The overhead in Python 3 for threading.Event().wait() is much larger than Python 2. I am trying to run a script at 60hz which worked correctly in Python2 but does not in Python 3. Here is a minimal example to demonstrate: 

#!/usr/bin/env python

import threading
import time

def sample_thread(stop_ev):
    while not stop_ev.is_set():
        t2 = time.time() 
        stop_ev.wait(0.016999959945)
        print((time.time() - t2))

def main():
    stop_ev = threading.Event()
    sample_t = threading.Thread(target=sample_thread, args=(stop_ev, ))
    sample_t.start()

    # Other stuff here, sleep is just dummy
    time.sleep(14)

    stop_ev.set()

    print('End reached.')

if __name__ == '__main__':
    main()

Python 2.7.0 consistently prints :

0.0169999599457
0.0169999599457
0.0170001983643
0.0169999599457
0.0169999599457
0.0169999599457
0.0169999599457
0.0169999599457

Python 3.8.2 waits much longer

0.031026363372802734
0.0320279598236084
0.031026363372802734
0.031026840209960938
0.031527042388916016
0.031026601791381836
0.03103041648864746
0.03302431106567383

Please provide the results of running the following with the two pythons you are using:

python -m test.pythoninfo

For what it's worth, running on macOS 10.15.5, I see a slight difference with more variability in the Python 3 results but nothing as pronounced as what you see. Also the Python 3 results were very similar across recent versions of 3.7.x, 3.8.x, and pre-release 3.9.

$ /usr/local/bin/python2.7 ~/test_41299.py | sort
0.017021894455
0.0170249938965
0.017028093338
0.017028093338
0.0170290470123
0.0170290470123
0.0170290470123
0.0170300006866
0.017030954361
0.0170311927795
[...]
0.0190448760986
0.0190479755402
0.0190498828888
0.0190510749817
0.0190589427948
0.019061088562
0.0190629959106
0.019079208374
0.0190861225128
0.0190970897675
End reached.0.0158078670502

$ /usr/local/bin/python3.8 ~/test_41299.py | sort
0.009722232818603516
0.017028093338012695
0.017033100128173828
0.017034053802490234
0.017034053802490234
0.017035961151123047
0.017037153244018555
0.017037153244018555
0.01703786849975586
0.017038822174072266
[...]
0.021351099014282227
0.021355152130126953
0.021355152130126953
0.02135610580444336
0.021361112594604492
0.021363258361816406
0.02136397361755371
0.021379947662353516
0.02138543128967285
0.021442890167236328
End reached.

I reproduced something similar on Python 3.9.0b1, Windows 64-bit version:

    py -m pyperf timeit -s "import threading; E = threading.Event()" "E.wait(<NUMBER>)"

NUMBER            Mean +- std dev
-------------------------------------------
0.0               5.79 us +- 0.13 us
0.000000000001    15.6 ms +- 0.1 ms
0.001             15.6 ms +- 0.1 ms
0.01              15.6 ms +- 0.6 ms
0.013             15.5 ms +- 0.6 ms
0.015             15.9 ms +- 0.9 ms
0.016             25.2 ms +- 0.5 ms
0.017             31.2 ms +- 0.2 ms
0.018             31.2 ms +- 0.4 ms
0.025             31.2 ms +- 1.0 ms
0.05              62.2 ms +- 0.8 ms
0.1               109 ms +- 2 ms
0.2               201 ms +- 3 ms
0.5               500 ms +- 0 ms
1.0               1.00 sec +- 0.00 sec

On the smaller scale, it looks quantized to multiples of ~15ms (?), but then it gets more accurate as the times get larger. I don't think it's a measurement error since the first measurement manages microseconds.

Perhaps this is just an OS-level thread-scheduling issue?

I have tested this both on vs code terminal and cygwin for windows.

