Re: Disk IO scheduling in XEN

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 03/24/2011 08:19 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?

There's no explicit Xen-specific disk scheduling.  Linux has a great
deal of intrinsic mechanisms for setting up disk scheduling, including
using cgroups to schedule groups of processes, so you can use those to
get any particular scheduling policy you desire.

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

I think there are many more options for disk scheduling than those.

For example: "is it better for guests to schedule their own requests, or
should they just pass requests straight through?".

    J