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Ouch. I had no idea that contended Arc could be that expensive.

I found a contention bug inside of Wine a few weeks ago. Something that is supposed to be "lockless" really had three nested spinlocks. With many threads contending for a lock, performance would drop to about 1% of normal.[1]

[1] https://bugs.winehq.org/show_bug.cgi?id=54979

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Lockless is not the same as lock-free, which is not the same as wait-free which seems to be what you are describing

>Lockless is not the same as lock-free

Can I get the HN comment length explanation of what this means?

Lock-free is a term of art with a formally defined meaning (at least one thread makes forward progress at any time). Lock-less doesn't really have a well defined meaning (first you would have to define what a lock is).

Sometimes I wish HN would have a term-of-art dictionary built in so users can check if there's a special meaning to reduce the chance of misinterpretation.

lockless is often the wrong term, goal, idea and solution. mutexes/futexes do very well, almost zero cost when not contended.

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