Re: [PATCH v3] mm: memcg: Use larger batches for proactive reclaim

Shakeel Butt
2024-02-04T16:17:39Z

On Fri, Feb 2, 2024 at 3:39 PM T.J. Mercier <tjmercier@google.com> 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@google.com>
> Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>

Acked-by: Shakeel Butt <shakeelb@google.com>