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This CL adds two related features enabled by default via compatibility GODEBUGs containermaxprocs and updatemaxprocs. On Linux, containermaxprocs makes the Go runtime consider cgroup CPU bandwidth limits (quota/period) when setting GOMAXPROCS. If the cgroup limit is lower than the number of logical CPUs available, then the cgroup limit takes precedence. On all OSes, updatemaxprocs makes the Go runtime periodically recalculate the default GOMAXPROCS value and update GOMAXPROCS if it has changed. If GOMAXPROCS is set manually, this update does not occur. This is intended primarily to detect changes to cgroup limits, but it applies on all OSes because the CPU affinity mask can change as well. The runtime only considers the limit in the leaf cgroup (the one that actually contains the process), caching the CPU limit file descriptor(s), which are periodically reread for updates. This is a small departure from the original proposed design. It will not consider limits of parent cgroups (which may be lower than the leaf), and it will not detection cgroup migration after process start. We can consider changing this in the future, but the simpler approach is less invasive; less risk to packages that have some awareness of runtime internals. e.g., if the runtime periodically opens new files during execution, file descriptor leak detection is difficult to implement in a stable way. For #73193. Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest Change-Id: I6a6a636c631c1ae577fb8254960377ba91c5dc98 Reviewed-on: https://go-review.googlesource.com/c/go/+/670497 LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>
API changes and other small changes to the standard library go here.