A worker message round trip cost about 12 µs, almost all of it in two
blocked waits: the poster signaled the waiting thread while it still held
the mailbox lock, so the woken thread went back to sleep on the lock, and
every wait paid a sleep and a wakeup. scr_mailbox_post and close now
signal after the unlock, and scr_mailbox_wait spins for up to 20 µs (or
the wait's timeout, if shorter) on a flag the queue keeps before it
blocks. A process that may run on only one CPU never spins.
A new bench:runtime workload, worker-messages, posts 100,000 round trips
between the main thread and a worker: 1296 -> 360 ms.
Chris Tate
8320cc2289