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@xxxxxxxxxx> > --- > 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.