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

On Fri, Aug 31, 2012 at 10:36:47AM -0700, George Dunlap wrote:
> 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

So what happens if you run with a 32-bit dom0? What is the kernel
version? There were some issues with extra traps being done due to the
cpuidle running (which it should not).

> 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

Can you try booting with 'nohz=off'. What does 'perf top' (you need to
run v3.4 or later) give you?

> 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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