Rewrite _PyTime_GetWinPerfCounter() for _PyTime_t

[vstinner]

BPO	31773
Nosy	@vstinner
PRs	bpo-31773: time.perf_counter() uses again double #3964 bpo-31773: _PyTime_GetPerfCounter() uses _PyTime_t #3983
Files	round.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2017-10-24.09:06:08.175>
created_at = <Date 2017-10-12.14:14:14.674>
labels = ['interpreter-core', 'type-feature', '3.7']
title = 'Rewrite _PyTime_GetWinPerfCounter() for _PyTime_t'
updated_at = <Date 2017-10-24.09:06:08.173>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2017-10-24.09:06:08.173>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2017-10-24.09:06:08.175>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2017-10-12.14:14:14.674>
creator = 'vstinner'
dependencies = []
files = ['47217']
hgrepos = []
issue_num = 31773
keywords = ['patch']
message_count = 5.0
messages = ['304239', '304263', '304327', '304477', '304877']
nosy_count = 1.0
nosy_names = ['vstinner']
pr_nums = ['3964', '3983']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31773'
versions = ['Python 3.7']

[vstinner]

The commit a997c7b of bpo-31415 moved the implementation of time.perf_counter() from Modules/timemodule.c to Python/pytime.c. The change not only moved the code, but also changed the internal type storing time from floatting point number (C double) to integer number (_PyTyime_t = int64_t).

The drawback of this change is that time.perf_counter() now converts QueryPerformanceCounter() / QueryPerformanceFrequency() double into a _PyTime_t (integer) and then back to double. Two useless conversions required by the _PyTime_t format used in Python/pytime.c. These conversions introduced a loss of precision.

Try attached round.py script which implements the double <=> _PyTime_t conversions and checks to check for precision loss. The script shows that we loose precision even with a single second for QueryPerformanceFrequency() == 3579545.

It seems like QueryPerformanceFrequency() now returns 10 ** 7 (10_000_000, resolution of 100 ns) on Windows 8 and newer, but returns 3,579,545 (3.6 MHz, resolution of 279 ns) on Windows 7. It depends maybe on the hardware clock, I don't know. Anyway, whenever possible, we should avoid precision loss of a clock.

[vstinner]

New changeset cba9a0c by Victor Stinner in branch 'master':
bpo-31773: time.perf_counter() uses again double (GH-3964)
cba9a0c

[vstinner]

I reopen the issue since I found a solution to only use integer in pytime.c for QueryPerformanceCounter() / QueryPerformanceFrequency() *and* prevent integer overflow.

[vstinner]

New changeset bdaeb7d by Victor Stinner in branch 'master':
bpo-31773: _PyTime_GetPerfCounter() uses _PyTime_t (GH-3983)
bdaeb7d

[vstinner]

Buildbots seem to be happy, I close the issue.

See my new PEP-564 for the next chapiter of this story ;-)