3.8.2 : 

python -m test.pythoninfo
Python debug information
========================

Py_DEBUG: No (sys.gettotalrefcount() missing)
_decimal.__libmpdec_version__: 2.4.2
builtins.float.double_format: IEEE, little-endian
builtins.float.float_format: IEEE, little-endian
datetime.datetime.now: 2020-07-15 09:09:34.936153
expat.EXPAT_VERSION: expat_2.2.8
locale.encoding: cp1252
os.cpu_count: 12
os.environ[APPDATA]: C:\Users\XXXX\AppData\Roaming
os.environ[COMPUTERNAME]: XXXX
os.environ[COMSPEC]: C:\Windows\system32\cmd.exe
os.environ[HOMEDRIVE]: C:
os.environ[HOMEPATH]: \Users\XXXX
os.environ[LANG]: en_GB.UTF-8
os.environ[PATHEXT]: .COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC;.PY;.PYW;.CPL
os.environ[PATH]: C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.2\bin;C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.2\libnvvp;C:\Program Files\SlikSvn\bin;C:\Python\Python38\Scripts\;C:\Python\Python38\;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\OpenSSH\;C:\Program Files (x86)\NVIDIA Corporation\PhysX\Common;C:\Program Files\dotnet\;C:\Program Files\TortoiseSVN\bin;C:\Program Files\CMake\bin;C:\Program Files (x86)\IncrediBuild;C:\Program Files\NVIDIA Corporation\NVIDIA NvDLISR;C:\Users\XXXX\AppData\Local\Microsoft\WindowsApps;;C:\Users\XXXX\AppData\Local\Programs\Microsoft VS Code\bin
os.environ[SYSTEMROOT]: C:\Windows
os.environ[TEMP]: C:\Users\XXXX\AppData\Local\Temp
os.environ[TMP]: C:\Users\XXXX\AppData\Local\Temp
os.environ[USERPROFILE]: C:\Users\XXXX
os.environ[WINDIR]: C:\Windows
os.getcwd: 
os.login: XXXX
os.name: nt
os.supports_bytes_environ: False
os.supports_effective_ids: []
os.supports_fd: ['stat', 'truncate']
os.supports_follow_symlinks: ['stat']
os.umask: 000
platform.architecture: 64bit WindowsPE
platform.platform: Windows-10-10.0.18362-SP0
platform.python_implementation: CPython
pymem.allocator: pymalloc
pymem.with_pymalloc: True
socket.hostname: 
sqlite3.sqlite_version: 3.28.0
sqlite3.version: 2.6.0
ssl.HAS_SNI: True
ssl.OPENSSL_VERSION: OpenSSL 1.1.1d  10 Sep 2019
ssl.OPENSSL_VERSION_INFO: (1, 1, 1, 4, 15)
ssl.OP_ALL: -0x7fffffac
ssl.OP_NO_TLSv1_1: 0x10000000
ssl.SSLContext.maximum_version: TLSVersion.MAXIMUM_SUPPORTED
ssl.SSLContext.minimum_version: TLSVersion.MINIMUM_SUPPORTED
ssl.SSLContext.options: Options.OP_NO_SSLv3|OP_CIPHER_SERVER_PREFERENCE|OP_ENABLE_MIDDLEBOX_COMPAT|OP_NO_COMPRESSION|OP_ALL
ssl.SSLContext.protocol: _SSLMethod.PROTOCOL_TLS
ssl.SSLContext.verify_mode: VerifyMode.CERT_NONE
ssl.default_https_context.maximum_version: TLSVersion.MAXIMUM_SUPPORTED
ssl.default_https_context.minimum_version: TLSVersion.MINIMUM_SUPPORTED
ssl.default_https_context.options: Options.OP_NO_SSLv3|OP_CIPHER_SERVER_PREFERENCE|OP_ENABLE_MIDDLEBOX_COMPAT|OP_NO_COMPRESSION|OP_ALL
ssl.default_https_context.protocol: _SSLMethod.PROTOCOL_TLS
ssl.default_https_context.verify_mode: VerifyMode.CERT_REQUIRED
ssl.stdlib_context.maximum_version: TLSVersion.MAXIMUM_SUPPORTED
ssl.stdlib_context.minimum_version: TLSVersion.MINIMUM_SUPPORTED
ssl.stdlib_context.options: Options.OP_NO_SSLv3|OP_CIPHER_SERVER_PREFERENCE|OP_ENABLE_MIDDLEBOX_COMPAT|OP_NO_COMPRESSION|OP_ALL
ssl.stdlib_context.protocol: _SSLMethod.PROTOCOL_TLS
ssl.stdlib_context.verify_mode: VerifyMode.CERT_NONE
subprocess._USE_POSIX_SPAWN: False
sys.api_version: 1013
sys.builtin_module_names: ('_abc', '_ast', '_bisect', '_blake2', '_codecs', '_codecs_cn', '_codecs_hk', '_codecs_iso2022', '_codecs_jp', '_codecs_kr', '_codecs_tw', '_collections', '_contextvars', '_csv', '_datetime', '_functools', '_heapq', '_imp', '_io', '_json', '_locale', '_lsprof', '_md5', '_multibytecodec', '_opcode', '_operator', '_pickle', '_random', '_sha1', '_sha256', '_sha3', '_sha512', '_signal', '_sre', '_stat', '_statistics', '_string', '_struct', '_symtable', '_thread', '_tracemalloc', '_warnings', '_weakref', '_winapi', '_xxsubinterpreters', 'array', 'atexit', 'audioop', 'binascii', 'builtins', 'cmath', 'errno', 'faulthandler', 'gc', 'itertools', 'marshal', 'math', 'mmap', 'msvcrt', 'nt', 'parser', 'sys', 'time', 'winreg', 'xxsubtype', 'zlib')
