Re: [PATCH] mm: memcg: add swapcache stat for memcg v2

[Shakeel Butt]

On Wed, Jan 6, 2021 at 6:53 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 31-12-20 18:39:55, Shakeel Butt wrote:
> > This patch adds swapcache stat for the cgroup v2. The swapcache
> > represents the memory that is accounted against both the memory and the
> > swap limit of the cgroup. The main motivation behind exposing the
> > swapcache stat is for enabling users to gracefully migrate from cgroup
> > v1's memsw counter to cgroup v2's memory and swap counters.
> >
> > Cgroup v1's memsw limit allows users to limit the memory+swap usage of a
> > workload but without control on the exact proportion of memory and swap.
> > Cgroup v2 provides separate limits for memory and swap which enables
> > more control on the exact usage of memory and swap individually for the
> > workload.
> >
> > With some little subtleties, the v1's memsw limit can be switched with
> > the sum of the v2's memory and swap limits. However the alternative for
> > memsw usage is not yet available in cgroup v2. Exposing per-cgroup
> > swapcache stat enables that alternative. Adding the memory usage and
> > swap usage and subtracting the swapcache will approximate the memsw
> > usage. This will help in the transparent migration of the workloads
> > depending on memsw usage and limit to v2' memory and swap counters.
>
> Could you expand a bit more on why memsw usage is important even in
> cgroup v2 land? How are you going to use the approximated value?
>

Two main benefits. First, it hides the underlying system's swap setup
from the applications. Applications with multiple instances running in
a datacenter with heterogeneous systems (some have swap and some
don't) will keep seeing a consistent view of their usage.

Second, most of the applications (at least in our prod) are not really
interested in two separate memory and swap usage metrics. A single
usage metric is more simple to use and reason about for these
applications.