Re: [PATCH RFC] mm/vmscan: try to protect active working set of cgroup from reclaim.

[Roman Gushchin <guro@fb.com>]

On Fri, Feb 22, 2019 at 08:58:25PM +0300, Andrey Ryabinin wrote:
> In a presence of more than 1 memory cgroup in the system our reclaim
> logic is just suck. When we hit memory limit (global or a limit on
> cgroup with subgroups) we reclaim some memory from all cgroups.
> This is sucks because, the cgroup that allocates more often always wins.
> E.g. job that allocates a lot of clean rarely used page cache will push
> out of memory other jobs with active relatively small all in memory
> working set.
> 
> To prevent such situations we have memcg controls like low/max, etc which
> are supposed to protect jobs or limit them so they to not hurt others.
> But memory cgroups are very hard to configure right because it requires
> precise knowledge of the workload which may vary during the execution.
> E.g. setting memory limit means that job won't be able to use all memory
> in the system for page cache even if the rest the system is idle.
> Basically our current scheme requires to configure every single cgroup
> in the system.
> 
> I think we can do better. The idea proposed by this patch is to reclaim
> only inactive pages and only from cgroups that have big
> (!inactive_is_low()) inactive list. And go back to shrinking active lists
> only if all inactive lists are low.

Hi Andrey!

It's definitely an interesting idea! However, let me bring some concerns:
1) What's considered active and inactive depends on memory pressure inside
a cgroup. Actually active pages in one cgroup (e.g. just deleted) can be colder
than inactive pages in an other (e.g. a memory-hungry cgroup with a tight
memory.max).

Also a workload inside a cgroup can to some extend control what's going
to the active LRU. So it opens a way to get more memory unfairly by
artificially promoting more pages to the active LRU. So a cgroup
can get an unfair advantage over other cgroups.

Generally speaking, now we have a way to measure the memory pressure
inside a cgroup. So, in theory, it should be possible to balance
scanning effort based on memory pressure.

Thanks!