Re: [PATCH] Documentation: Clarify usage of memory limits

[Waiman Long <longman@redhat.com>]

On 6/1/23 15:53, Johannes Weiner wrote:
> On Thu, Jun 01, 2023 at 03:15:28PM -0400, Waiman Long wrote:
>> On 6/1/23 14:38, Dan Schatzberg wrote:
>>> The existing documentation refers to memory.high as the "main mechanism
>>> to control memory usage." This seems incorrect to me - memory.high can
>>> result in reclaim pressure which simply leads to stalls unless some
>>> external component observes and actions on it (e.g. systemd-oomd can be
>>> used for this purpose). While this is feasible, users are unaware of
>>> this interaction and are led to believe that memory.high alone is an
>>> effective mechanism for limiting memory.
>>>
>>> The documentation should recommend the use of memory.max as the
>>> effective way to enforce memory limits - it triggers reclaim and results
>>> in OOM kills by itself.
>> That is not how my understanding of memory.high works. When memory usage
>> goes past memory.high, memory reclaim will be initiated to reclaim the
>> memory back. Stall happens when memory.usage keep increasing like by
>> consuming memory faster than what memory reclaim can recover. When
>> memory.max is reached, OOM killer will then kill off the tasks.
> This was the initial plan indeed: Slow down the workload and thus slow
> the growth; hope that the workload recovers with voluntary frees; set
> memory.max as a safety if it keeps going beyond.
>
> This never panned out. Once workloads are stuck, they might not back
> down on their own. By increasingly slowing growth, it becomes harder
> and harder for them to reach the memory.max intervention point.
>
> It's a very brittle configuration strategy. Unless you very carefully
> calibrate memory.high and memory.max together with awareness of the
> throttling algorithm, workloads that hit memory.high will just go to
> sleep indefinitely. They require outside intervention that either
> adjusts limits or implements kill policies based on observed sleeps
> (they're reported as pressure via psi).
>
> So the common usecases today end up being that memory.max is for
> enforcing kernel OOM kills, and memory.high is a tool to implement
> userspace OOM killing policies.
>
> Dan is right to point out the additional expectations for userspace
> management when memory.high is in used. And memory.max is still the
> primary, works-out-of-the-box method of memory containment.

Thanks for clarification. I have to reset my false assumption.

Cheers,
Longman