Strange delays on G5

Hello,

I've noticed sluggishness for a while but now I finally found some data
to at least somewhat narrow it down. Here's what I see:
- disk IO on G5 is slower than it used to be ( or should be ) - 60MB/s
  from a SATA drive that used to get around 100MB/s a few years ago.
  It's not just onboard SATA, I see the same with an ahcisata. No such
  thing on any of my G4s.
- shell startup takes several seconds on a 2GHz 970MP while on an
  800MHz G4 it's more or less instant.
- finally, during gooeymux hackery I noticed that it's not just shell
  startup that's slow - by the look of it all tty throughput eats up a
  lot more CPU time than doing the same thing on a much slower G4.

I don't see any differences in interrupt frequency, and I see the same
symptoms on G5s with HT PIC and regular OpenPIC, so that's not it
either.

I've seen this on several quite different machines:
- a powermac 11,2 and 7,2 - one PCIe / HT PIC / 970MP, the other PCI-X
  / openpic / single 970. Both show symptoms.
- on the other hand, I've been using a quicksilver, two different
  alubooks, a pismo, an xserve G4, a beige G3 and a performa 6330 - all
  of them work as expected.

Changes I remember around the time:
- there were tty locking changes, but I don't think our locks behave
  much different on G5 vs. everything else.
- pmap changes a while ago, IIRC they were specifically for G5
  stability. Since we don't have BATs on G5 I'd expect some more
  activity there, but nothing quite as noticeable.

Does that ring a bell with anyone?

have fun
Michael