René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"GC performance regression in free threaded build","articleBody":"# Bug report\n\n### Bug description:\n\nI've identified a significant performance regression when using Python's free-threaded mode with shared list appends. In my test case, simply appending to a shared list causes a 10-15x performance decrease compared to normal Python operation.\n\nTest Case:\n```python\nimport itertools\nimport time\n\ndef performance_test(n_options=5, n_items=5, iterations=50):\n    list = []\n    \n    def expensive_operation():\n        # Create lists of tuples\n        data = []\n        for _ in range(n_options):\n            data.append([(f\"a{i}\", f\"b{i}\") for i in range(n_items)])\n        \n        # Generate all combinations and create result tuples\n        results = []\n        for combo in itertools.product(*data):\n            result = tuple((x[0], x[1], f\"name_{i}\") for i, x in enumerate(combo))\n            results.append(result)\n        \n        # Commenting the following line solves the performance regression in free-threaded mode\n        list.append(results)\n        return results\n\n    start = time.time()\n    for _ in range(iterations):\n        result = expensive_operation()\n    \n    duration = time.time() - start\n    print(f\"n_options={n_options}, n_items={n_items}, iterations={iterations}\")\n    print(f\"Time: {duration:.4f}s, Combinations: {len(result)}\")\n    return duration\n\nif __name__ == \"__main__\":\n    print(\"Python Performance Regression Test\")\n    print(\"-\" * 40)\n    performance_test()\n```\n\nResults:\n- Standard Python3.13: 0.1290s\n- Free-threaded Python3.13t: 2.1643s\n- Free-threaded Python 3.14.0a7: 2.1923s\n- Free-threaded Python3.13t with list.append commented out: 0.1332s\n\nThe regression appears to be caused by contention on the per-list locks and reference count fields when appending to a shared list in free-threaded mode.\n\n### CPython versions tested on:\n\n3.14\n\n### Operating systems tested on:\n\nLinux\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-133399\n* gh-133464\n* gh-133508\n* gh-133544\n* gh-133718\n* gh-134692\n* gh-134802\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/gptsarthak","@type":"Person","name":"gptsarthak"},"datePublished":"2025-04-25T09:55:02.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":13},"url":"https://github.com/132917/cpython/issues/132917"}