Potentially serious regression in garbage collector performance in Python 3.14 #139951
Description
Bug report
Bug description:
The following code, consisting of a bunch of dict lookups and writes, runs in
- < 1 second on Python 3.13
- > 2 seconds on Python 3.14
n = 1000 d = {} for i in range(n): d[(i, i)] = d[(i, 0)] = d[(0, i)] = 0 for i in range(1, n): for j in range(1, n): d[(i, j)] = i + j + d[(i - 1, j)] + d[(i, j - 1)] + d[(i - 1, j - 1)] print(d[(n - 1, n - 1)])Tests are run on a Dell Precision 5490, running Ubuntu:
Linux precision 6.14.0-33-generic #33-Ubuntu SMP PREEMPT_DYNAMIC Wed Sep 17 23:22:02 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux Ubuntu 25.04 (Plucky) The cpython is from python-build-standalone
cpython-3.14.0+20251010-x86_64-unknown-linux-gnu-install_only.tar.gzcpython-3.13.8+20251010-x86_64-unknown-linux-gnu-install_only.tar.gz
Log
[~/regression-testing]$ hyperfine --warmup=10 "cp313/python/bin/python3.13 dicttest.py" "cp314/python/bin/python3.14 dicttest.py" Benchmark 1: cp313/python/bin/python3.13 dicttest.py Time (mean ± σ): 880.8 ms ± 119.7 ms [User: 730.1 ms, System: 150.1 ms] Range (min … max): 713.3 ms … 1066.2 ms 10 runs Benchmark 2: cp314/python/bin/python3.14 dicttest.py Time (mean ± σ): 2.235 s ± 0.035 s [User: 2.066 s, System: 0.168 s] Range (min … max): 2.168 s … 2.295 s 10 runs Summary cp313/python/bin/python3.13 dicttest.py ran 2.54 ± 0.35 times faster than cp314/python/bin/python3.14 dicttest.py It seems to scale very poorly too. With the same code, but n = 2000 (instead of n = 1000), leads to 3.3 sec (Python 3.13) vs 20 seconds (Python 3.14)! I.e., Python 3.13 being 5 times faster.
[~/regression-testing]$ hyperfine --warmup=10 "cp313/python/bin/python3.13 dicttest.py" "cp314/python/bin/python3.14 dicttest.py" Benchmark 1: cp313/python/bin/python3.13 dicttest.py Time (mean ± σ): 3.458 s ± 0.048 s [User: 2.889 s, System: 0.568 s] Range (min … max): 3.375 s … 3.521 s 10 runs Benchmark 2: cp314/python/bin/python3.14 dicttest.py Time (mean ± σ): 20.084 s ± 0.230 s [User: 19.503 s, System: 0.577 s] Range (min … max): 19.749 s … 20.345 s 10 runs Summary cp313/python/bin/python3.13 dicttest.py ran 5.81 ± 0.10 times faster than cp314/python/bin/python3.14 dicttest.py 
Plotting time (in seconds) of the script run with n = 0 ... 2000.
CPython versions tested on:
3.14
Operating systems tested on:
Linux
Linked PRs
- gh-139951: Do not track immutable tuples in GC #140204
- GH-139951: Fix major GC performance regression #140262
- gh-139951: Test on GC collection disabled if threshold is zero #140304
- [3.14] gh-139951: Test on GC collection disabled if threshold is zero (GH-140304) #140362
- [3.13] gh-139951: Test on GC collection disabled if threshold is zero (GH-140304) #140363
- [3.14] GH-139951: Fix major GC performance regression (GH-140262) #140423
- [3.14] GH-139951: Fix major GC performance regression. Backport of GH-140262 #140447
- gh-139951: Tests on tuple tracking #140575