mypy is slow when type checking torch

[hauntsaninja]

λ mypy --version          
mypy 1.11.2 (compiled: yes)

λ uv pip show torch       
Using Python 3.11.8 environment at /Users/shantanu/.virtualenvs/openai-wfht
Name: torch
Version: 2.1.0
Location: /Users/shantanu/.virtualenvs/openai-wfht/lib/python3.11/site-packages
Requires: filelock, fsspec, jinja2, networkx, sympy, typing-extensions
Required-by: ...

λ time mypy -c 'import torch' --no-incremental
Success: no issues found in 1 source file
mypy -c 'import torch' --no-incremental  33.09s user 2.73s system 98% cpu 36.391 total

λ time mypy -c 'import torch'
Success: no issues found in 1 source file
mypy -c 'import torch'  6.24s user 0.88s system 95% cpu 7.454 total

We use a lot of torch at work, performance is probably the biggest reason folks at work switch to a different type checker.

[hauntsaninja]

If this is accurate, maybe the fscache exception handling is really slowing us down in the mypyc build.

mypyc:
native

interpreted:
interpreted

[JukkaL]

mypy -v produces details about processed files, and this seems important:

LOG:  Processing SCC of size 945 (torch.onnx._globals torch._inductor.exc torch._inductor.runtime.hi
nts torch.utils._traceback torch.utils._sympy.functions ... <long output snipped>

Mypy detects an import cycle with 945 modules.

Overall 1380 files were parsed, so 68% of processed files are in this one SCC. I've seen this pattern in other third-party packages as well -- the majority of the implementation is a single SCC.

A potential way to make the SCC smaller would be to process imports lazily in third-party modules (where this is possible, since errors aren't reported). It may be tricky to implement though, but I'll think about it more.

[hauntsaninja]

Yeah, lazy import resolution could be a massive perf win

[hauntsaninja]

#17924 is the issue for tracking lazy resolution

Jukka's times in #17920 (comment) are much better than mine. #17948 is the issue for tracking performance improvements in my work environment.