[Xen-users] Default CPU allocation

[Paul Stimpson <paul@xxxxxxxxxxxxxxxxxxxx>]

Hi,

We have a machine with a 4-core Hyperthreading-capable CPU that appears as 8 logical CPUs (0-7) to Xen.

The Dom0 is Ubuntu 12.04 with Xen 4.2.0. There are two guests: A low-load Ubuntu 12.04 Linux guest that is used for maintenance purposes. An HVM Windows Server guest that is doing all the heavy work but isn't currently heavily loaded.

Allocation is:

CPU0 - Dom0 shared with low-load maintenance guest.
CPU1 - Low-load maintenance guest shared with Windows guest
CPU2-7 - Windows guest exclusive

The following interesting questions just surfaced during a discussion:

How does the default Xen scheduler allocate cores? Is it intelligent enough to use the exclusive cores for the Windows guest before it resorts to the core it shares with the maintenance guest or does it go in core order? If it's going in core order, it looks like the sharing of its lowest numbered core would cause needless context switching even when Windows was lightly loaded. Would we be better to make core 7 the shared core instead of core 1?

I would value your opinions on this.

Thanks,
Paul.

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