Re: [BUG] The usage of memory cgroup is not consistent with processes when using THP

[Yang Shi <shy828301@gmail.com>]

On Tue, Sep 28, 2021 at 12:15 AM 台运方 <yunfangtai09@gmail.com> wrote:
>
> Yang Shi <shy828301@gmail.com> 于2021年9月28日周二 上午1:28写道：
> > IMHO I don't think this is a bug. The disparity reflects the
> > difference in how the page life cycle is viewed between process and
> > cgroup. The usage of process comes from the rss_counter of mm. It
> > tracks the per-process mapped memory usage. So it is updated once the
> > page is zapped.
> >
> > But from the point of cgroup, the page is charged when it is allocated
> > and uncharged when it is freed. The page may be zapped by one process,
> > but there might be other users pin the page to prevent it from being
> > freed. The pin may be very transient or may be indefinite. THP is one
> > of the pins. It is gone when the THP is split, but the split may
> > happen a long time after the page is zapped due to deferred split.
> Thank you for reply. I agree that it reflects the difference between
> process and cgroup. The memory usage of cgroup is usually used to
> indicate the memory usage of the container. It can be used to avoid
> the OOM and etc. The disparity will cause that the memory usage of
> containers with the same processes are randomly different (we found
> more than 30GB different). It is hard to manage them. Of course,
> disable THP is a way to solve it. Can it have another way to solve it
> ?

I don't quite get what exactly you want to manage. If you want to get
rid of the disparity, I don't have good idea other than splitting THP
in place instead of using deferred split. But AFAIK it is not quite
feasible due to some locking problems.