# \[Regression in rc3\] Can you decline threads you've been assigned, e.g. GC threads? Or should it be possible?

**URL:** <https://discourse.julialang.org/t/regression-in-rc3-can-you-decline-threads-youve-been-assigned-e-g-gc-threads-or-should-it-be-possible/119445>\
**Category:** Offtopic\
**Created:** [September 15, 2024, 9:17pm UTC](https://discourse.julialang.org/t/regression-in-rc3-can-you-decline-threads-youve-been-assigned-e-g-gc-threads-or-should-it-be-possible/119445 "2024-09-15T21:17:08Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Palli](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/palli/32/3380_2.png) [@Palli](https://discourse.julialang.org/u/Palli)\
**Post date:** [September 15, 2024, 9:17pm UTC](https://discourse.julialang.org/t/regression-in-rc3-can-you-decline-threads-youve-been-assigned-e-g-gc-threads-or-should-it-be-possible/119445/1 "2024-09-15T21:17:09Z")

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Can anyone first confirm my 2.2% slowdown, on 1.11 rc3:

```julia
$ time julia +1.11 -O3 -t1 --gcthreads=4 pidigits.jl 10000 >/dev/null

real	0m1,205s
user	0m2,165s
sys	0m0,072s

$ time julia -O3 -t1 --gcthreads=4 pidigits.jl 10000 >/dev/null

real	0m1,178s
user	0m1,718s
sys	0m0,458s

```

and explain why `user` is higher (it seems though it plus sys isn’t much changed, but should be closer to user?)? I think I do by below.

I had noticed at the Benchmark Game:

> [**Julia** #3](https://benchmarksgame-team.pages.debian.net/benchmarksgame/q6600/program/pidigits-julia-3.html) 2.06 169,012 506 2.55 99% 6% 13% 6%

And “busy” is 23% slower than the sec. column. While nobody is competing on “busy” I think, I believe it’s explained by the 3 “unused” cores giving those “6% 13% 6%”. But that’s a single-threaded program and those should ideally be 0% each or close to (the ideal nobody gets, or may not be able to get, see my other theory):

What I think could be happening is that the GC is running on those other cores and/or that the one you use overheats, and/or the program moved to another core by the CPU, and thus to a different L1 cache… as for the fastest C program “67% 0% 2% 33%”.

The Benchmark Game insists on all the same setting for all the program, multi-threaded or not, i.e. `-t 4`, so I’m thinking when I know it’s not needed, could I add at the top of my script `GC_cores(1)`, when I know the program is actually single-threaded. I’m not sure it’s wise, since hypothetically more GC threads should help then maybe `GC_cores(1, 4)` and if the latter number is omitted then keep the default GC threads?

I tried to limit load on my machine (suspending firefox), and hope my timing isn’t off, also good to know if Julia is less immune to a loaded machine.

```julia
$ hyperfine 'julia +1.11 -O3 -t4 pidigits.jl 10000 >/dev/null'
Benchmark 1: julia +1.11 -O3 -t4 pidigits.jl 10000 >/dev/null
  Time (mean ± σ): 1.249 s ± 0.034 s [User: 2.159 s, System: 0.071 s]
  Range (min … max): 1.203 s … 1.315 s 10 runs

$ hyperfine 'julia -O3 -t4 pidigits.jl 10000 >/dev/null'
Benchmark 1: julia -O3 -t4 pidigits.jl 10000 >/dev/null
  Time (mean ± σ): 1.230 s ± 0.031 s [User: 1.676 s, System: 0.503 s]
  Range (min … max): 1.189 s … 1.284 s 10 runs

$ time julia -O3 -t1 --gcthreads=2 pidigits.jl 10000 >/dev/null

real	0m1,215s
user	0m1,651s
sys	0m0,508s

```

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**Author:** ![carstenbauer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/carstenbauer/32/4981_2.png) [@carstenbauer](https://discourse.julialang.org/u/carstenbauer)\
**Post date:** [September 16, 2024, 2:22am UTC](https://discourse.julialang.org/t/regression-in-rc3-can-you-decline-threads-youve-been-assigned-e-g-gc-threads-or-should-it-be-possible/119445/2 "2024-09-16T02:22:25Z")

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What is pidigits.jl?

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<div class="post-metadata">

**Author:** ![Palli](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/palli/32/3380_2.png) [@Palli](https://discourse.julialang.org/u/Palli)\
**Post date:** [September 16, 2024, 5:51pm UTC](https://discourse.julialang.org/t/regression-in-rc3-can-you-decline-threads-youve-been-assigned-e-g-gc-threads-or-should-it-be-possible/119445/3 "2024-09-16T17:51:01Z")

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It’s linked above (the “#3” code, in case you missed it), i.e. [pidigits Julia&nbsp;#3 program | Q6600 Benchmarks Game](https://benchmarksgame-team.pages.debian.net/benchmarksgame/q6600/program/pidigits-julia-3.html)

[FYI: This would help with that benchmark (and the whole PR with mandelbrot too), but otherwise unrelated to the question here: [Add misc precompiles (for benchmarks) by PallHaraldsson · Pull Request #55784 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/pull/55784/commits/5823f1c2b550e6638b59a868c0092dec79b6ccd5) ]