Re: [PATCH] mm/memcg: support control THP behaviour in cgroup

[CGEL <cgel.zte@gmail.com>]

On Tue, May 10, 2022 at 12:34:20PM -0700, Yang Shi wrote:
> On Mon, May 9, 2022 at 6:43 PM CGEL <cgel.zte@gmail.com> wrote:
> >
> > On Mon, May 09, 2022 at 01:48:39PM +0200, Michal Hocko wrote:
> > > On Mon 09-05-22 11:26:43, CGEL wrote:
> > > > On Mon, May 09, 2022 at 12:00:28PM +0200, Michal Hocko wrote:
> > > > > On Sat 07-05-22 02:05:25, CGEL wrote:
> > > > > [...]
> > > > > > If there are many containers to run on one host, and some of them have high
> > > > > > performance requirements, administrator could turn on thp for them:
> > > > > > # docker run -it --thp-enabled=always
> > > > > > Then all the processes in those containers will always use thp.
> > > > > > While other containers turn off thp by:
> > > > > > # docker run -it --thp-enabled=never
> > > > >
> > > > > I do not know. The THP config space is already too confusing and complex
> > > > > and this just adds on top. E.g. is the behavior of the knob
> > > > > hierarchical? What is the policy if parent memcg says madivise while
> > > > > child says always? How does the per-application configuration aligns
> > > > > with all that (e.g. memcg policy madivise but application says never via
> > > > > prctl while still uses some madvised - e.g. via library).
> > > > >
> > > >
> > > > The cgroup THP behavior is align to host and totally independent just likes
> > > > /sys/fs/cgroup/memory.swappiness. That means if one cgroup config 'always'
> > > > for thp, it has no matter with host or other cgroup. This make it simple for
> > > > user to understand or control.
> > >
> > > All controls in cgroup v2 should be hierarchical. This is really
> > > required for a proper delegation semantic.
> > >
> >
> > Could we align to the semantic of /sys/fs/cgroup/memory.swappiness?
> > Some distributions like Ubuntu is still using cgroup v1.
> 
> Other than enable flag, how would you handle the defrag flag
> hierarchically? It is much more complicated.

Refer to memory.swappiness for cgroup, this new interface better be independent.
> >
> > > > If memcg policy madivise but application says never, just like host, the result
> > > > is no THP for that application.
> > > >
> > > > > > By doing this we could promote important containers's performance with less
> > > > > > footprint of thp.
> > > > >
> > > > > Do we really want to provide something like THP based QoS? To me it
> > > > > sounds like a bad idea and if the justification is "it might be useful"
> > > > > then I would say no. So you really need to come with a very good usecase
> > > > > to promote this further.
> > > >
> > > > At least on some 5G(communication technology) machine, it's useful to provide
> > > > THP based QoS. Those 5G machine use micro-service software architecture, in
> > > > other words one service application runs in one container.
> > >
> > > I am not really sure I understand. If this is one application per
> > > container (cgroup) then why do you really need per-group setting?
> > > Does the application is a set of different processes which are only very
> > > loosely tight?
> > >
> > For micro-service architecture, the application in one container is not a
> > set of loosely tight processes, it's aim at provide one certain service,
> > so different containers means different service, and different service
> > has different QoS demand.
> >
> > The reason why we need per-group(per-container) setting is because most
> > container are managed by compose software, the compose software provide
> > UI to decide how to run a container(likes setting swappiness value). For
> > example the docker compose:
> > https://docs.docker.com/compose/#compose-v2-and-the-new-docker-compose-command
> >
> > To make it clearer, I try to make a summary for why container needs this patch:
> >     1.one machine can run different containers;
> >     2.for some scenario, container runs only one service inside(can be only one
> > application);
> >     3.different containers provide different services, different services have
> > different QoS demands;
> >     4.THP has big influence on QoS. It's fast for memory access, but eat more
> > memory;
> 
> I have been involved in this kind of topic discussion offline a couple
> of times. But TBH I don't see how you could achieve QoS by this flag.
> THP allocation is *NOT* guaranteed. And the overhead may be quite
> high. It depends on how fragmented the system is.

For THP, the word 'QoS' maybe too absolute, so let's describe it in the way why user
need THP: seeking for better memory performance.
Yes as you said THP may be quite overhead, and madvise() may not be suitable some time
(see PR_SET_THP_DISABLE https://man7.org/linux/man-pages/man2/prctl.2.html).

So I think this is just the reason why we need the patch: give user a method to use
THP with more precise range(only the performance sensitive containers) and reduce
overhead.

> 
> >     5.containers usually managed by compose software, which treats container as
> > base management unit;
> >     6.this patch provide cgroup THP controller, which can be a method to adjust
> > container memory QoS.
> >
> > > > Container becomes
> > > > the suitable management unit but not the whole host. And some performance
> > > > sensitive containers desiderate THP to provide low latency communication.
> > > > But if we use THP with 'always', it will consume more memory(on our machine
> > > > that is about 10% of total memory). And unnecessary huge pages will increase
> > > > memory pressure, add latency for minor pages faults, and add overhead when
> > > > splitting huge pages or coalescing normal sized pages into huge pages.
> > >
> > > It is still not really clear to me how do you achieve that the whole
> > > workload in the said container has the same THP requirements.
> > > --
> > > Michal Hocko
> > > SUSE Labs