Re: [PATCH] mm, memcg: do full scan initially in force_empty

[Michal Hocko <mhocko@suse.com>]

On Fri 31-07-20 09:50:04, Yafang Shao wrote:
> On Thu, Jul 30, 2020 at 7:26 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Tue 28-07-20 03:40:32, Yafang Shao wrote:
> > > Sometimes we use memory.force_empty to drop pages in a memcg to work
> > > around some memory pressure issues. When we use force_empty, we want the
> > > pages can be reclaimed ASAP, however force_empty reclaims pages as a
> > > regular reclaimer which scans the page cache LRUs from DEF_PRIORITY
> > > priority and finally it will drop to 0 to do full scan. That is a waste
> > > of time, we'd better do full scan initially in force_empty.
> >
> > Do you have any numbers please?
> >
> 
> Unfortunately the number doesn't improve obviously, while it is
> directly proportional to the numbers of total pages to be scanned.

Your changelog claims an optimization and that should be backed by some
numbers. It is true that reclaim at a higher priority behaves slightly
and subtly differently but that urge for even more details in the
changelog.

> But then I notice that force_empty will try to write dirty pages, that
> is not expected by us, because this behavior may be dangerous in the
> production environment.

I do not understand your claim here. Direct reclaim doesn't write dirty
page cache pages directly. And it is even less clear why that would be
dangerous if it did.

> What do you think introducing per memcg drop_cache ?

I do not like the global drop_cache and per memcg is not very much
different. This all shouldn't be really necessary because we do have
means to reclaim memory in a memcg.
-- 
Michal Hocko
SUSE Labs