possible GC regression vs 1.10 when calling with >1 thread, even if threads unused

[adienes]

versioninfo

Julia Version 1.10.0
Commit 3120989f39b (2023-12-25 18:01 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin22.4.0)
  CPU: 10 × Apple M1 Max
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-15.0.7 (ORCJIT, apple-m1)
  Threads: 1 on 8 virtual cores
Environment:
  JULIA_LOAD_PATH = @:@stdlib

vs current master (b4f7263)

full MWE

using BenchmarkTools

using Base.Threads: @threads
using Random: shuffle

function sample_vote!(_rb, chop_counts)
    pts = rand(length(chop_counts))

    N = length(_rb)
    _srt = 4245
    partialsortperm!(_rb, pts, 1:_srt; lt = <, rev = true)
    while sum(@views chop_counts[_rb[1:_srt]]) ≤ 5660
        _srt = min(2 * _srt, N)
        partialsortperm!(_rb, pts, 1:_srt; lt = <, rev = true)
    end
end

function parallel_scores(chop_counts)
    for k ∈ 1:8 # @threads in real code, but not needed for mwe. do need -t8 though
        _rb = collect(1:length(chop_counts))

        # the bigger this number, the more % GC time
        for _ ∈ 1:1000
            sample_vote!(_rb, chop_counts)
        end
    end
end

# kind of arbitrary, but approximates my data
chop_counts = shuffle(trunc.(Int, 6500 ./ (50:100_000)))

# @benchmark parallel_scores($(chop_counts)) evals = 1

note that the regression only appears on my machine when called with multiple threads, e.g. -t8, even if I do not use them to multithread the loop

[oscardssmith]

Does this reproduce if you pass --gcthreads=1?

[adienes]

nope

huh

[gbaraldi]

I can reproduce, my GC time goes from 1.5% to 13% from 1 gc thread to 10.