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So the >   open question here is do we care about overhead of the OOM killer task >   selection ? > > - I understanding that some people implement their own OOM killer in >   userspace based on RSS values exposed through /proc. Because those >   RSS values are the precise counts (split-counter sums), there should >   be no difference there compared to the upstream implementation, but >   there would be no performance gain as well. It may be interesting >   to eventually expose the counter approximations (and the accuracy >   intervals) to userspace so it could speed up its task selection >   eventually. Not really a drawback, more something to keep in mind as >   future improvement. > > - I took care not to add additional memory allocation to the mm >   allocation/free code because it regresses some benchmarks. >   Still it's good to keep an eye out for bot reports about those >   regressions. > > - The intermediate tree levels counters use extra memory. This is >   a tradeoff between compactness and cache locality of the counters. >   I currently used cache-aligned integers (thus favored cache locality >   and eliminating false-sharing), but I have other prototypes which >   use packed bytes for the intermediate levels. For instance, on a >   256 core machine, we have 37 intermediate levels nodes, for a total >   of 2368 bytes (that's in addition to the 1024 bytes of per-cpu memory >   for the per-cpu counters). If we choose to instead go for the packed >   bytes approach, the 37 intermediate levels nodes will use 37 bytes >   of memory, but there will be false-sharing across those counters. > >   An alternative approach there is to use a strided allocator [1] with a >   byte counter set allocator on top [2]. This way we can benefit from the >   memory savings of byte counters without the false-sharing. One more point: - The choice of constants is an educated guess at best and would require testing/feedback on real-world workloads: - The batch size (32), - The n-arity of the counter tree for each power-of-two number of CPUs (per_nr_cpu_order_config). - The choice of making the number of intermediate level counter bits match the n-arity of the counters aggregated into that level is also arbitrary. - The precise badness sums limit (16) for an OOM killer task selection. This is perhaps something that could become a sysctl tunable. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com