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Re: [Xen-devel] Disk IO scheduling in XEN

To: Paresh Nakhe <paresh.nakhe@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Disk IO scheduling in XEN
From: John Weekes <lists.xen@xxxxxxxxxxxxxxxxxx>
Date: Thu, 24 Mar 2011 13:25:37 -0700
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On 3/24/2011 1:19 PM, Paresh Nakhe wrote:

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.

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