Spin Or Sleep

A microbenchmark on one lock. The critical section takes $C = 60$ ns. When a waiter blocks instead of spinning, the futex sleep, the later wake, and the two context switches cost $H = 3.0$ microseconds, and that cost is paid once per handoff. Eight threads call the lock at the same instant and each runs the critical section exactly once.

Under a pure spin lock the k-th thread to acquire waits

$t_{\text{spin}}(k) = (k-1),C$

and under a pure sleeping lock each handoff also pays the block-and-wake cost

$t_{\text{sleep}}(k) = (k-1),(C + H)$

How long does the last of the eight threads wait under each policy?