time.sleep (floatsleep()) should use clock_nanosleep() on Linux

[shankarunni]

BPO	21302
Nosy	@vstinner, @4kir4, @1st1, @eryksun, @Livius90
PRs	[3.10] bpo-21302: Add clock_nanosleep() implementation for time.sleep() in Linux #28077 bpo-21302: Add clock_nanosleep() implementation for time.sleep() #28111 bpo-21302: Update time.sleep() doc for clock_nanosleep() #28311 bpo-21302: Add Waitable timer implementation for time.sleep() in Windows #28341 bpo-21302: Add _PyTime_AsNanoseconds() #28350 bpo-21302: time.sleep() uses waitable timer on Windows #28483 bpo-21302: Add nanosleep() implementation for time.sleep() in Unix #28526 bpo-21302: Add nanosleep() implementation for time.sleep() in Unix #28545
Files	wait.py bench.py

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

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GitHub fields:

assignee = None
closed_at = <Date 2021-09-25.12:37:55.425>
created_at = <Date 2014-04-18.20:12:03.043>
labels = ['type-bug', 'library', '3.11']
title = 'time.sleep (floatsleep()) should use clock_nanosleep() on Linux'
updated_at = <Date 2021-10-11.08:30:28.782>
user = 'https://bugs.python.org/shankarunni'

bugs.python.org fields:

activity = <Date 2021-10-11.08:30:28.782>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2021-09-25.12:37:55.425>
closer = 'vstinner'
components = ['Library (Lib)']
creation = <Date 2014-04-18.20:12:03.043>
creator = 'shankarunni'
dependencies = []
files = ['50289', '50294']
hgrepos = []
issue_num = 21302
keywords = ['patch']
message_count = 32.0
messages = ['216799', '216814', '217225', '217226', '217227', '217233', '217235', '401108', '401701', '401706', '401824', '402279', '402437', '402438', '402440', '402441', '402442', '402448', '402618', '402619', '402630', '402632', '403528', '403544', '403550', '403568', '403569', '403581', '403583', '403610', '403632', '403633']
nosy_count = 7.0
nosy_names = ['vstinner', 'akira', 'python-dev', 'yselivanov', 'eryksun', 'shankarunni', 'Livius']
pr_nums = ['28077', '28111', '28311', '28341', '28350', '28483', '28526', '28545']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue21302'
versions = ['Python 3.11']

[shankarunni]

I know that an earlier request to use nanosleep() has been rejected as "wontfix", but I'm filing this one for a different reason.

Today, timemodule.c:floatsleep() calls select() on platforms that support it. On Linux, select() with a timeout has an unfortunate property that it is very sensitive to clock jumps, because it computes a sleep end time based on the current kernel timestamp.

If the system clock is yanked back (by ntpd, or other processes), then the process can end up sleeping for a very long time. (E.g. if the clock is yanked back by half an hour while we are in the middle of, say, a sleep(10), then the process will sleep until "original_kernel_clock+10", which will turn into a half-hour sleep.

Yes, systems shouldn't jerk their clocks around, but we can't often control this sort of thing on end-user environments.

Using clock_nanosleep(CLOCK_MONOTONIC, 0, <timespec>, NULL) makes the sleep a much more reliable thing, and mostly insensitive to such jumps. (It'll still be affected by any adjtime(), but that's OK in this case).

[shankarunni]

I'm working on a patch, but I noticed a similar issue in Condition.wait(), which also keeps re-evaluating the "remaining sleep time" based on the current kernel clock, with similar effects.

I'll try to address both issues, or we could open a separate bug for the latter..

[vstinner]

I know that an earlier request to use nanosleep() has been rejected as "wontfix"

It was the issue bpo-13981. I created this issue while I worked on the PEP-410 (nanosecond timestamp). I closed the issue myself, it doesn't mean that Python must not use the function, just that I didn't want to work on it anymore at this time.

[vstinner]

"I'm working on a patch, but I noticed a similar issue in Condition.wait(), which also keeps re-evaluating the "remaining sleep time" based on the current kernel clock, with similar effects."

I see that Lock.acquire(timeout) uses the C function gettimeofday() to recompute the timeout if acquiring the lock was interrupted (C error "EINTR"). It would be better to use a monotonic clock here, but please open a new issue because it's unrelated to nanosleep().

Or did you another bug?

By the way, you didn't mention the Python version. Are you working on Python 2.7 or 3.5?

See also the PEP-418.

[vstinner]

If you want to modify time.sleep(), you must be careful of the portability: Windows, Linux, but also Mac OS X, FreeBSD, Solaris, etc.

Try to describe the behaviour of each underlying C function on each platform to be able to describe the "portable behaviour" on all platforms, especially the expected behaviour when the system clock is changed (is time.sleep impacted or not? always?) and the expected behaviour when the system is suspended.

For example, it looks like nanosleep() uses a different clock depending on OS (Linux uses CLOCK_MONOTONIC, other UNIX platforms use CLOCK_REALTIME).
http://lists.gnu.org/archive/html/bug-coreutils/2012-08/msg00087.html

I know that you suggest to use clock_nanosleep(), but this function is not available on all platforms. For example, I would not use it on Windows.

Another example (on Fedora?): "sleep() ignores time spent with a suspended system"
http://mjg59.dreamwidth.org/7846.html

You should also decide how to handle interrupted sleep (C error "EINTR"). Currently, the sleep is interrupted, no error is raised.

I began to describe all these functions in the PEP-418, even if I didn't change the implementation with the PEP:
http://legacy.python.org/dev/peps/pep-0418/#sleep

[shankarunni]

If you want to modify time.sleep(), you must be careful of the portability: Windows, Linux, but also Mac OS X, FreeBSD, Solaris, etc.

Oh, I totally agree. What I'm trying to do is to define another autoconf flag (HAVE_CLOCK_NANOSLEEP), that does a feature test and enable that flag, and just use that if available.

Now that's a good point that if we have clock_nanosleep() on another platform (non-Linux) and it does the wrong thing, then I might have to add further discrimination.

For now, one sticking point that I've stumbled across is that clock_nanosleep() requires "-lrt". Complicates the autoconf check a bit.

[vstinner]

2014-04-27 2:26 GMT+02:00 Shankar Unni <report@bugs.python.org>:

> If you want to modify time.sleep(), you must be careful of the portability: Windows, Linux, but also Mac OS X, FreeBSD, Solaris, etc.

Oh, I totally agree. What I'm trying to do is to define another autoconf flag (HAVE_CLOCK_NANOSLEEP), that does a feature test and enable that flag, and just use that if available.

I'm talking about the expected behaviour which can be found in the
documentation of the function:
https://docs.python.org/dev/library/time.html#time.sleep

[Livius90]

Can you review my final implementation?
#28111

[vstinner]

New changeset 85a4748 by Livius in branch 'main':
bpo-21302: Add clock_nanosleep() implementation for time.sleep() (GH-28111)
85a4748

[vstinner]

New changeset 85dc53a by Victor Stinner in branch 'main':
bpo-21302: Update time.sleep() doc for clock_nanosleep() (GH-28311)
85dc53a

[vstinner]

New changeset b49263b by Victor Stinner in branch 'main':
bpo-21302: Add _PyTime_AsNanoseconds() (GH-28350)
b49263b

[vstinner]

wait.py: script to test the time.sleep() function. Press CTRL+C multiple times during the sleep to check that even if the sleep is interrupted, time.sleep() sleeps the expected duration.