sys.byteorder: little
sys.dont_write_bytecode: False
sys.executable: C:\Python\Python38\python.exe
sys.filesystem_encoding: utf-8/surrogatepass
sys.flags: sys.flags(debug=0, inspect=0, interactive=0, optimize=0, dont_write_bytecode=0, no_user_site=0, no_site=0, ignore_environment=0, verbose=0, bytes_warning=0, quiet=0, hash_randomization=1, isolated=0, dev_mode=False, utf8_mode=0)
sys.float_info: sys.float_info(max=1.7976931348623157e+308, max_exp=1024, max_10_exp=308, min=2.2250738585072014e-308, min_exp=-1021, min_10_exp=-307, dig=15, mant_dig=53, epsilon=2.220446049250313e-16, radix=2, rounds=1)
sys.float_repr_style: short
sys.hash_info: sys.hash_info(width=64, modulus=2305843009213693951, inf=314159, nan=0, imag=1000003, algorithm='siphash24', hash_bits=64, seed_bits=128, cutoff=0)
sys.hexversion: 50856688
sys.implementation: namespace(cache_tag='cpython-38', hexversion=50856688, name='cpython', version=sys.version_info(major=3, minor=8, micro=2, releaselevel='final', serial=0))
sys.int_info: sys.int_info(bits_per_digit=30, sizeof_digit=4)
sys.maxsize: 9223372036854775807
sys.maxunicode: 1114111
sys.path: ['C:\\Python\\Python38\\python38.zip', 'C:\\Python\\Python38\\DLLs', 'C:\\Python\\Python38\\lib', 'C:\\Python\\Python38', 'C:\\Users\\XXXX\\AppData\\Roaming\\Python\\Python38\\site-packages', 'C:\\Python\\Python38\\lib\\site-packages']
sys.platform: win32
sys.prefix: C:\Python\Python38
sys.stderr.encoding: utf-8/backslashreplace
sys.stdin.encoding: utf-8/surrogateescape
sys.stdout.encoding: utf-8/surrogateescape
sys.thread_info: sys.thread_info(name='nt', lock=None, version=None)
sys.version: 3.8.2 (tags/v3.8.2:7b3ab59, Feb 25 2020, 23:03:10) [MSC v.1916 64 bit (AMD64)]
sys.version_info: sys.version_info(major=3, minor=8, micro=2, releaselevel='final', serial=0)
sys.windowsversion: sys.getwindowsversion(major=10, minor=0, build=18363, platform=2, service_pack='')
sys.winver: 3.8
sysconfig[prefix]: C:\Python\Python38
test_socket.HAVE_SOCKET_ALG: False
test_socket.HAVE_SOCKET_CAN: False
test_socket.HAVE_SOCKET_CAN_ISOTP: False
test_socket.HAVE_SOCKET_QIPCRTR: False
test_socket.HAVE_SOCKET_RDS: False
test_socket.HAVE_SOCKET_VSOCK: False
test_support.IPV6_ENABLED: True
test_support._is_gui_available: True
test_support.python_is_optimized: False
time.altzone: 14400
time.daylight: 1
time.get_clock_info(monotonic): namespace(adjustable=False, implementation='GetTickCount64()', monotonic=True, resolution=0.015625)
time.get_clock_info(perf_counter): namespace(adjustable=False, implementation='QueryPerformanceCounter()', monotonic=True, resolution=1e-07)
time.get_clock_info(process_time): namespace(adjustable=False, implementation='GetProcessTimes()', monotonic=True, resolution=1e-07)
time.get_clock_info(thread_time): namespace(adjustable=False, implementation='GetThreadTimes()', monotonic=True, resolution=1e-07)
time.get_clock_info(time): namespace(adjustable=True, implementation='GetSystemTimeAsFileTime()', monotonic=False, resolution=0.015625)
time.time: 1594818575.0377405
time.timezone: 18000
time.tzname: ('Eastern Standard Time', 'Eastern Summer Time')
tkinter.TCL_VERSION: 8.6
tkinter.TK_VERSION: 8.6
tkinter.info_patchlevel: 8.6.9
windows.RtlAreLongPathsEnabled: True
windows.dll_path: C:\Python\Python38\python38.dll
zlib.ZLIB_RUNTIME_VERSION: 1.2.11
zlib.ZLIB_VERSION: 1.2.11


