Re: [PATCH 1/4] Add kswapd descriptor.

[Ying Han <yinghan@google.com>]

On Tue, Nov 30, 2010 at 12:54 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Tue, 30 Nov 2010 17:27:10 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>
>> On Tue, 30 Nov 2010 17:15:37 +0900
>> Minchan Kim <minchan.kim@gmail.com> wrote:
>>
>> > Ideally, I hope we unify global and memcg of kswapd for easy
>> > maintainance if it's not a big problem.
>> > When we make patches about lru pages, we always have to consider what
>> > I should do for memcg.
>> > And when we review patches, we also should consider what the patch is
>> > missing for memcg.
>> > It makes maintainance cost big. Of course, if memcg maintainers is
>> > involved with all patches, it's no problem as it is.
>> >
>> I know it's not. But thread control of kswapd will not have much merging point.
>> And balance_pgdat() is fully replaced in patch/3. The effort for merging seems
>> not big.

I intended to separate out the logic of per-memcg kswapd logics and
not having it
interfere with existing code. This should help for merging.

>>
>
> kswapd's balance_pgdat() is for following
>  - reclaim pages within a node.
>  - balancing zones in a pgdat.
>
> memcg's background reclaim needs followings.
>  - reclaim pages within a memcg
>  - reclaim pages from arbitrary zones, if it's fair, it's good.
>    But it's not important from which zone the pages are reclaimed from.
>    (I'm not sure we can select "the oldest" pages from divided LRU.)

The current implementation is simple, which it iterates all the nodes
and reclaims pages from the per-memcg-per-zone LRU. As long as the
wmarks is ok, the kswapd is done. Meanwhile, in order to not wasting
cputime on "unreclaimable: nodes ( a node is unreclaimable if all the
zones are unreclaimable), I used the nodemask to record that from the
last scan, and the bit is reset as long as a page is returned back.
This is a similar logic used in the global kswapd.

A potential improvement is to remember the last node we reclaimed
from, and starting from the next node for the next kswapd wake_up.
This avoids the case all the memcg kswapds are reclaiming from the
small node ids on large numa machines.

>
> Then, merging will put 2 _very_ different functionalities into 1 function