On Thu, Jul 25, 2024 at 04:20:45PM GMT, Yosry Ahmed wrote: > On Mon, Jul 22, 2024 at 3:53 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote: > > > > Linux kernel does not expose memory.current on the root memcg and there > > are applications which have to traverse all the top level memcgs to > > calculate the total memory charged in the system. This is more expensive > > (directory traversal and multiple open and reads) and is racy on a busy > > machine. As the kernel already have the needed information i.e. root's > > memory.current, why not expose that? > > > > However root's memory.current will have a different semantics than the > > non-root's memory.current as the kernel skips the charging for root, so > > maybe it is better to have a different named interface for the root. > > Something like memory.children_usage only for root memcg. > > > > Now there is still a question that why the kernel does not expose > > memory.current for the root. The historical reason was that the memcg > > charging was expensice and to provide the users to bypass the memcg > > charging by letting them run in the root. However do we still want to > > have this exception today? What is stopping us to start charging the > > root memcg as well. Of course the root will not have limits but the > > allocations will go through memcg charging and then the memory.current > > of root and non-root will have the same semantics. > > > > This is an RFC to start a discussion on memcg charging for root. > > I vaguely remember when running some netperf tests (tcp_rr?) in a > cgroup that the performance decreases considerably with every level > down the hierarchy. I am assuming that charging was a part of the > reason. If that's the case, charging the root will be similar to > moving all workloads one level down the hierarchy in terms of charging > overhead. No, the workloads running in non-root memcgs will not see any difference. Only the workloads running in root will see charging overhead.
