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Re: [Xen-devel] unfair servicing of DomU vbd requests

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxx>
Subject: Re: [Xen-devel] unfair servicing of DomU vbd requests
From: Keir Fraser <keir.xen@xxxxxxxxx>
Date: Thu, 03 Mar 2011 08:30:07 +0000
Cc: xen devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Delivery-date: Thu, 03 Mar 2011 00:32:09 -0800
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Thread-topic: [Xen-devel] unfair servicing of DomU vbd requests
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On 03/03/2011 08:28, "James Harper" <james.harper@xxxxxxxxxxxxxxxx> 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

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