What is the PREEMPTION option good for?

[Ivan Voras]

Robert Watson wrote:

> There's a known performance regression with PREEMPTION and loopback
> network traffic on UP or UP-like systems due to a poor series of context
> switches occuring in the network stack.  If your benchmark involves the
> above web load over the loopback, that could be the source of what
> you're seeing.  If it's not loopback traffic, then that's not the source
> of the problem.

The dynamic stuff is accessing the database (fairly intensively) over
the loopback.

> You might try fiddling with kern.sched.ipiwakeup.enabled and see what
> the effect is, btw -- this controls whether or not the scheduler wakes
> up another idle CPU to run a thread when waking up that thread, rather
> than queuing it to run which may occur on the other CPU at the next
> clock tick.

Try this with or without PREEMPTION?