Re: [PATCH -v2 changelog updated] mm: do batched scans for mem_cgroup

[KOSAKI Motohiro]

2009/8/21 Wu Fengguang <fengguang.wu@xxxxxxxxx>:
> For mem_cgroup, shrink_zone() may call shrink_list() with nr_to_scan=1,
> in which case shrink_list() _still_ calls isolate_pages() with the much
> larger SWAP_CLUSTER_MAX.  It effectively scales up the inactive list
> scan rate by up to 32 times.
>
> For example, with 16k inactive pages and DEF_PRIORITY=12, (16k >> 12)=4.
> So when shrink_zone() expects to scan 4 pages in the active/inactive
> list, the active list will be scanned 4 pages, while the inactive list
> will be (over) scanned SWAP_CLUSTER_MAX=32 pages in effect. And that
> could break the balance between the two lists.
>
> It can further impact the scan of anon active list, due to the anon
> active/inactive ratio rebalance logic in balance_pgdat()/shrink_zone():
>
> inactive anon list over scanned => inactive_anon_is_low() == TRUE
>                                => shrink_active_list()
>                                => active anon list over scanned
>
> So the end result may be
>
> - anon inactive  => over scanned
> - anon active    => over scanned (maybe not as much)
> - file inactive  => over scanned
> - file active    => under scanned (relatively)
>
> The accesses to nr_saved_scan are not lock protected and so not 100%
> accurate, however we can tolerate small errors and the resulted small
> imbalanced scan rates between zones.
>

Looks good to me.
  Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
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