alt Hacker News> eventfd always pays a syscall on both sides (~200-400ns) regardless of load.
It’s fairly standard to make the waiting side spin a bit after processing some data, and only issue another wait syscall if no more data arrives during the spin period.
(For instance, io_uring, which does this kind of IPC with a kernel thread on the receiving side, literally lets you configure how long said kernel thread should spin[1].)
Fair point. The real difference is the narrower: with a futex the producer can inspect consumer_sleeping directly in shared memory and skip the FUTEX_WAKE entirely if the consumer is still spinning. With eventfd you need a write() regardless, or you add shared state to gate it, which is essentially rebuilding futex. Same idea but slightly less clean.