Re: [RFC PATCH] cgroup: introduce dynamic protection for memcg

[Zhaoyang Huang <huangzhaoyang@gmail.com>]

On Thu, Mar 31, 2022 at 7:35 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Thu 31-03-22 19:18:58, Zhaoyang Huang wrote:
> > On Thu, Mar 31, 2022 at 5:01 PM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Thu 31-03-22 16:00:56, zhaoyang.huang wrote:
> > > > From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> > > >
> > > > For some kind of memcg, the usage is varies greatly from scenarios. Such as
> > > > multimedia app could have the usage range from 50MB to 500MB, which generated
> > > > by loading an special algorithm into its virtual address space and make it hard
> > > > to protect the expanded usage without userspace's interaction.
> > >
> > > Do I get it correctly that the concern you have is that you do not know
> > > how much memory your workload will need because that depends on some
> > > parameters?
> > right. such as a camera APP will expand the usage from 50MB to 500MB
> > because of launching a special function(face beauty etc need special
> > algorithm)
> > >
> > > > Furthermore, fixed
> > > > memory.low is a little bit against its role of soft protection as it will response
> > > > any system's memory pressure in same way.
> > >
> > > Could you be more specific about this as well?
> > As the camera case above, if we set memory.low as 200MB to keep the
> > APP run smoothly, the system will experience high memory pressure when
> > another high load APP launched simultaneously. I would like to have
> > camera be reclaimed under this scenario.
>
> OK, so you effectivelly want to keep the memory protection when there is
> a "normal" memory pressure but want to relax the protection on other
> high memory utilization situations?
>
> How do you exactly tell a difference between a steady memory pressure
> (say stream IO on the page cache) from "high load APP launched"? Should
> you reduce the protection on the stram IO situation as well?
We can take either system's io_wait or PSI_IO into consideration for these.
>
> [...]
> > > One very important thing that I am missing here is the overall objective of this
> > > tuning. From the above it seems that you want to (ab)use memory->low to
> > > protect some portion of the charged memory and that the protection
> > > shrinks over time depending on the the global PSI metrict and time.
> > > But why this is a good thing?
> > 'Good' means it meets my original goal of keeping the usage during a
> > period of time and responding to the system's memory pressure. For an
> > android like system, memory is almost forever being in a tight status
> > no matter how many RAM it has. What we need from memcg is more than
> > control and grouping, we need it to be more responsive to the system's
> > load and could  sacrifice its usage  under certain criteria.
>
> Why existing tools/APIs are insufficient for that? You can watch for
> both global and memcg memory pressure including PSI metrics and update
> limits dynamically. Why is it necessary to put such a logic into the
> kernel?
Poll and then React method in userspace requires a polling interval
and response time. Take PSI as an example, it polls ten times during
POLLING_INTERVAL while just report once, which introduce latency in
some extend.
>
> --
> Michal Hocko
> SUSE Labs