2.7.18 : 

 python -m test.pythoninfo
Python debug information
========================

Py_DEBUG: No (sys.gettotalrefcount() missing)
builtins.float.double_format: IEEE, little-endian
builtins.float.float_format: IEEE, little-endian
datetime.datetime.now: 2020-07-15 09:28:00.404000
expat.EXPAT_VERSION: expat_2.2.8
locale.encoding: cp1252
os.cwd: C:\Python\Python27
os.environ[APPDATA]: C:\Users\XXXX\AppData\Roaming
os.environ[COMPUTERNAME]: 
os.environ[COMSPEC]: C:\Windows\system32\cmd.exe
os.environ[HOMEDRIVE]: C:
os.environ[HOMEPATH]: \Users\XXXX
os.environ[PATHEXT]: .COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC;.PY;.PYW
os.environ[PATH]: C:\Program Files\ConEmu\ConEmu\Scripts;C:\Program Files\ConEmu;C:\Program Files\ConEmu\ConEmu;C:\Python\Python384\Scripts\;C:\Python\Python384\;C:\Users\XXXX\AppData\Local\Microsoft\WindowsApps;;C:\Users\XXXX\AppData\Local\Programs\Microsoft VS Code\bin;C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.2\bin;C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.2\libnvvp;C:\Program Files\SlikSvn\bin;C:\Python\Python38\Scripts\;C:\Python\Python38\;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\OpenSSH\;C:\Program Files (x86)\NVIDIA Corporation\PhysX\Common;C:\Program Files\dotnet\;C:\Program Files\TortoiseSVN\bin;C:\Program Files\CMake\bin;C:\Program Files (x86)\IncrediBuild;C:\Program Files\NVIDIA Corporation\NVIDIA NvDLISR
os.environ[SYSTEMROOT]: C:\Windows
os.environ[TEMP]: C:\Users\XXXX\AppData\Local\Temp
os.environ[TMP]: C:\Users\XXXX\AppData\Local\Temp
os.environ[USERPROFILE]: C:\Users\XXXX
os.environ[WINDIR]: C:\Windows
os.name: nt
os.umask: 000
platform.architecture: 64bit WindowsPE
platform.platform: Windows-10-10.0.18362
platform.python_implementation: CPython
socket.hostname: 
sqlite3.sqlite_version: 3.28.0
sqlite3.version: 2.6.0
ssl.HAS_SNI: True
ssl.OPENSSL_VERSION: OpenSSL 1.0.2t  10 Sep 2019
ssl.OPENSSL_VERSION_INFO: (1, 0, 2, 20, 15)
ssl.OP_ALL: -0x7ffffc01
ssl.OP_NO_TLSv1_1: 0x10000000
ssl.SSLContext(PROTOCOL_TLS).options: OP_NO_SSLv3|OP_NO_SSLv2|OP_CIPHER_SERVER_PREFERENCE|OP_SINGLE_DH_USE|OP_SINGLE_ECDH_USE|OP_NO_COMPRESSION|OP_ALL
ssl.SSLContext(PROTOCOL_TLS).protocol: PROTOCOL_TLS
ssl.SSLContext(PROTOCOL_TLS).verify_mode: CERT_NONE
ssl.default_https_context.options: OP_NO_SSLv3|OP_NO_SSLv2|OP_CIPHER_SERVER_PREFERENCE|OP_SINGLE_DH_USE|OP_SINGLE_ECDH_USE|OP_NO_COMPRESSION|OP_ALL
ssl.default_https_context.protocol: PROTOCOL_TLS
ssl.default_https_context.verify_mode: CERT_REQUIRED
ssl.stdlib_context.options: OP_NO_SSLv3|OP_NO_SSLv2|OP_CIPHER_SERVER_PREFERENCE|OP_SINGLE_DH_USE|OP_SINGLE_ECDH_USE|OP_NO_COMPRESSION|OP_ALL
