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

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>, xen devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] unfair servicing of DomU vbd requests
From: Keir Fraser <keir.xen@xxxxxxxxx>
Date: Thu, 03 Mar 2011 07:29:55 +0000
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Thread-topic: [Xen-devel] unfair servicing of DomU vbd requests
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On 03/03/2011 02:25, "James Harper" <james.harper@xxxxxxxxxxxxxxxx> wrote:

> A user of GPLPV (see thread "blue screen in windows balloon driver") is
> getting a bug check in Windows under extremely high memory usage and
> swapfile thrashing tests across multiple DomU's. Responses to my query
> on the ntdev mailing list say that this would happen if an IO request is
> not completed after 70 seconds during high memory/pagefile pressure,
> which is what is happening.
> 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. When a pending event is found, the corresponding
blkfront has a batch of requests pulled down and submitted into Linux's
block subsystem at which point we have no more control over scheduling (it
could generally be configured though -- Linux has an admin mechanism for

 -- Keir

> Thanks
> James
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