time.monotonic vs time.perf_counter: What's the difference?

Gina Häußge // @foosel

EuroPython 2021

What do the docs say?

time.monotonic() → float
Return the value (in fractional seconds) of a monotonic clock, i.e. a clock that cannot go backwards. The clock is not affected by system clock updates. The reference point of the returned value is undefined, so that only the difference between the results of two calls is valid.

time.perf_counter() → float
Return the value (in fractional seconds) of a performance counter, i.e. a clock with the highest available resolution to measure a short duration. It does include time elapsed during sleep and is system-wide. The reference point of the returned value is undefined, so that only the difference between the results of two calls is valid.

https://docs.python.org/3/library/time.html

What does time.get_clock_info say?

Windows Linux MacOS
time.monotonic 64 Hz 👀 1,000,000,000 Hz 1,000,000,000 Hz
time.perf_counter 10,000,000 Hz 1,000,000,000 Hz 1,000,000,000 Hz

Implementation: GetTickCount64/QueryPerformanceCounter (Windows) vs clock_gettime(CLOCK_MONOTONIC) (Linux) vs mach_absolute_time (MacOS)

Monotonic: true

Let's have a closer look at the resolution

Test: Loop with 1,000,000,000 cycles, number of changes of returned value.

Windows Linux MacOS
time.monotonic 6811 👀 1,000,000,000 1,000,000,000
time.perf_counter 1,000,000,000 1,000,000,000 1,000,000,000

And what about performance?

timeit, 5,000,000 loops, best of 5.

Windows Linux MacOS
time.monotonic 54.2 nsec 👀 74 nsec 98.8 nsec
time.perf_counter 84.1 nsec 73.3 nsec 94.3 nsec

Summary

  1. time.monotonic and time.perf_counter are both monotonic.
  2. Under Windows, time.monotonic has lower resolution and higher performance than time.perf_counter. Under Linux and MacOS they are identical.

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