It's not really bypassing the kernel if you're using futexes, is it?

Exactly, futexes are only used if the consumer opts-in for the sleep phase. The low-latency hot path uses pure spin mode which completely bypasses kernel and fences.

	▲	sunnypq 2 days ago \| parent [-]
		Thanks for clarifying, I didn't spot that waiting on a futex was optional. I wonder to what extent the performance would be affected with a middle-ground option to spin a few times and then call sched_yield() syscall before spinning again.