calling `buffer.par_sort_unstable_by_key` from a task calls the task itself about rayon HOT 4 CLOSED

I removed inner parallelism instead, it was easier to implement and didn't have a significant impact on performance in my workload.

Thanks again for your help

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Reproduced on an AMD Ryzen 9 7950X, since @progval was worried about their CPU being bugged. ;)

Add this to the top of the file for use with cargo play:

//# stderrlog = '*'
//# thread_local = '*'
//# rayon = '*'

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This is an instance of #592 -- the call to par_sort may lead to work-stealing while it waits for sub-tasks to finish, and that may steal another piece of your task that wants to use that mutex, making you re-entrant.

As a workaround, maybe your TLS can have an Option<Vec<T>> for the buffer that you take out while you're working with it, rather than holding the Mutex locked? Then store it back as Some when you're done. When re-entrancy finds it None, you can allocate a new one instead. Maybe you can even keep Vec<Vec<T>> to save multiple allocations just in case, with pop to use and then push it back (or push the new one).

(I'm assuming the buffer is to save allocation time, not needing re-used data between instances.)

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This is an instance of #592

Ouch. I noticed that issue but assumed I wouldn't be affected because of the thread-specific mutexes.

may lead to work-stealing while it waits for sub-tasks to finish

Indeed, that matches experience of the deadlocks often occurring toward the end of the execution.

As a workaround, maybe your TLS can have an Option<Vec> for the buffer that you take out while you're working with it, rather than holding the Mutex locked? Then store it back as Some when you're done. When re-entrancy finds it None, you can allocate a new one instead.

Makes sense. This should be pretty rare so the overhead won't be too much. (I'll need to keep the mutexes instead of UnsafeCells I initially used, though.)

Thanks!

(I'm assuming the buffer is to save allocation time, not needing re-used data between instances.)

Correct, my actual code .clear()s it after sorting.

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