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.
