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Can you have the runtime run a job every second, to trim thread stack depths and return memory to the kernel (whatever was meant by https://www.youtube.com/watch?v=kPR8h4-qZdk&t=1150s) while holding a FFI mutex (or a sharded one), and have every C function call first lock the mutex and expand the stack to the necessary depth?

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Yes, you can use madvise(MADV_FREE) to return the RAM on Linux. I've seen a thread pool library that does this. I don't remember how this part works, but I think it's possible to maintain a high water mark in some inexpensive fashion to know when it's worth making this call.

Fucking with memory maps like that is super expensive and I wouldn’t advise it in any part that’s performance critical due to the cross CPU TLB flush that syscall will entail.

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