Somebody Got It The Wrong Way Round ...

From
<https://www.infoworld.com/article/4018856/4-tips-for-getting-started-with-free-threaded-python.html>:

    As an example, if you have a job that writes a lot of files,
    having each job in its own thread is less effective if each job
    also writes the file. This is because writing files is an
    inherently serial operation. A better approach would be to divide
    jobs across threads and use one thread for writing to disk. As
    each job finishes, it sends work to the disk-writing job. This
    way, jobs don’t block each other and aren’t themselves blocked by
    file writing.

Actually, blocking system calls (whether for I/O or something else)
only block the current thread. So having each thread do its own I/O
should be faster than funnelling it all through one bottleneck thread.

If you don’t want your worker threads blocked waiting for I/O to
complete, then each worker context can be a pair of threads: one does
the CPU-intensive stuff, while the other handles the blocking I/O.