SCHED_ULE makes 256Mbyte i386 unusable

[Rick Macklem]

I decided to start a new thread on current related to SCHED_ULE, since I see
more than just performance degradation and on a recent current kernel.
(I cc'd a couple of the people discussing performance problems in freebsd-stable
 recently under a subject line of "Re: kern.sched.quantum: Creepy, sadistic scheduler".

When testing a pNFS server on a single core i386 with 256Mbytes using a Dec. 2017
current/head kernel, I would see about a 30% performance degradation (elapsed
run time for a kernel build over NFSv4.1) when the server kernel was built with
options SCHED_ULE
instead of
options SCHED_4BSD

Now, with a kernel from a couple of days ago, the
options SCHED_ULE
kernel becomes unusable shortly after starting testing.
I have seen two variants of this:
- Became essentially hung. All I could do was ping the machine from the network.
- Reported "vm_thread_new: kstack allocation failed
  and then any attempt to do anything gets "No more processes".
with the only difference being a kernel built with
options SCHED_4BSD
everything works and performs the same as the Dec 2017 kernel.

I can try rolling back through the revisions, but it would be nice if someone
could suggest where to start, because it takes a couple of hours to build a
kernel on this system.

So, something has made things worse for a head/current kernel this winter, rick