Start julia with multiple worker threads by default

[jtrakk]

This will help new Julia programmers who don't know about JULIA_NUM_THREADS=auto get fast execution. It also symbolizes how Julia supports the increasing parallelism of modern technical computing. Anybody who needs sequential execution, e.g. for debugging, can use JULIA_NUM_THREADS=1.

• Rust's Rayon uses the number of logical cores by default.

• Intel® Cilk™ Plus uses the number of hardware threads.

• Crystal defaults to 4.

• Fortran OMP defaults to 1.

[ViralBShah]

At the very least need a good way to get the physical core count as discussed in #43692

[fonsp]

We could use min(1, Sys.CPU_THREADS ÷ 2) as a safe default while waiting on that issue? I also think giving more than one thread by default would be great for new users!

[gbaraldi]

There is also the issue that new cpus have big.Little architectures (M1 mac, 12th gen intel) so we would have to choose either just performance cores (probably the best) or all cores.

[lassepe]

It seems like this issue got stuck. Is there any fundamental downside of having JULIA_NUM_THREADS=auto,auto be the default over the current 1,0? I understand that the auto-detection of the CPU core count is not perfect right now but setting to 1,0 seems to provide a worse default behavior (especially for new users) almost every time (since Threads.@spawn and friends will not warn about having only a single thread so it is just silently refusing the multi-thread.)

[IanButterworth]

I'm tentatively adding this to the 1.12 milestone, as it seems the direction to go in and would help REPL issues like #55434

My understanding is the main issue is Distributed not being threadsafe? But that may have improved?

[ViralBShah]

I do not believe Distributed has seen any major contributions in a long time.

[jpsamaroo]

DistributedNext.jl has seen thread-safety changes merged, so users have options if we wanted to switch this on.

That said, if we're going to block this on Distributed.jl becoming thread-safe, then we should have someone from the core team help review JuliaLang/Distributed.jl#101. The changes are not really too complicated, especially if you just review everything before JuliaLang/Distributed.jl@0377cda (since this commit and those following it are not necessary to improve the thread-safety of Distributed).

[ViralBShah]

Should we link to DistributedNext.jl in the Distributed.jl repo?

[jpsamaroo]

That would be great! We're registering 1.0 now: JuliaRegistries/General#120499. (We picked 1.0, rather than a pre-release version, since it's just Distributed.jl with fixes on top right now).

[DilumAluthge]

Should import Distributed print a warning if the number of threads is > 1?