ssl.stdlib_context.protocol: PROTOCOL_TLS
ssl.stdlib_context.verify_mode: CERT_NONE
sys.api_version: 1013
sys.builtin_module_names: ('__builtin__', '__main__', '_ast', '_bisect', '_codecs', '_codecs_cn', '_codecs_hk', '_codecs_iso2022', '_codecs_jp', '_codecs_kr', '_codecs_tw', '_collections', '_csv', '_functools', '_heapq', '_hotshot', '_io', '_json', '_locale', '_lsprof', '_md5', '_multibytecodec', '_random', '_sha', '_sha256', '_sha512', '_sre', '_struct', '_subprocess', '_symtable', '_warnings', '_weakref', '_winreg', 'array', 'audioop', 'binascii', 'cPickle', 'cStringIO', 'cmath', 'datetime', 'errno', 'exceptions', 'future_builtins', 'gc', 'imageop', 'imp', 'itertools', 'marshal', 'math', 'mmap', 'msvcrt', 'nt', 'operator', 'parser', 'signal', 'strop', 'sys', 'thread', 'time', 'xxsubtype', 'zipimport', 'zlib')
sys.byteorder: little
sys.dont_write_bytecode: False
sys.executable: C:\Python\Python27\python.exe
sys.filesystem_encoding: mbcs
sys.flags: sys.flags(debug=0, py3k_warning=0, division_warning=0, division_new=0, inspect=0, interactive=0, optimize=0, dont_write_bytecode=0, no_user_site=0, no_site=0, ignore_environment=0, tabcheck=0, verbose=0, unicode=0, bytes_warning=0, hash_randomization=0)
sys.float_info: sys.float_info(max=1.7976931348623157e+308, max_exp=1024, max_10_exp=308, min=2.2250738585072014e-308, min_exp=-1021, min_10_exp=-307, dig=15, mant_dig=53, epsilon=2.220446049250313e-16, radix=2, rounds=1)
sys.float_repr_style: short
sys.hexversion: 34018032
sys.maxsize: 9223372036854775807
sys.maxunicode: 65535
sys.path: ['', 'C:\\Windows\\SYSTEM32\\python27.zip', 'C:\\Python\\Python27\\DLLs', 'C:\\Python\\Python27\\lib', 'C:\\Python\\Python27\\lib\\plat-win', 'C:\\Python\\Python27\\lib\\lib-tk', 'C:\\Python\\Python27', 'C:\\Python\\Python27\\lib\\site-packages']
sys.platform: win32
sys.prefix: C:\Python\Python27
sys.stderr.encoding: cp850
sys.stdin.encoding: cp850
sys.stdout.encoding: cp850
sys.version: 2.7.18 (v2.7.18:8d21aa21f2, Apr 20 2020, 13:25:05) [MSC v.1500 64 bit (AMD64)]
sys.version_info: sys.version_info(major=2, minor=7, micro=18, releaselevel='final', serial=0)
sys.windowsversion: sys.getwindowsversion(major=6, minor=2, build=9200, platform=2, service_pack='')
sys.winver: 2.7
sysconfig[prefix]: C:\Python\Python27
test_support.IPV6_ENABLED: True
test_support._is_gui_available: True
test_support.python_is_optimized: False
time.altzone: 14400
time.daylight: 1
time.time: 1594819680.4
time.timezone: 18000
time.tzname: ('Eastern Standard Time', 'Eastern Summer Time')
tkinter.TCL_VERSION: 8.5
tkinter.TK_VERSION: 8.5
zlib.ZLIB_VERSION: 1.2.11

