responsiveness during IO tasks

[Julian Elischer]

> Doug Barton wrote:
> No problem, just let's hunt things down. I'll wait for that larger post.
> In meantime, if it is related to eventtimers, it would be good to
> collect more detailed information. You could try to make timer run
> during idle (kern.eventtimer.idletick). You could try to switch timer
> from one-shot to periodic mode (kern.eventtimer.periodic). You could
> also try to switch to another timer (kern.eventtimer.timer).

kern.eventtimer.periodic needs to be disabled to run 9.x on xen
(as of a few months ago)