On Mon 25-09-23 10:11:05, Yosry Ahmed wrote: > On Mon, Sep 25, 2023 at 6:50 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > On Fri 22-09-23 17:57:38, Yosry Ahmed wrote: > > > While working on adjacent code [1], I realized that the values passed > > > into memcg_rstat_updated() to keep track of the magnitude of pending > > > updates is consistent. It is mostly in pages, but sometimes it can be in > > > bytes or KBs. Fix that. > > > > What kind of practical difference does this change make? Is it worth > > additional code? > > As explained in patch 2's commit message, the value passed into > memcg_rstat_updated() is used for the "flush only if not worth it" > heuristic. As we have discussed in different threads in the past few > weeks, unnecessary flushes can cause increased global lock contention > and/or latency. > > Byte-sized paths (percpu, slab, zswap, ..) feed bytes into the > heuristic, but those are interpreted as pages, which means we will > flush earlier than we should. This was noticed by code inspection. How > much does this matter in practice? I would say it depends on the > workload: how many percpu/slab allocations are being made vs. how many > flushes are requested. > > On a system with 100 cpus, 25M of stat updates are needed for a flush > usually, but ~6K of slab/percpu updates will also (mistakenly) cause a > flush. This surely depends on workload and that is understandable. But it would be really nice to provide some numbers for typical workloads which exercise slab heavily. -- Michal Hocko SUSE Labs
