodes and cgroups over which the pages are spread. As such, the bigger
> the request, the bigger the absolute overreclaim error. Historic
> in-kernel users of reclaim have used fixed, small sized requests to
> approach an appropriate reclaim rate over time. When we reclaim a user
> request of arbitrary size, use decaying batch sizes to manage error while
> maintaining reasonable throughput.
>
> 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@xxxxxxxxxx>

LGTM with a nit below:
Reviewed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>

>
> ---
> v2: Simplify the request size calculation per Johannes Weiner and Michal Koutný
>
>  mm/memcontrol.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 46d8d02114cf..e6f921555e07 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6965,6 +6965,9 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
>         while (nr_reclaimed < nr_to_reclaim) {
>                 unsigned long reclaimed;
>
> +               /* Will converge on zero, but reclaim enforces a minimum */
> +               unsigned long batch_size = (nr_to_reclaim - nr_reclaimed) / 4;
> +
>                 if (signal_pending(current))
>                         return -EINTR;
>
> @@ -6977,7 +6980,7 @@ 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),
> +                                       batch_size,
>                                         GFP_KERNEL, reclaim_options);

I think the above two lines should now fit into one.