On Wed 22-04-20 10:15:14, Johannes Weiner wrote: > On Wed, Apr 22, 2020 at 03:26:32PM +0200, Michal Hocko wrote: > > That being said I believe our discussion is missing an important part. > > There is no description of the swap.high semantic. What can user expect > > when using it? > > Good point, we should include that in cgroup-v2.rst. How about this? > > diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst > index bcc80269bb6a..49e8733a9d8a 100644 > --- a/Documentation/admin-guide/cgroup-v2.rst > +++ b/Documentation/admin-guide/cgroup-v2.rst > @@ -1370,6 +1370,17 @@ PAGE_SIZE multiple when read back. > The total amount of swap currently being used by the cgroup > and its descendants. > > + memory.swap.high > + A read-write single value file which exists on non-root > + cgroups. The default is "max". > + > + Swap usage throttle limit. If a cgroup's swap usage exceeds > + this limit, allocations inside the cgroup will be throttled. Hm, so this doesn't talk about which allocatios are affected. This is good for potential future changes but I am not sure this is useful to make any educated guess about the actual effects. One could expect that only those allocations which could contribute to future memory.swap usage. I fully realize that we do not want to be very specific but we want to provide something useful I believe. I am sorry but I do not have a good suggestion on how to make this better. Mostly because I still struggle on how this should behave to be sane. I am also missing some information about what the user can actually do about this situation and call out explicitly that the throttling is not going away until the swap usage is shrunk and the kernel is not capable of doing that on its own without a help from the userspace. This is really different from memory.high which has means to deal with the excess and shrink it down in most cases. The following would clarify it for me "Once the limit is exceeded it is expected that the userspace is going to act and either free up the swapped out space or tune the limit based on needs. The kernel itself is not able to do that on its own. " > + > + This slows down expansion of the group's memory footprint as > + it runs out of assigned swap space. Compare to memory.swap.max, > + which stops swapping abruptly and can provoke kernel OOM kills. > + > memory.swap.max > A read-write single value file which exists on non-root > cgroups. The default is "max". -- Michal Hocko SUSE Labs
