Re: unfair servicing of DomU vbd requests

[Keir Fraser <keir.xen@gmail.com>]

On 03/03/2011 08:28, "James Harper" <james.harper@bendigoit.com.au> wrote:

>>>> It appears that Dom0 is not servicing vbd requests from DomU's fairly so
>>>> one or two end up getting stalled while the others are mostly okay. How
>>>> are vbd requests supposed to be serviced? Is there potential for one to
>>>> be overlooked for a long period of time? Is there some settings that
>>>> could be changed to avoid this happening?
>>> 
>>> Dom0 does round-robin scanning of pending event channels these days, which
>>> helps fairness a fair bit.
>> 
>> I have a feeling this isn't true of pvops kernels...
>> 
>> looks like we need to pull 324:7fe1c6d02a2b (and subsequent fixes) out
>> of 2.6.18-xen.hg into the pvops world.
>> 
> 
> I hope that's true and easy to fix. It would certainly explain why one DomU
> can starve enough to the point where it's IO doesn't get serviced for >70
> seconds.

Without the round-robin servicing, unfairness to the point of starvation is
a distinct possibility.

 -- Keir

> Thanks for looking at this.
> 
> James