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

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, Aug 20, 2009 at 1:05 PM, Wu Fengguang<fengguang.wu@intel.com> wrote:
> On Thu, Aug 20, 2009 at 11:13:47AM +0800, KAMEZAWA Hiroyuki wrote:
>> On Thu, 20 Aug 2009 10:49:29 +0800
>> Wu Fengguang <fengguang.wu@intel.com> wrote:
>>
>> > 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, it will be scanned SWAP_CLUSTER_MAX=32 pages in effect.
>> >
>> > 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.
>> >
>> > This batching won't blur up the cgroup limits, since it is driven by
>> > "pages reclaimed" rather than "pages scanned". When shrink_zone()
>> > decides to cancel (and save) one smallish scan, it may well be called
>> > again to accumulate up nr_saved_scan.
>> >
>> > It could possibly be a problem for some tiny mem_cgroup (which may be
>> > _full_ scanned too much times in order to accumulate up nr_saved_scan).
>> >
>> > CC: Rik van Riel <riel@redhat.com>
>> > CC: Minchan Kim <minchan.kim@gmail.com>
>> > CC: Balbir Singh <balbir@linux.vnet.ibm.com>
>> > CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
>> > CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

It looks better than now :)

I hope you will rewrite description and add test result in changelog. :)
Thanks for your great effort.

-- 
Kind regards,
Minchan Kim

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