Thanks for the version info.

Windows and/or threading experts: any ideas?

At a guess, it's probably the signal emulation (a.k.a. Ctrl+C support). We could save some time by checking the requested handle first without blocking, and only beginning the blocking call if it's not available.

> On the smaller scale, it looks quantized to multiples of ~15ms (?), 
> but then it gets more accurate as the times get larger. I don't 
> think it's a measurement error since the first measurement manages
> microseconds.

NT's default system timer resolution for thread dispatching is 15.625 ms. If a wait is for 17 ms, expect to see actual wait times mostly in the range 17 ms up to 31.25 ms. A few outliers may wait longer because the due time on the kernel wait is when the thread is ready to be dispatched, but other threads may preempt it.

The system timer resolution can be increased to about 1 ms (or 500 us with the undocumented NtSetSystemTime system call). But stricter timing still requires combining a dispatcher wait for long course-grained waits with a performance-counter busy wait for short precise waits.

---

But something else is also involved here. I lowered the system timer resolution to 1 ms, and this allowed time.sleep(0.017) to wait for 17-18 ms in about 95% of cases. But the wait for acquiring a _thread.lock stubbornly refused to cooperate. Ultimately it's just calling WaitForSingleObjectEx on a semaphore, so I used ctypes to make a simple alternative lock via CreateSemaphoreW, ReleaseSemaphore, and WaitForSingleObject. This simple implementation performed exactly like the time.sleep wait with regard to the system timer resolution, so the difference is in the _thread.lock wait. I traced it to the following code in EnterNonRecursiveMutex in Python/thread_nt.h:

        /* wait at least until the target */
        ULONGLONG now, target = GetTickCount64() + milliseconds;
        while (mutex->locked) {
            if (PyCOND_TIMEDWAIT(&mutex->cv, &mutex->cs, (long long)milliseconds*1000) < 0) {
                result = WAIT_FAILED;
                break;
            }
            now = GetTickCount64();
            if (target <= now)
                break;
            milliseconds = (DWORD)(target-now);
        }

GetTickCount64 is documented to be "limited to the resolution of the system timer", but what they don't say is that changing the resolution of the system timer has no effect on the minimum increment of the tick count. It still increments by 15-16 ms even if the system timer resolution is set to 1 ms. 

OTOH, the value of QueryInterruptTime [1] and QueryUnbiasedInterruptTime [2] is incremented with the timer interrupt. The biased version [1] is preferred for long waits that may straddle a system sleep or hibernate, but it's only available in Windows 10. I patched EnterNonRecursiveMutex to call QueryInterruptTime instead of GetTickCount64, and this did enable increased precision when waiting on a lock. For example (patched behavior):

    >>> lock = _thread.allocate_lock()
    >>> lock.acquire()
    True
    >>> setup = 'from __main__ import lock'
    >>> stmt = 'lock.acquire(True, 0.017)'

15.625 ms system timer:

    >>> timeit.timeit(stmt, setup, number=1000)
    30.173713599999985

1 ms system timer:

    >>> with set_timer_resolution(0.001):
    ...     timeit.timeit(stmt, setup, number=1000)
    ...
    17.66828049999998

That said, increasing the timer resolution is discouraged in most cases, so  we may simply document that lock waits are limited to the default system timer resolution of 15.625 ms, and increasing the system timer resolution has no effect on this limit.

[1]: https://docs.microsoft.com/en-us/windows/win32/api/realtimeapiset/nf-realtimeapiset-queryinterrupttime
[2]: https://docs.microsoft.com/en-us/windows/win32/api/realtimeapiset/nf-realtimeapiset-queryunbiasedinterrupttime

---

Note that this is unrelated to cancel support via Ctrl+C. Windows Python has no support for canceling a wait on a _thread.lock. It's just a single-object wait in _PyCOND_WAIT_MS, not a multiple-object wait that we set up to include the SIGINT event when called on the main thread (or a variant that I like, which queues a user APC to the main thread for SIGINT instead of using an event, and switches to using alertable waits with SleepEx, WaitForSingleObjectEx, and without needing a wait slot in WaitForMultipleObjectsEx). It's possible to implement a Ctrl+C cancel as long as the lock implementation waits on a kernel semaphore object. However, some effort has gone into developing a different implementation based on condition variables and SRW locks. I don't know whether there's a way to cancel SleepConditionVariableSRW, or whether maybe a different implementation could be used for _thread.lock instead of sharing an implementation with the GIL.

> or 500 us with the undocumented NtSetSystemTime system call

Umm... that should be NtSetTimerResolution. ;-)

So there is no advised way around this then. I am currently using time.sleep() to hit my 60hz.

>So there is no advised way around this then.

Changing the implementation of EnterNonRecursiveMutex() to use WinAPI QueryInterruptTime() instead of GetTickCount64() would allow increasing the wait precision by setting the system timer resolution to the minimum supported period via timeBeginPeriod() [1]. It seems reasonable to at least support doing this.

That said, changing the system timer resolution shouldn't be encouraged since it affects the entire system and can increase power consumption. If you have results in 2.7 similar what's shown in msg373660, check clockres in a separate terminal to see the current timer interval [2].

