[Issue 19513] New: Use sched_getaffinity(2) to get the number of CPU cores if available

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19513

          Issue ID: 19513
           Summary: Use sched_getaffinity(2) to get the number of CPU
                    cores if available
           Product: D
           Version: D2
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: kubo39 at gmail.com

Currently std.parallelism.totalCPUs is implemented by
sysconf(_SC_NPROCESSORS_ONLN).
However, on GNU/Linux, usable number of processoes may be restricted if a
process runs in containers.
In case it's better to use sched_getaffinity(2).

How to implement:

1. std.parallelism.totalCPUs uses sched_getaffinity(2) internally.
2. Add new API (and considering function name).
  - std.parallelism.sched_getaffinity(pid, mask)? (Like Python)
  - std.parallelism.nprocessors? (Like Ruby)
  - or berrer name

Libraries exposing sched_getaffinity:

- Python >= 3.3 provides os.get_affinity(pid, mask) function.
- Ruby >= 2.2 provides Etc.nprocessors function.
- GNU coreutils provides nproc(1) command.
- Rust's num_cpus crate provides num_cpus::get() function.

Links:

- https://github.com/golang/go/issues/3921
- https://docs.python.org/3/library/os.html#os.sched_getaffinity
- https://bugs.ruby-lang.org/issues/10267
- http://man7.org/linux/man-pages/man1/nproc.1.html

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