[Bug]: High memory usage in `Figure.savefig` · Issue #29434 · matplotlib/matplotlib · GitHub

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

This bisects to 3ceba40; you can set image.interpolation_stage to data to avoid it switching to the rgba stage.

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 🙂).

Some what ironically, the big images are where the RGBA interpolation is probably the most useful.

Yeah, I'm not quite sure why that is not in the release notes. Sorry for not catching that it was missing.

[Bug]: High memory usage in `Figure.savefig` · Issue #29434 · matplotlib/matplotlib · GitHub

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[Bug]: High memory usage in Figure.savefig #29434

Bug summary

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Additional information

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[Bug]: High memory usage in `Figure.savefig` · Issue #29434 · matplotlib/matplotlib · GitHub

Repository navigation

[Bug]: High memory usage in Figure.savefig #29434

Description

Bug summary

Code for reproduction

Actual outcome

Expected outcome

Additional information

Operating system

Matplotlib Version

Matplotlib Backend

Python version

Jupyter version

Installation

Activity

QuLogic commented on Jan 9, 2025

zqianem commented on Jan 9, 2025

tacaswell commented on Jan 9, 2025

jklymak commented on Jan 9, 2025

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