---
[1] https://docs.microsoft.com/en-us/windows/win32/api/timeapi/nf-timeapi-timebeginperiod
[2] https://docs.microsoft.com/en-us/sysinternals/downloads/clockres

I just ran into this. GetTickCount64() is a bad choice even without improving the Windows timer resolution, as every mutex wait will have 16ms of jitter. Here are some lock.acquire(timeout=0.001) times measured with time.perf_counter():

elapsed=21.215ms
elapsed=30.960ms
elapsed=21.686ms
elapsed=30.998ms
elapsed=30.794ms

Here's the same lock.acquire(timeout=0.001) with CPython patched to use QueryPerformanceCounter() instead of GetTickCount64(). Notice this is less overhead than even the original post's Python 2.x times.

elapsed=9.554ms
elapsed=14.516ms
elapsed=13.985ms
elapsed=13.434ms
elapsed=13.724ms

Here's the QueryPerformanceCounter() test in a timeBeginPeriod(1) block:

elapsed=1.135ms
elapsed=1.204ms
elapsed=1.189ms
elapsed=1.052ms
elapsed=1.052ms

I'd like to submit a PR to fix the underlying issue by switching to QueryPerformanceCounter() in EnterNonRecursiveMutex().

QueryInterruptTime() is a bad candidate because it's only supported on Windows 10, and CPython still supports Windows 8. Improvements based on QueryPerformanceCounter() can be backported to at least 3.8 (3.8 dropped Windows XP support, which was the last Windows version where QueryPerformanceCounter() could fail).

I checked and the only other use of GetTickCount64() seems to be in time.monotonic(). Honestly I would vote to change time.monotonic() to QueryPerformanceCounter() as well, as QueryPerformanceCounter() can no longer fail on any Windows newer than XP (which is no longer supported by Python), but that's probably a topic for a new BPO.

Ok, I filed a PR for this. I used pytime's interface to avoid duplicating the QueryPerformanceFrequency() code.

I found a StackOverflow answer that says QueryPerformance functions will only fail if you pass in an unaligned pointer: https://stackoverflow.com/a/27258700

Per that, I used Py_FatalError to catch this case, as there is probably something wrong with the compiler at that point, and the other recovery options would be likely to result in incorrect program behavior (e.g. dead lock).

Change looks good. 3.8 is security fixes only at this stage, but it can go back to 3.9

New changeset 449e6f0ef395231e3abe467f910b02d7f075c27f by Ryan Hileman in branch 'main':
bpo-41299: Reduce lag in Windows threading timeouts by using a higher precision time source (GH-26568)
https://github.com/python/cpython/commit/449e6f0ef395231e3abe467f910b02d7f075c27f

New changeset 57b3ca7f0aef4d180038d475398f809d3fcdd8be by Miss Islington (bot) in branch '3.9':
bpo-41299: Reduce lag in Windows threading timeouts by using a higher precision time source (GH-26568)
https://github.com/python/cpython/commit/57b3ca7f0aef4d180038d475398f809d3fcdd8be

I suggest to remove the (now <= 0) check.

        _PyTime_t now = _PyTime_GetPerfCounter();
        if (now <= 0) {
            Py_FatalError("_PyTime_GetPerfCounter() == 0");
        }

Please don't add such Py_FatalError() in the middle of a function. If this case happens, it means that Python immediately exits and creates a debug report (ex: coredump on Unix, popup on Windows asking to open a debugger), and worse: it exits the process immediately.

I'm not aware of anyone reporting that _PyTime_GetPerfCounter() returns 0. Do you fear that the clock is broken and *always* report 0? If yes, please add at check when Python starts. But why do you think that it could happen?

I agree with not throwing fatal errors, but that check is unlikely to actually be hit, and you removed the startup checks covering the underlying clocks here: https://github.com/python/cpython/commit/ae6cd7cfdab0599139002c526953d907696d9eef

I think if the time source is broken, a user would probably prefer an exception or fatal error to the deadlock they will otherwise get (as time not advancing means it's impossible to timeout), so it doesn't make sense to remove the check without doing something else about it.

There are three places win_perf_counter_frequency() can fail: https://github.com/python/cpython/blob/bb3e0c240bc60fe08d332ff5955d54197f79751c/Python/pytime.c#L974

I mention the QueryPerformanceFrequency error case here (stack misalignment): https://bugs.python.org/issue41299#msg395237

The other option, besides startup checks, would be to better parameterize the timer used (mentioned in bpo-44328): Prefer QueryUnbiasedInterruptTimePrecise if available (Win 10+ via GetProcAddress), then QPC, then QueryUnbiasedInterruptTime (which has the original bug wrt jitter but should never be used as QPC is unlikely to be broken).

