[issue37859] time.process_time() constant / erratic on Windows

[STINNER Victor]

STINNER Victor  added the comment:

> I see that now. The behaviour was different in Linux, though, I suppose it 
> may benefit from a more precise counter, but since in Windows it also has a 
> precise counter with time.perf_counter_ns(), I was expecting to see that 
> value change, but it was mainly a confusion with the older time.clock().

On Windows, time.clock() was implemented with QueryPerformanceCounter(). This 
function became time.perf_counter() in Python 3.4. time.clock() was removed. 
Use time.get_clock_info('perf_counter') ;-)

The PEP 418 introduces new well defined clocks, since time.clock() was not 
portable.

perf_counter and process_time have very different properties. process_time is 
stopped when the process sleeps, for example.

https://docs.python.org/dev/library/time.html#time.perf_counter
https://docs.python.org/dev/library/time.html#time.process_time

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[issue37859] time.process_time() constant / erratic on Windows

[STINNER Victor]

STINNER Victor  added the comment:

> Process times [1] are stored as a 64-bit integer in units of 100 ns (1e-7). 
> But the kernel schedules threads based on a timer that ticks every 15.625 ms 
> by default. It can be lowered to about 0.5 ms, but this degrades battery life.

Patches are welcome to enhance time.get_clock_info() :-)

Don't rely too much on time.get_clock_info() on Linux neither: it basically 
always say 1 ns, even if the effective resolution is way worse.

See the PEP 418 for some numbers:
https://www.python.org/dev/peps/pep-0418/#process-time

I wrote these programs to write this PEP ;-)

https://github.com/python/peps/tree/master/pep-0418

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[issue37859] time.process_time() constant / erratic on Windows

[Eryk Sun]

Eryk Sun  added the comment:

> I suppose it may benefit from a more precise counter, but since in 
> Windows it also has a precise counter with time.perf_counter_ns(), 
> I was expecting to see that value change, but it was mainly a 
> confusion with the older time.clock().

Don't read too much into the clock info here:

>>> time.get_clock_info('process_time').resolution
1e-07

Process times [1] are stored as a 64-bit integer in units of 100 ns (1e-7). But 
the kernel schedules threads based on a timer that ticks every 15.625 ms by 
default. It can be lowered to about 0.5 ms, but this degrades battery life.

[1] 
https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-getprocesstimes

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[issue37859] time.process_time() constant / erratic on Windows

[Red Glyph]

Red Glyph  added the comment:

I see that now. The behaviour was different in Linux, though, I suppose it may 
benefit from a more precise counter, but since in Windows it also has a precise 
counter with time.perf_counter_ns(), I was expecting to see that value change, 
but it was mainly a confusion with the older time.clock().

Thanks!

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[issue37859] time.process_time() constant / erratic on Windows

[Zachary Ware]

Change by Zachary Ware :


--
resolution:  -> not a bug
stage:  -> resolved
status: pending -> closed

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[issue37859] time.process_time() constant / erratic on Windows

[Zachary Ware]

Zachary Ware  added the comment:

Try this:

while time.process_time() < 0.5:
print('.', flush=True, end='')

Or:

>>> time.process_time()
0.03125
>>> len(str(2**500_000))
150515
>>> time.process_time()
0.484375


Basically, process_time() (and process_time_ns()) measure CPU time used by the 
process, and you aren't making the CPU do anything between your checks.

--
resolution: not a bug -> 
stage: resolved -> 
status: closed -> pending

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[issue37859] time.process_time() constant / erratic on Windows

[Red Glyph]

Change by Red Glyph :


--
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue37859] time.process_time() constant / erratic on Windows

[Red Glyph]

New submission from Red Glyph :

Tested with 
- Python 3.7.4, 64-bit, Windows version (installer version)
- python-3.8.0b3-embed-amd64.zip
- python-3.7.2.post1-embed-amd64.zip 
on Windows 7 x64 Professional

time.process_time() always returns the same value. If I check the value of 
time.process_time_ns(), sometimes it is constant, sometimes I observe a few 
changes, then it becomes constant too.

Here is a log of an example session, I'm waiting at least 1-2 seconds between 
each command:

Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 20:34:20) [MSC v.1916 64 bit 
(AMD64)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> import time
>>> time.process_time()
0.1092007
>>> time.process_time()
0.1092007
>>> time.process_time_ns()
109200700
>>> time.process_time_ns()
124800800
>>> time.process_time_ns()
124800800
>>> time.process_time()
0.1248008
>>> time.process_time()
0.1248008
>>> time.process_time()
0.1248008
>>> time.process_time_ns()
124800800
>>> time.process_time_ns()
124800800
>>> time.process_time_ns()
124800800
>>> time.process_time_ns()
124800800
>>> time.clock()

Warning (from warnings module):
  File "__main__", line 1
DeprecationWarning: time.clock has been deprecated in Python 3.3 and will be 
removed from Python 3.8: use time.perf_counter or time.process_time instead
77.006126126
>>> time.clock()
79.245575778
>>> time.clock()
80.801103036
>>> time.process_time()
0.1248008
>>>

--
components: Library (Lib), Windows
messages: 349745
nosy: Red Glyph, paul.moore, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: time.process_time() constant / erratic on Windows
type: behavior
versions: Python 3.7, Python 3.8

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