Re: [RFC PATCH] mm: memcontrol: memory+swap accounting for cgroup-v2

[Shakeel Butt <shakeelb@google.com>]

On Tue, Dec 19, 2017 at 4:49 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Mon 18-12-17 16:01:31, Shakeel Butt wrote:
>> The memory controller in cgroup v1 provides the memory+swap (memsw)
>> interface to account to the combined usage of memory and swap of the
>> jobs. The memsw interface allows the users to limit or view the
>> consistent memory usage of their jobs irrespectibe of the presense of
>> swap on the system (consistent OOM and memory reclaim behavior). The
>> memory+swap accounting makes the job easier for centralized systems
>> doing resource usage monitoring, prediction or anomaly detection.
>>
>> In cgroup v2, the 'memsw' interface was dropped and a new 'swap'
>> interface has been introduced which allows to limit the actual usage of
>> swap by the job. For the systems where swap is a limited resource,
>> 'swap' interface can be used to fairly distribute the swap resource
>> between different jobs. There is no easy way to limit the swap usage
>> using the 'memsw' interface.
>>
>> However for the systems where the swap is cheap and can be increased
>> dynamically (like remote swap and swap on zram), the 'memsw' interface
>> is much more appropriate as it makes swap transparent to the jobs and
>> gives consistent memory usage history to centralized monitoring systems.
>>
>> This patch adds memsw interface to cgroup v2 memory controller behind a
>> mount option 'memsw'. The memsw interface is mutually exclusive with
>> the existing swap interface. When 'memsw' is enabled, reading or writing
>> to 'swap' interface files will return -ENOTSUPP and vice versa. Enabling
>> or disabling memsw through remounting cgroup v2, will only be effective
>> if there are no decendants of the root cgroup.
>>
>> When memsw accounting is enabled then "memory.high" is comapred with
>> memory+swap usage. So, when the allocating job's memsw usage hits its
>> high mark, the job will be throttled by triggering memory reclaim.
>
> From a quick look, this looks like a mess.

The main motivation behind this patch is to convince that memsw has
genuine use-cases. How to provide memsw is still in RFC stage.
Suggestions and comments are welcomed.

> We have agreed to go with
> the current scheme for some good reasons.

Yes I agree, when the swap is a limited resource the current 'swap'
interface should be used to fairly distribute it between different
jobs.

> There are cons/pros for both
> approaches but I am not convinced we should convolute the user API for
> the usecase you describe.
>

Yes, there are pros & cons, therefore we should give users the option
to select the API that is better suited for their use-cases and
environment. Both approaches are not interchangeable. We use memsw
internally for use-cases I mentioned in commit message. This is one of
the main blockers for us to even consider cgroup-v2 for memory
controller.

>> Signed-off-by: Shakeel Butt <shakeelb@google.com>

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