Re: [External] Re: [PATCH] mm/memcontrol: Add the drop_cache interface for cgroup v2

[Shakeel Butt <shakeelb@google.com>]

On Tue, Sep 22, 2020 at 5:37 AM Chunxin Zang <zangchunxin@bytedance.com> wrote:
>
> On Tue, Sep 22, 2020 at 6:42 PM Chris Down <chris@chrisdown.name> wrote:
> >
> > Chunxin Zang writes:
> > >On Tue, Sep 22, 2020 at 5:51 PM Chris Down <chris@chrisdown.name> wrote:
> > >>
> > >> Chunxin Zang writes:
> > >> >My usecase is that there are two types of services in one server. They
> > >> >have difference
> > >> >priorities. Type_A has the highest priority, we need to ensure it's
> > >> >schedule latency、I/O
> > >> >latency、memory enough. Type_B has the lowest priority, we expect it
> > >> >will not affect
> > >> >Type_A when executed.
> > >> >So Type_A could use memory without any limit. Type_B could use memory
> > >> >only when the
> > >> >memory is absolutely sufficient. But we cannot estimate how much
> > >> >memory Type_B should
> > >> >use. Because everything is dynamic. So we can't set Type_B's memory.high.
> > >> >
> > >> >So we want to release the memory of Type_B when global memory is
> > >> >insufficient in order
> > >> >to ensure the quality of service of Type_A . In the past, we used the
> > >> >'force_empty' interface
> > >> >of cgroup v1.
> > >>
> > >> This sounds like a perfect use case for memory.low on Type_A, and it's pretty
> > >> much exactly what we invented it for. What's the problem with that?
> > >
> > >But we cannot estimate how much memory Type_A uses at least.
> >
> > memory.low allows ballparking, you don't have to know exactly how much it uses.
> > Any amount of protection biases reclaim away from that cgroup.
> >
> > >For example:
> > >total memory: 100G
> > >At the beginning, Type_A was in an idle state, and it only used 10G of memory.
> > >The load is very low. We want to run Type_B to avoid wasting machine resources.
> > >When Type_B runs for a while, it used 80G of memory.
> > >At this time Type_A is busy, it needs more memory.
> >
> > Ok, so set memory.low for Type_A close to your maximum expected value.
>
> Please forgive me for not being able to understand why setting
> memory.low for Type_A can solve the problem.
> In my scene, Type_A is the most important, so I will set 100G to memory.low.
> But 'memory.low' only takes effect passively when the kernel is
> reclaiming memory. It means that reclaim Type_B's memory only when
> Type_A  in alloc memory slow path. This will affect Type_A's
> performance.
> We want to reclaim Type_B's memory in advance when A is expected to be busy.
>

How will you know when to reclaim from B? Are you polling /proc/meminfo?

From what I understand, you want to proactively reclaim from B, so
that A does not go into global reclaim and in the worst case kill B,
right?

BTW you can use memory.high to reclaim from B by setting it lower than
memory.current of B and reset it to 'max' once the reclaim is done.
Since 'B' is not high priority (I am assuming not a latency sensitive
workload), B hitting temporary memory.high should not be an issue.
Also I am assuming you don't much care about the amount of memory to
be reclaimed from B, so I think memory.high can fulfil your use-case.
However if in future you decide to proactively reclaim from all the
jobs based on their priority i.e. more aggressive reclaim from B and a
little bit reclaim from A then memory.high is not a good interface.

Shakeel