Use logical CPUs instead of physical by default

[seanmonstar]

Some reasons to prefer logical count as the default:

• Chips reporting many logical CPUs vs physical, such as via
  hyperthreading, probably know better than us about the workload the CPUs
  can handle.
• The logical count (num_cpus::get()) takes into consideration
  schedular affinity, and cgroups CPU quota, in case the user wants to
  limit the amount of CPUs a process can use.

Closes #2269

[jebrosen]

Looks like this also closes #2269.

[hawkw]

lgtm! freebsd build is just busted.

[jonhoo]

This is fantastic! So many hacks to set the thread pool size when I want to run with fewer cores can go away 🎉

[Vagelis-Prokopiou]

@seanmonstar @jonhoo I had to look into this matter too, in the context of actix-web.

In my specific case, the configuration with workers = number of physical cpus outperformed the configuration with workers = number of logical cpus. You can check this issue for specifics: actix/actix-web#957

I believe that some amount of benchmarks should be the judge in this matter, because there is a possibility that the performance is degraded out of the box with this configuration.

[jonhoo]

@Vagelis-Prokopiou I suspect that's because hyperthreading is generally bad for performance (at least in my experience). It's a tricky trade-off. What I'd really like for us to do is to respect cgroups/affinity, but ignore hyperthreading, but I don't know that that's feasible to implement 😅

[Vagelis-Prokopiou]

@jonhoo I am not that aware of the internals in order to provide a good proposal.

What I am proposing is:

a) Some benchmarks by various people, in order to decide the best default option from the currently available options.

b) If option a is not possible, at least better documentation for the current setup (I created a PR for this #4330).