Scheduling of I/O domains

[Rob Gardner <rob.gardner@hp.com>]

I have been looking for the code in xen that handles scheduling of I/O
domains, but have not succeeded in finding any. Only after some time did
I start thinking that maybe there wasn't any. So I did the following
simple experiment:
In domain 1:
  time dd if=/dev/hda of=/dev/null bs=1024k count=100
and this this took about 2 seconds.
Then I ran a cpu-consuming process in domain 2, and repeated the dd
command above. The result was more than 8 seconds! This strongly
suggests that domain 0 is not being treated specially by the scheduler,
and is being made to wait before servicing I/O interrupts, thereby
killing I/O performance in the presence of cpu-bound domain activity. It
seems to me that I/O domains need to be scheduled and dispatched
immediately upon receipt of an I/O interrupt.

Am I missing something here? How come nobody has noticed this behavior
before? Is somebody working on this? I

Rob Gardner






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