Perhaps the simplest initial fix would be to move that check down to PyThread__init_thread() in the same file. I'm not sure what the cpython convention for that kind of init error is, would it just be the same Py_FatalError block or is there a better pattern?

I'm fine with adding _PyTime_Init() again, and maybe only implement checks on Windows.

On Linux, I only saw a single failure in _PyTime_Init() once, when Python was run in a sandbox, and the time syscalls were not allowed. It was a corner case about the new 64-bit time_t syscalls on a 32-bit system (container):
https://bugzilla.redhat.com/show_bug.cgi?id=1887445

> I mention the QueryPerformanceFrequency error case here (stack misalignment): https://bugs.python.org/issue41299#msg395237

Do you think that pytime.c has the bug? I don't think so.

> There are three places win_perf_counter_frequency() can fail: https://github.com/python/cpython/blob/bb3e0c240bc60fe08d332ff5955d54197f79751c/Python/pytime.c#L974

In theory yes, in practice we got zero error reports. So it sounds like it cannot happen.

I don't think that it's a good pratice to start to add checks in all functions using a clock "just in case" the clock might fail.

> Do you think that pytime.c has the bug? I don't think so.

No, a misaligned stack would be an issue in the caller or compiler, not pytime.c. I have hit misaligned stack in practice, but it should be rare enough to check on init only.

> In theory yes, in practice we got zero error reports. So it sounds like it cannot happen.
> I don't think that it's a good practice to start to add checks in all functions using a clock "just in case" the clock might fail.

My read is that as long as we're not confident enough to remove those checks from pytime.c, a caller should assume they're reachable. If the pytime checks need to stay, adding a Windows only pytime init check to make sure that locks won't deadlock sounds fine to me.

> My read is that as long as we're not confident enough to remove those checks from pytime.c, a caller should assume they're reachable. If the pytime checks need to stay, adding a Windows only pytime init check to make sure that locks won't deadlock sounds fine to me.

If you consider that the risk is non-zero, you can add a check at startup.

Do you expect that a startup check is enough, or do you expect failures later?

I understood that if the first call succeed, later calls are very unlikely to fail. The first call should be enough to check for all possible errors.

New changeset 8673b77e251e42874501a47b1df86c6bde4fe1d2 by Miss Islington (bot) in branch '3.10':
bpo-41299: Reduce lag in Windows threading timeouts by using a higher precision time source (GH-26568) (GH-26580)
https://github.com/python/cpython/commit/8673b77e251e42874501a47b1df86c6bde4fe1d2

I created PR #28548 to remove the Py_FatalError() code path.

Calling Py_FatalError() must be avoided by any mean. I spent significant time to remove Py_FatalError() at Python startup in my PEP 587 implementation: https://www.python.org/dev/peps/pep-0587/

I also removed Py_FatalError() calls to report errors to the caller: https://bugs.python.org/issue38631

Calling Py_FatalError() is really bad when Python is embedded in an application. It kills the whole process, the caller has no way to catch the error.

New changeset f35ddf24227e834c9b6b39ad23a0ec382b4de48b by Victor Stinner in branch 'main':
bpo-41299: QueryPerformanceFrequency() cannot fail (GH-28552)
https://github.com/python/cpython/commit/f35ddf24227e834c9b6b39ad23a0ec382b4de48b

New changeset d639e3170506a1a80c4b2f1b3a6250bb95ff190a by Victor Stinner in branch 'main':
bpo-41299: Fix EnterNonRecursiveMutex() (GH-28548)
https://github.com/python/cpython/commit/d639e3170506a1a80c4b2f1b3a6250bb95ff190a

New changeset 6bc89116cb121c176453b9745e3daa2cc5df43ab by Victor Stinner in branch 'main':
bpo-41299: Mark private thread_nt.h functions as static (GH-28553)
https://github.com/python/cpython/commit/6bc89116cb121c176453b9745e3daa2cc5df43ab

I close the issue.

> bpo-41299: Fix EnterNonRecursiveMutex() (GH-28548)

I don't think that it's worth it to backport this change to 3.9 and 3.10. In practice, Py_FatalError() will never be called, and _PyTime_GetMonotonicClock() result must always be greater than 0.

But I wanted to remove it in the main branch to reduce the number of functions calling Py_FatalError().