`-Zthreads` is slower the more cores I add

[oriongonza]

In my crates I haven't actually gotten a speedup from more threads. In fact, quite the opposite!
The more threads I add the slower it becomes, 1 being the fastest.

rustc --version
rustc 1.82.0-nightly (91376f416 2024-08-12)

[the8472]

This is very project-dependent. If possible please provide some steps to reproduce.

[lolbinarycat]

how many hardware threads do you have? if you have 2, and 1 of them is busy running something else, adding software threads will indeed slow things down.

[oriongonza]

output of lscpu:

CPU(s):                   16
  On-line CPU(s) list:    0-15