[Regression] New multithreaded reformat function slower by default than with just 1 thread used in PyAV < 17.0.0

[timminator]

In the latest version, to_ndarray(format='rgb24') is significantly slower by default. Forcing threads=1 restores the original performance inline with PyAV 16.1.0. Seems to only occur consistently on unique video frames, hence the startup code in the script underneath. I expected a performance increase from this new update. :-)
@lgeiger I hope its fine if I tag you here - you added this new functionality which is really appreciated and you might know whats going on here. :-)

Example script:

import timeit
import numpy as np

n = 500

setup_code = f"""
import av
frames = [av.VideoFrame(1920, 1080, 'yuv420p') for _ in range({n})]
frame_iter = iter(frames)
"""

stmt_code = "next(frame_iter).to_ndarray(format='rgb24')"
stmt_code_1thread = "next(frame_iter).to_ndarray(format='rgb24', threads=1)"

# Run benchmark
times = timeit.repeat(stmt_code, setup=setup_code, repeat=10, number=n)
times_per_call = np.array(times) / n
times = timeit.repeat(stmt_code_1thread, setup=setup_code, repeat=10, number=n)
times_per_call_1thread = np.array(times) / n

print(f"Multithreaded (new default): {times_per_call.mean() * 1e3:.4f} ± {times_per_call.std() * 1e3:.4f} ms")
print(f"Single threaded (PyAV < 17.0.0): {times_per_call_1thread.mean() * 1e3:.4f} ± {times_per_call_1thread.std() * 1e3:.4f} ms")

Result:

Multithreaded (new default): 2.5456 ± 0.0552 ms
Single threaded (PyAV < 17.0.0): 1.8747 ± 0.0305 ms

Operating System: Windows 11
Python version: 3.12.8

[WyattBlue]

You might be just be measuring 3.11 limited api perf regression, which is known issue. You should try using 3.14t

[lgeiger]

I can't seem to reproduce this regression on my machine. I'm using the wheel from pypi with 3.12.13 on macOS:

Multithreaded (new default): 0.6496 ± 0.0289 ms
Single threaded (PyAV < 17.0.0): 1.4913 ± 0.0045 ms

@timminator Maybe this is an issue related to Windows? Can you also reproduce this on Linux?

[timminator]

Thanks for the quick replies!

@lgeiger Yes, im also seeing this on Linux:

(myenv) gpuvm@gpuvm-Virtual-Machine:~/test$ echo "CPU: $(lscpu | grep 'Model name' | sed 's/Model name:[[:space:]]*//')" && echo "OS: $(grep PRETTY_NAME /etc/os-release | cut -d= -f2 | tr -d '"')" && echo "Python: $(python -V 2>&1)"
CPU: AMD Ryzen 9 5900X 12-Core Processor
OS: Ubuntu 22.04.5 LTS
Python: Python 3.12.8
(myenv) gpuvm@gpuvm-Virtual-Machine:~/test$ python test.py
Multithreaded (new default): 1.9957 ± 0.1240 ms
Single threaded (PyAV < 17.0.0): 0.7005 ± 0.0184 ms

I dont have a mac so I cant test there aswell.
@WyattBlue I have not tested on 3.14t but I think something else is going on here. On Linux and Windows its considerably slower.

Edit: FYI: I've noticed this in my program that makes use of PyAV where the whole runtime in my testcase increased from 42 seconds to over 60 seconds after upgrading to PyAV 17.0.0.