Re: Disk IO scheduling in XEN

[John Weekes <lists.xen@nuclearfallout.net>]

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On 3/24/2011 1:19 PM, Paresh Nakhe wrote:
> Hi,
>
> We are working on a project to modify the disk scheduling mechanism in 
> XEN a bit in an attempt to improve it. In the paper "Xen and the Art 
> of Virtualization"  following is mentioned.
>
> /"Xen services batches of requests from competing domains in a
> simple round-robin fashion; these are then passed to a standard elevator
> scheduler before reaching the disk hardware/"
>
> We were going through linux-jeremy source code in an attempt to map 
> the above in code. We could not however do so. On the contrary we came 
> to the conclusion that there is no such mechanism. Domain 0 services 
> requests as soon as it receives a hypercall from a guest domain. Are 
> we right?

Ian Campbell forward-ported the 2.6.18-xen.hg patches for this but I 
don't think that they've been merged into the main git 2.6.32 stable 
tree yet. Check the list archives for the subject line "unfair servicing 
of DomU vbd requests" for more information on this.

> Which one would be better first come first serve or round robin and why?

Of the two, RR, because domains can become starved otherwise.

-John

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