fix(Multiprocessing): Fully disable multiprocessing when not used

[Simpag]

This PR fully disables multiprocessing when max_processes = 1. This speeds up execution on windows based systems as the overhead of spawning processing using spawn context is large. For this reason, this PR also disabled the multiprocessing checkpointing tests for all systems not running Linux since Linux by default uses fork which comes with less overhead. Finally, a better error message is given when users runs unguarded benchmarks (main script is not wrapped in if __name__ == '__main__') while using multiprocessing (max_processes > 1).

closes #279

[Copilot]

Pull request overview

This PR reduces multiprocessing overhead by avoiding multiprocessing.Manager()/spawn setup when max_processes == 1, adjusts progress-bar coordination to work without a manager, and updates checkpointing tests to skip multi-process cases on non-Linux platforms.

Changes:

• Disable multiprocessing initialization (manager/log listener/spawn context) when max_processes == 1.
• Allow ProgressBarController to run with manager=None by using a local in-process queue.
• Skip max_processes > 1 checkpointing test variants on non-Linux, and refactor CPU-core skip logic.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
decent_bench/benchmark/_benchmark.py	Avoid multiprocessing init for single-process runs; improve spawn main-guard error message; make log listener optional.
decent_bench/utils/progress_bar.py	Support manager=None by falling back to a local queue to avoid manager overhead.
test/utils/test_checkpoints.py	Skip multi-process checkpoint tests on non-Linux; centralize CPU-count-based skipping.

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[adrianardv]

Maybe it would also be worth adding a short note in the user guide about the Windows spawn behavior and the if name == "main": guard?

[Simpag]

Maybe it would also be worth adding a short note in the user guide about the Windows spawn behavior and the if name == "main": guard?

Good idea, the user docs are very outdated but its still good so we remember to include it

[Simpag]

I have not touched anything with the documentation and RTD fails... Why is complaining now about the class tagger?

[nicola-bastianello]

thank you for fixing this and the RTD issue!

I confirm the tests now take very little time on my machine too (<10s). merging now