mm: memcg: use larger batches for proactive reclaim

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.  The
restart cost is amortized over more pages with larger batch sizes, and
becomes a significant component of the runtime if the batch size is too
small.

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.

MGLRU enabled - memcg LRU used
root - full reclaim       pages/sec   time (sec)
pre-0388536ac291      :    68047        10.46
post-0388536ac291     :    13742        inf
(reclaim-reclaimed)/4 :    67352        10.51

MGLRU enabled - memcg LRU not used
/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

MGLRU enabled - memcg LRU not used
/uid_0 - full reclaim     pages/sec   time (sec)
pre-0388536ac291      :    72334        7.09
post-0388536ac291     :    38105        14.45
(reclaim-reclaimed)/4 :    72914        6.96

[[email protected]: v4]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 0388536ac291 ("mm:vmscan: fix inaccurate reclaim during proactive reclaim")
Signed-off-by: T.J. Mercier <[email protected]>
Reviewed-by: Yosry Ahmed <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Reviewed-by: Michal Koutny <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Efly Young <[email protected]>
Cc: Yu Zhao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions