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

Zhaoyang Huang
2022-04-02T05:18:10Z

On Fri, Apr 1, 2022 at 7:34 PM Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org> wrote:
>
> On Fri 01-04-22 09:34:02, Zhaoyang Huang wrote:
> > On Thu, Mar 31, 2022 at 7:35 PM Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org> wrote:
> > >
> > > On Thu 31-03-22 19:18:58, Zhaoyang Huang wrote:
> > > > On Thu, Mar 31, 2022 at 5:01 PM Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org> wrote:
> > > > >
> > > > > On Thu 31-03-22 16:00:56, zhaoyang.huang wrote:
> > > > > > From: Zhaoyang Huang <zhaoyang.huang-1tVvrHeaX6nQT0dZR+AlfA@public.gmane.org>
> > > > > >
> > > > > > 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.
>
> I do not follow. Let's say you have a stream IO workload which is mostly
> RO. Reclaiming those pages means effectivelly to drop them from the
> cache so there is no IO involved during the reclaim. This will generate
> a constant flow of reclaim that shouldn't normally affect other
> workloads (as long as kswapd keeps up with the IO pace). How does your
> scheme cope with this scenario? My understanding is that it will simply
> relax the protection.
You are right. This scheme treats the system's memory pressure
equally, no matter if it comes from in-kernel page allocation with
high order or cache drop by IO like things. The decay_factor composed
of PSI_SOME and PSI_FULL which represent the system is tight on
memory, every entity has the obligation to donate to solve this issue.
>
> > > [...]
> > > > > 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.
>
> Do workload transitions happen so often in your situation that the
> interval really matters? As Suren already pointed out starting a new
> application is usually an explicit event which can pro-activelly update
> limits.
Yes. As my reply to Suren's comment, even a positive monitor service
which could be aware of the activity starting(APP launching etc) at
the very first time, has to 1. read PSI and memcg->watermark/usage 2.
make a decision. 3. write memcg->memory.low to adjust memory
allowance. Furthermore, monitors could not supervise the APP for whole
life time, while the reclaiming could arise at any time.

> --
> Michal Hocko
> SUSE Labs