Re: [Xen-devel] Scheduling of I/O domains

["G. Milos" <gm281@xxxxxxxxxxxxxxxx>]

I've just checked in another set of changes to the scheduling code.
Now the default scheduler (BVT) handles I/O bound domains much better. I 
run the test suggested by Rob:
time dd if=/dev/sda1 of=/dev/null bs=1024k count=1000
with the folloving results:
dd alone:
run 1: 16.830s
run 2: 17.058s
run 3: 16.999s
avg:   16.962s

dd against cpu bound process in another domain:
run 1: 17.348s
run 2: 16.973s
run 3: 16.931s
avg:   17.084s

difference: 0.72% (small enough)

As soon as the code gets pushed in to bkbits it will be available for 
tests.
Currently the warp mechanism does not work (it has never been implemented 
properly). Changing the warp parameters will have small effect on the 
scheduler's behaviour. As soon as I get this to work we can arrange for 
domain 0 to run warped, thus receiving CPU in "privilaged" manner.
Cheers
Gregor


> 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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