Re: [RFC PATCH 0/2] mm/memcontrol: Finer-grained memory control

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Apr 11, 2019 at 10:02:16AM -0400, Waiman Long wrote:
> On 04/10/2019 03:54 PM, Michal Hocko wrote:
> > On Wed 10-04-19 15:13:19, Waiman Long wrote:
> >> The current control mechanism for memory cgroup v2 lumps all the memory
> >> together irrespective of the type of memory objects. However, there
> >> are cases where users may have more concern about one type of memory
> >> usage than the others.
> >>
> >> We have customer request to limit memory consumption on anonymous memory
> >> only as they said the feature was available in other OSes like Solaris.
> > Please be more specific about a usecase.
> 
> From that customer's point of view, page cache is more like common goods
> that can typically be shared by a number of different groups. Depending
> on which groups touch the pages first, it is possible that most of those
> pages can be disproportionately attributed to one group than the others.
>
> Anonymous memory, on the other hand, are not shared and so can more
> correctly represent the memory footprint of an application. Of course,
> there are certainly cases where an application can have large private
> files that can consume a lot of cache pages. These are probably not the
> case for the applications used by that customer.

I don't understand what the goal is. What do you accomplish by only
restricting anon memory? Are you trying to contain malfunctioning
applications? Malicious applications?

Cache can apply as much pressure to the system as anon can. So if you
are in the position to ask your applications to behave wrt cache,
surely you can ask them to behave wrt anon as well...?

This also answers only one narrow question out of the many that arise
when heavily sharing cache. The accounting isn't done right,
memory.current of the participating cgroups will make no sense, IO
read and writeback burden is assigned to random cgroups.

> >> For simplicity, the limit is not hierarchical and applies to only tasks
> >> in the local memory cgroup.
> > This is a no-go to begin with.
> 
> The reason for doing that is to introduce as little overhead as
> possible. We can certainly make it hierarchical, but it will complicate
> the code and increase runtime overhead. Another alternative is to limit
> this feature to only leaf memory cgroups. That should be enough to cover
> what the customer is asking for and leave room for future hierarchical
> extension, if needed.

I agree with Michal, this is a no-go. It involves userspace ABI that
we have to maintain indefinitely, so it needs to integrate properly
with the overall model of the cgroup2 interface.

That includes hierarchical support, but as per above it includes wider
questions of how this is supposed to integrate with the concepts of
comprehensive resource control. How it integrates with the accounting
(if you want to support shared pages, they should also be accounted as
shared and not to random groups), the relationships with connected
resources such as IO (in a virtual memory system that can do paging,
memory and IO are fungible, so if you want to be able to share one,
you have to be able to share the other as well to the same extent),
how it integrates with memory.low protection etc.

As it stands, I don't see this patch set addressing any of these.