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

T.J. Mercier
2024-02-02T23:39:13Z

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>

---
v3: Formatting fixes per Yosry Ahmed and Johannes Weiner. No functional
changes.
v2: Simplify the request size calculation per Johannes Weiner and Michal Koutný

mm/memcontrol.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 46d8d02114cf..f6ab61128869 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6976,9 +6976,11 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
if (!nr_retries)
lru_add_drain_all();

+ /* Will converge on zero, but reclaim enforces a minimum */
+ unsigned long batch_size = (nr_to_reclaim - nr_reclaimed) / 4;
+
reclaimed = try_to_free_mem_cgroup_pages(memcg,
- min(nr_to_reclaim - nr_reclaimed, SWAP_CLUSTER_MAX),
- GFP_KERNEL, reclaim_options);
+ batch_size, GFP_KERNEL, reclaim_options);

if (!reclaimed && !nr_retries--)
return -EAGAIN;
--
2.43.0.594.gd9cf4e227d-goog