I/O or Threading Suffer

[jesk]

hi,
i figured out that the performanceloss only really occur if the process is
heavily writing on the filesystem.
dd if=/dev/zero of=/dev/null bs=128k doesnt hurt much in responsetime
of parallel processes, but when dd operates on the filesystem with of=foo
every process will be affect in executiontime.
a simple ps or ls meanwhile dding onto the disk will be hang for dozen of
seconds.

i tested it with 4BSD and ULE with and without Preemption.