Re: [PATCH v2] mm, memcg: skip killing processes under memcg protection at first scan

[Michal Hocko <mhocko@suse.com>]

On Wed 21-08-19 16:15:54, Yafang Shao wrote:
> On Wed, Aug 21, 2019 at 4:05 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Wed 21-08-19 15:26:56, Yafang Shao wrote:
> > > On Wed, Aug 21, 2019 at 2:44 PM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > On Wed 21-08-19 09:00:39, Yafang Shao wrote:
> > > > [...]
> > > > > More possible OOMs is also a strong side effect (and it prevent us
> > > > > from using it).
> > > >
> > > > So why don't you use low limit if the guarantee side of min limit is too
> > > > strong for you?
> > >
> > > Well, I don't know what the best-practice of memory.min is.
> >
> > It is really a workload reclaim protection. Say you have a memory
> > consumer which performance characteristics would be noticeably disrupted
> > by any memory reclaim which then would lead to SLA disruption. This is a
> > strong requirement/QoS feature and as such comes with its demand on
> > configuration.
> >
> > > In our plan, we want to use it to protect the top priority containers
> > > (e.g. set the memory.min same with memory limit), which may latency
> > > sensive. Using memory.min may sometimes decrease the refault.
> > > If we set it too low, it may useless, becasue what memory.min is
> > > protecting is not specified. And if there're some busrt anon memory
> > > allocate in this memcg, the memory.min may can't protect any file
> > > memory.
> >
> > I am still not seeing why you are considering guarantee (memory.min)
> > rather than best practice (memory.low) here?
> 
> Let me show some examples for you.
> Suppose we have three containers with different priorities, which are
> high priority, medium priority and low priority.
> Then we set memory.low to these containers as bellow,
> high prioirty: memory.low same with memory.max
> medium priroity: memory.low is 50% of memory.max
> low priority: memory.low is 0
> 
> When all relcaimable pages withouth protection are reclaimed, the
> reclaimer begins to reclaim the protected pages, but unforuantely it
> desn't know which pages are belonging to high priority container and
> which pages are belonging to medium priority container. So the
> relcaimer may reclaim the high priority contianer first, and without
> reclaiming the medium priority container at all.

Hmm, it is hard to comment on this configuration without knowing what is
the overall consumption of all the three. In any case reclaiming all of
the reclaimable memory means that you have actually reclaimed full of
the low and half of the medium container to even start hitting on high
priority one. When there are only low priority protected containers then
they will get reclaimed proportionally to their size.
-- 
Michal Hocko
SUSE Labs