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

[Yafang Shao <laoar.shao@gmail.com>]

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.

Thanks
Yafang