Use `std::thread::available_parallelism()` instead of `num_cpus`

[c410-f3r]

Requires a to bump 1.64 but it will allow the decrease of the number of transient dependencies for downstream crates.

1.63 only makes use of cgroupv2 and such exclusive behavior led to issues like rust-lang/rust#97549. In 1.64, cgroupv1 is also supported (rust-lang/rust#97925, https://releases.rs/docs/1.64.0/).

cc #6210

[seanmonstar]

Worth noting that available_parallelism() is worse for users on older rustc. An additional point that I don't have the answer to: does it support all the targets that num_cpus does?

[carllerche]

@c410-f3r are you able to address @seanmonstar's points re: num_cpus. I know that that num_cpus has had updates over the years to address edge cases that are not initially obvious (e.g. reducing available CPUs to a process via cgroups)

[c410-f3r]

!!! This report is a best effort subject to possible errors !!!

Targets found in the rustc repository.

• Unsupported: itron, sgx, teeos, wasi

• Supported: aix, android, dragonfly, emscripten, freebsd, fuchsia, haiku, hermit, hurd, illumos, ios, linux, macos, solaris, tvos, netbsd, nto, openbsd, windows, xous

Targets founds in the num_cpus repository:

• Unsupported: None, everything that is not supported has a fixed 1 value.
  Supported: aix, android, dragonfly, freebsd, fuchsia, haiku, hermit, illumos, ios, linux, macos, nacl, netbsd, openbsd, solaris, windows

In regards to implementation, they differ in some points. For example, aarch64 uses libc::_SC_NPROCESSORS_CONF in num_cpus (https://github.com/seanmonstar/num_cpus/blob/7c03fc930cc47a9b94e8ca66ca44ef1a454c8f51/src/lib.rs#L368) but rustc uses libc::_SC_NPROCESSORS_ONLN in non-android platforms (https://github.com/rust-lang/rust/blob/d253bf61ad38a59cc579aee688f81a06c31283d3/library/std/src/sys/unix/thread.rs#L347).

That said, it seems that the current Rustc version supports everything but nacl.

cc https://github.com/tokio-rs/mio/blob/c0919cc4cee75904904c2042f5066726b2dc9397/Makefile#L2

[carllerche]

@seanmonstar You have thoughts? We would need to wait until our MSRV includes the method, but besides that?

[c410-f3r]

Closing due to inactivity.

[Darksonn]

There's no need to close the issue. Even if we can't upgrade right now, we will be able to in the future when we bump the MSRV.

[c410-f3r]

Closing due to inactivity.

If desirable, please create another issue to track this feature.