Everything you need to know about Python 3.13 – JIT and GIL went up the hill

Original Article ↗ Hacker News Discussion ↗

Free-threaded CPython & GIL Removal

  • Strong debate over removing the GIL: some expect more complexity, subtle bugs, and slower single-threaded code; others see it as necessary for a multicore future.
  • Many existing programs (including C extensions) implicitly rely on the GIL for thread safety; removing it risks race conditions and hard-to-debug issues.
  • A free-threaded build is seen as strategically important for scientific computing, ML, GUI apps, and workloads with large shared data where multiprocessing is too memory‑heavy.

Multithreading vs Multiprocessing

  • Several commenters say they’ve “never” hit GIL limits and happily use multiple processes (or even MPI) instead.
  • Others report regular pain points where multiprocessing overhead, data copying, and shared-memory problems make it inadequate.
  • ML and high-throughput inference (especially GPU-backed) are highlighted as domains where threads with shared memory are far superior to processes.

Performance & Correctness Concerns

  • Some claim no‑GIL builds are 20–50% slower for single-threaded code compared to older CPython, calling the tradeoff disappointing and politically driven.
  • Others point to public benchmarks showing ongoing overall speedups since 3.10 and argue the slowdown is expected early-stage cost that will be improved.
  • There is consensus that data races and memory unsafety like in C/C++ are unacceptable; Python’s refcounting especially cannot simply “ignore” races.

JIT in 3.13

  • JIT is experimental and off by default; current tier‑2 optimizer reportedly slows code ~20%, with JIT mostly regaining that loss.
  • Some want official binaries with JIT enabled to encourage real‑world testing; others argue forcing source builds is appropriate for such an alpha feature.

PyPy & Alternatives

  • PyPy’s mature JIT is praised and considered underused; some see CPython’s JIT as NIH duplication.
  • Major barrier: incomplete or slower compatibility with the CPython C API and widespread C extensions.
  • PyPy also still has a GIL; past attempts at STM and GIL removal stalled due to overhead and complexity.

Syntax, Tooling, and Miscellaneous

  • A side discussion argues over Python’s indentation-as-syntax; some wish for an end keyword for easier auto-formatting, others see braces/keywords as noise.
  • Packaging/binaries: Linux binaries are tricky due to libc and ecosystem diversity; users often rely on distros or third-party portable builds.