New submission from Ken Healy <healykb+pythonb...@gmail.com>:
The documentation for time.perf_counter() indicates it should return a system-wide value: https://docs.python.org/3/library/time.html#time.perf_counter This is not true on Windows, as a process-specific offset is subtracted from the underlying QueryPerformanceCounter() value. The code comments indicate this is to reduce precision loss: https://github.com/python/cpython/blob/master/Python/pytime.c#L995-L997 This is relevant in multiprocess applications, where accurate timing is required across multiple processes. Here is a simple test case: ----------------------------------------------------------- import concurrent.futures import time def worker(): return time.perf_counter() if __name__ == '__main__': pool = concurrent.futures.ProcessPoolExecutor() futures = [] for i in range(3): print('Submitting worker {:d} at time.perf_counter() == {:.3f}'.format(i, time.perf_counter())) futures.append(pool.submit(worker)) time.sleep(1) for i, f in enumerate(futures): print('Worker {:d} started at time.perf_counter() == {:.3f}'.format(i, f.result())) ----------------------------------------------------------- Output: ----------------------------------------------------------- C:\...>Python37\python.exe -VV Python 3.7.3 (v3.7.3:ef4ec6ed12, Mar 25 2019, 22:22:05) [MSC v.1916 64 bit (AMD64)] C:\...>Python37\python.exe perf_counter_across_processes.py Submitting worker 0 at time.perf_counter() == 0.376 Submitting worker 1 at time.perf_counter() == 1.527 Submitting worker 2 at time.perf_counter() == 2.529 Worker 0 started at time.perf_counter() == 0.380 Worker 1 started at time.perf_counter() == 0.956 Worker 2 started at time.perf_counter() == 1.963 ----------------------------------------------------------- See my stackoverflow question for a comparison with Linux: https://stackoverflow.com/questions/56502111/should-time-perf-counter-be-consistent-across-processes-in-python-on-windows ---------- assignee: docs@python components: Documentation, Library (Lib), Windows messages: 345057 nosy: docs@python, kh90909, paul.moore, steve.dower, tim.golden, vstinner, zach.ware priority: normal severity: normal status: open type: behavior versions: Python 3.5, Python 3.6, Python 3.7, Python 3.8, Python 3.9 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue37205> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
