SCHED_ULE should not be the default

[perryh at pluto.rain.com]

Jeremy Chadwick <freebsd at jdc.parodius.com> wrote:

> It is already easy to switch schedulers.  You change the
> option in your kernel config, rebuild kernel (world isn't
> necessary as long as you haven't csup'd between your last
> rebuild and now), make installkernel, shutdown -r now,
> done.

and you have thereby shot freebsd-update in the foot,
because you are no longer using a generic kernel.

> If what you're proposing is to make the scheduler changeable
> in real-time?  I think that would require a **lot** of work
> for something that very few people would benefit from ...

Switching on the fly sounds frightfully difficult, as long as
4BSD and ULE are separate code bases.  (It might not be so bad
if a tunable or 3 could be added to ULE, so that it could be
configured to behave like 4BSD.)

However, the freebsd-update complication could in principle be
relieved by building both schedulers into the generic kernel,
with the choice being configurable in loader.conf.  It would
still take a reboot to switch, but not a kernel rebuild.  Of
course there may be practical issues, e.g. name collisions.