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[Bug]: High memory usage in Figure.savefig #29434
Description
Bug summary
When saving figures with large data images (imshow), matplotlib version 3.10.0 uses ~4.2 times the amount of memory, and runs ~2.5 times slower.
Code for reproduction
#!/usr/bin/env -S uv run --with humanize --with numpy --with "matplotlib==3.10"
import tracemalloc
import humanize
import matplotlib.pyplot as plt
import numpy as np
tracemalloc.start()
fig, ax = plt.subplots()
ax.imshow(np.zeros((10_000, 10_000)))
fig.savefig("test.png")
_, peak = tracemalloc.get_traced_memory()
print(f"Memory usage peaked at {humanize.naturalsize(peak)}")Actual outcome
Memory usage peaked at 7.2 GB
Expected outcome
Memory usage peaked at 1.7 GB
This is the result when the shebang in the code example is changed to "matplotlib<3.10", or when the fig.savefig("test.png") line is removed.
Additional information
May be related to #28904, although switching the backend to Cairo doesn't affect the results.
Operating system
Arch
Matplotlib Version
3.10.0
Matplotlib Backend
agg
Python version
Python 3.13.0
Jupyter version
No response
Installation
None
Activity
[-][Bug]: High memory usage by `Figure.savefig` starting in version 3.10.0[/-][+][Bug]: High memory usage by `Figure.savefig`[/+][-][Bug]: High memory usage by `Figure.savefig`[/-][+][Bug]: High memory usage in `Figure.savefig`[/+]QuLogic commentedon Jan 9, 2025
This bisects to 3ceba40; you can set
image.interpolation_stagetodatato avoid it switching to thergbastage.zqianem commentedon Jan 9, 2025
Thanks for tracking this down!
Would it be worth adding #28061, the image resampling docs, and this workaround to the 3.10 release notes? I could see this affecting quite of bit of code with non-obvious slowdowns and OOM errors (which is how I found it in the first place 🙂).
tacaswell commentedon Jan 9, 2025
Some what ironically, the big images are where the RGBA interpolation is probably the most useful.
jklymak commentedon Jan 9, 2025
Yeah, I'm not quite sure why that is not in the release notes. Sorry for not catching that it was missing.