Here are some faulthandler outputs. I couldn’t make heads or tails of the first one (fork), and went so far as to write my own home-grown version of it, which suffered the same fate anyway! :blobfoxupsidedowndizzy: The second one looks like it makes sense, so now I can actually track down what I think is a deadlock. :blobfoxthonking: :blobfoxthumbsup: