Dispatch latency

[Andrew Doran <ad%netbsd.org@localhost>]

Hi,

I'm posting this for no other reason than I think it's kind of interesting.
It's a histogram generated from src/regress/sys/kern/latency2.

There are two SCHED_FIFO threads. Both are bound to a single CPU that is not
handling device interrupts, is otherwise online. There are 8 CPUs and the
system is running build.sh so the threads have to contend for scheduler
locks, cpu cache and the system bus. One thread grabs a timestamp, wakes the
other causing a kernel preemption, and then the receiver measures how long
the wakeup+preemption took.

        http://www.netbsd.org/~ad/rthist.png

Andrew