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GeezerVison is showing github.com — gh-108724: Use _PyTime_GetMonotonicClock() in parking_lot.c by vstinner · Pull Request #112222 · python/cpython

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@vstinner

@vstinner vstinner commented Nov 17, 2023 •
edited by bedevere-app Bot
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Using a system clock causes issues when it's being updated by the system administrator, NTP, Daylight Saving Time, or anything else. Sadly, on Windows, the monotonic clock resolution is around 15.6 ms.

Example: #85876

  • Issue: Add lightweight locking C API #108724

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@vstinner
pythongh-108724: Use _PyTime_GetMonotonicClock() in parking_lot.c
71a000c 
Using a system clock causes issues when it's being updated by the
system administrator, NTP, Daylight Saving Time, or anything
else. Sadly, on Windows, the monotonic clock resolution is around
15.6 ms.

Example: python#85876
@vstinner vstinner added the skip news label Nov 17, 2023
@bedevere-app bedevere-app Bot added the awaiting core review label Nov 17, 2023
@bedevere-app bedevere-app Bot mentioned this pull request Nov 17, 2023
Add lightweight locking C API #108724
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@vstinner

vstinner commented Nov 17, 2023

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@colesbury @ericsnowcurrently: Please don't use the system clock for locks. It causes too many issues. Using a monotonic clock is more reliable. Sadly, on Windows, the monotonic clock has a resolution of 15.6 ms.
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colesbury
colesbury requested changes Nov 17, 2023
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@colesbury colesbury left a comment

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We should use the monotonic clock when available, but this change isn't sufficient:

  • sem_timedwait does not work with CLOCK_MONOTONIC. (Need sem_clockwait if available)
  • pthread_cond_t needs to be initialized with the right attribute

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colesbury
colesbury reviewed Nov 17, 2023
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Comment thread Python/parking_lot.c
Comment on lines +121 to 124
_PyTime_t deadline = _PyTime_Add(_PyTime_GetMonotonicClock(), timeout);
_PyTime_AsTimespec(deadline, &ts);

err = sem_timedwait(&sema->platform_sem, &ts);

@colesbury colesbury Nov 17, 2023

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Maybe:

# ifdef HAVE_SEM_CLOCKWAIT
        _PyTime_t deadline = _PyDeadline_Init(timeout);
        _PyTime_AsTimespec_clamp(deadline, &ts);
        err = sem_clockwait(&sema->platform_sem, CLOCK_MONOTONIC, &ts);
# else
        _PyTime_t deadline = _PyTime_Add(_PyTime_GetSystemClock(), timeout);
        _PyTime_AsTimespec_clamp(deadline, &ts);
        err = sem_timedwait(&sema->platform_sem, &ts);
 #endif

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@vstinner

vstinner commented Nov 17, 2023

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Oh. I made a mechanical change to avoid _PyTime_GetSystemClock(). I didn't see that timestamps are passed to existing functions such as pthread_cond_timedwait(). I mark my PR as a draft for now and will try to update it to address Sam's review
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@vstinner
vstinner marked this pull request as draft November 17, 2023 23:24
@bedevere-app bedevere-app Bot removed the awaiting core review label Nov 17, 2023
@vstinner vstinner closed this Dec 1, 2023
@vstinner
vstinner deleted the monotonic_parking branch December 1, 2023 18:26
@vstinner

vstinner commented Dec 1, 2023

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I don't have time to fix my PR right now. I may try again later ;-)
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@colesbury

colesbury commented Dec 1, 2023 •
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I wrote it up as an issue so that I don't forget about this and in case someone else wants to tackle it first: #112606
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@colesbury colesbury colesbury requested changes

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@vstinner @colesbury
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