"swiN: clock sio" process taking 75% CPU

[Gareth McCaughan]

I wrote:
> About 6 minutes after booting (on two occasions; I don't
> guarantee that this doesn't vary), a process that appears
> in the output of "ps" as "[swi4: clock sio]" begins to
> use about 3/4 of the machine's CPU. I think it does so
> more or less instantaneously. It continues to do so
> indefinitely, so far as I can tell.
[etc]

No ideas? I'm willing to help track this down, and the machine
in question is sufficiently little used that I can do so without
gross inconvenience; but I don't have enough FreeBSD kernel
expertise to feel like diving in blind.

                             *

A little more information, in case it's useful to anyone:

  | $ echo; sysctl debug | egrep to_
  | debug.to_avg_mpcalls: 2890
  | debug.to_avg_mtxcalls: 0
  | debug.to_avg_gcalls: 768
  | debug.to_avg_depth: 3815

That's with HZ = 100. Here are some numbers from a message
in freebsd-ia64, from Marcel Moolenaar, in 2004-07, to someone
seeing symptoms like mine. They're meant to be typical healthy
numbers. Mine above look somewhat worse, but not insanely so;
surely not enough to explain the difference between using
0.3% cpu and using 75%. Marcel also had HZ=100.

  | % sysctl debug | grep to_avg
  | debug.to_avg_depth: 2500
  | debug.to_avg_gcalls: 1003
  | debug.to_avg_mpcalls: 1255

                             *

It would be a shame if the only conclusion to be drawn from this
were "sometimes a machine running FreeBSD is just 4x slower than
it should be, and no one knows why".

-- 
g