Feature #21797: Make Etc.nprocessors cgroup-aware on Linux

• Status: Open
• Assignee: -
• Target version: -

[moznion (Taiki Kawakami)]

Currently, Etc.nprocessors ignores cgroup CPU quotas. This causes issues in containers where CPU limits differ from the host CPU count.
I have written a gem for this purpose (https://github.com/moznion/maxprocs-ruby), but it would be preferable if the Ruby core implementation respected cgroup configuration.

Additionally, concurrent-ruby provides similar functionality, but if the language itself offered this capability, language users would not need to implement it individually.
And some parts of the Ruby language handle the number of processors using hard-coded values (e.g., https://github.com/ruby/ruby/blob/8efaf5e6b6a25e0d237f3d71b75865661ae98268/thread_pthread.c#L1737-L1738), so this could also be useful for Ruby language development.

Extending Etc.nprocessors to respect cgroups is one option, but that would be a breaking change, so adding a new API (e.g., Etc.cpu_quota or something?) might be a better approach.

Prior Art

• Go 1.25: runtime GOMAXPROCS (Issue #73193)
• uber-go/automaxprocs
• concurrent-ruby: https://github.com/ruby-concurrency/concurrent-ruby/blob/129cf004294af68ac53e53a2f1197621b303570a/lib/concurrent-ruby/concurrent/utility/processor_counter.rb

[moznion (Taiki Kawakami)]

• Description updated (diff)

[hsbt (Hiroshi SHIBATA)]

RubyGems 4.0.x support -j option for building C extension gem. But It causes in container environment like Circle CI.

https://github.com/ruby/rubygems/issues/9170

If cgroup provides the correct number of CPU for Cicle CI or others, I'm positive to support this.

[moznion (Taiki Kawakami)]

• Description updated (diff)

[osyoyu (Daisuke Aritomo)]

It'd be nice if RUBY_MAX_CPU would be autoconfigured based on this, just like Go 1.25 GOMAXPROCS.

Its default value is currently fixed to 8, but not many cloud containers have 8 cores worth of processors.
https://github.com/ruby/ruby/blob/8efaf5e6b6a25e0d237f3d71b75865661ae98268/thread_pthread.c#L1737-L1738