Re: [PATCH 4/4] Memory controller soft limit reclaim on contention (v5)

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> * KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> [2009-03-13 13:50:26]:
> 
> > > > > I have two objection to this.
> > > > > 
> > > > > - "if (!order || !did_some_progress)" mean no call try_to_free_pages()
> > > > >   in order>0 and did_some_progress>0 case.
> > > > >   but mem_cgroup_soft_limit_reclaim() don't have lumpy reclaim.
> > > > >   then, it break high order reclaim.
> > > > 
> > > > I am sending a fix for this right away. Thanks, the check should be
> > > > if (order || !did_some_progress)
> > > 
> > > No.
> > > 
> > > it isn't enough.
> > > after is does, order-1 allocation case twrice reclaim (soft limit shrinking
> > > and normal try_to_free_pages()).
> > > then, order-1 reclaim makes slower about 2 times.
> > > 
> > > unfortunately, order-1 allocation is very frequent. it is used for
> > > kernel stack.
> > 
> > in normal order-1 reclaim is:
> > 
> > 1. try_to_free_pages()
> > 2. get_page_from_freelist()
> > 3. retry if order-1 page don't exist
> > 
> > Coundn't you use the same logic?
> 
> Sorry, forgot to answer this question earlier.
> 
> I assume that by order-1 you mean 2 pages (2^1).

Yes, almost architecutre's kernel stack use 2 pages.


> Are you suggesting that if soft limit reclaim fails, we retry? Not
> sure if I understand your suggestion completely.

sorry. my last explanation is too poor.

if we need only 2pages, soft_limit_shrink() typically success
reclaim it.
then

1. mem_cgroup_soft_limit_reclaim()
2. get_page_from_freelist() if mem_cgroup_soft_limit_reclaim() return >0.
3. goto got_pg if if get_page_from_freelist() is successed.
4. try_to_free_pages()
5. get_page_from_freelist() if try_to_free_pages() return >0.
6. goto 1 if order-1 page don't exist

obiously, this logic make slower more higher order because
(2) is often fail in higher order.
but that's ok. higher order reclaim is very slow. additional
overhead don't observe (maybe).

What do you think?



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