New submission from STINNER Victor: The article https://atleastfornow.net/blog/not-all-bytes/ says that bytes.join(sequence) is slower on Python 3 compared to Python 2.
I compared Python 2 and Python 3 code: the main different seems to be that Python 3 uses the Py_buffer API to support more types than only bytes, and that it allocates a buffer (on the C stack or on the heap memory) of Py_buffer objects. Attached patch makes bytes.join(sequence) up to 29% faster. The patch avoids the Py_buffer API and the allocation of the temporary array of Py_buffer if all items are bytes or bytearray. I'm not 100% sure that it's worth it to optimize bytes.join(). On Python 2, bytes += bytes uses an hack in Python/ceval.c to optimize this instruction. On Python 3, the optimization is only applied to str += str (unicode), bytes += bytes is inefficient. To get best performances on Python 2 and Python 3, bytes.join(sequence) is better than bytes+=bytes. Microbenchmark commands: $ ./python -m perf timeit -s "sep=b''; seq=(b'hello', b'world')" 'sep.join(seq)' $ ./python -m perf timeit -s "sep=b''; seq=(b'hello', b'world', b'. ') * 100" 'sep.join(seq)' Python 3.6 => patched Python3.6: * 2 items: 92.1 ns +- 1.8 ns => 90.3 ns +- 3.1 ns (-2%) * 300 items: 3.11 us +- 0.07 us => 2.22 us +- 0.11 us (-29%) -- I'm not sure that Python 3 is really slower than Python 2 :-/ Python 3.5 is 10 ns slower tha Python 2.7 for sequence of 2 items, but it's 6% faster for 300 items. So the question is if it's worth it to optimize bytes.join(). Python 2: * 2 items: 87.7 ns +- 3.7 ns * 300 items: 3.25 us +- 0.11 us Python 3.5: * 2 items: 97.4 ns +- 9.0 ns * 300 items: 3.06 us +- 0.20 us ---------- files: bytes_join.patch keywords: patch messages: 274855 nosy: haypo, serhiy.storchaka priority: normal severity: normal status: open title: Optimize bytes.join(sequence) type: performance versions: Python 3.6 Added file: http://bugs.python.org/file44444/bytes_join.patch _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue28004> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
