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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2011-03-03 at 07:29 +0000, Keir Fraser wrote:
> 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.

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'll take a look shortly if no one beats me to it.

>  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
> that).
> 
>  -- Keir
> 
> > Thanks
> > 
> > James
> > 
> > _______________________________________________
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> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> 
> 
> 
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