Re: [Xen-devel] Xen 4.2 TODO / Release Plan

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Tue, Aug 28, 2012 at 3:06 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
>     * [BUG] qemu-traditional has 50% cpu utilization on an idle
>       Windows system if USB is enabled. Not 100% clear whether this is
>       Xen or qemu.  George Dunlap is performing initial
>       investigations.

So it's hard to get directly comparable results, but I think that
early indications are that the biggest chunk of this is due to the
extra syscall overhead for a 64-bit dom0.  Data points are:
1. Ubuntu 12.04, 64-bit, pvops Ubuntu kernel, Xen 4.2-rc2, older AMD
system: qemu uses 50% on an idle system
2. XenServer built with Xen-4.2; (32-bit 2.6.32 dom0), Nehalem system:
<qemu uses 2% on an idle system
3. Debian wheezy with the squeeze 2.6.32 32-bit kernel, older AMD
system: qemu uses 10% on an idle system

Looking at the traces, it seems that on the AMD box there were just a
whole lot more USB-related IO accesses than on the Nehalem system.  #2
had far fewer USB-related accesses than #1, but #3 had about twice as
many as #1.  So it seems likely to be a combination between something
weird that the USB driver in the guest is doing under AMD, and the
extra overhead of a 64-bit kernel.

So I think this is probably OK to take off the blocker list (although
it's probably something we want to look into further).

 -George

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