Re: [Xen-users] ntpd under Xen Dom0 exhibits extremely high jitter/noise? runs stable/quiet under non-xen kernel.

["Fajar A. Nugraha" <fajar@xxxxxxxxx>]

On Thu, Jan 14, 2010 at 11:32 PM, mail ignored <0.bugs.only.0@xxxxxxxxx> wrote:
> hi,
>
> On Wed, Jan 13, 2010 at 1:55 PM, Pasi Kärkkäinen <pasik@xxxxxx> wrote:
>> like Andy pointed out on xen-devel, the xen.gz option is called
>> dom0_max_vcpus nowadays.
>>
>> I verified dom0_max_vcpus=1 works OK with Xen 3.4.2.
>
> I've just been told @novell bugzilla,
>
>  "Please be also advised that dom0_max_vcpus=1 is not recommended (and
> not supported); a minimum of 2 should be set here."
>
> Any thoughts/comments?  Should it be used @ =1, or not?  confusing, atm.


Your original problem was that ntpd experience high noise/jitter. IMHO
dedicated dom0 cpu is not the fix for that, so that's two different
issues.

First, about ntpd. Since you're using Opensuse kernel, your best bet
is novell guys, since users on this list does not know what cause or
fix that issue. A quick workaround is to use Redhat's kernel, which
worked fine for me with both Intel and AMD cpus. But from your posts I
gather you want to stick with Opensuse kernel.

Second, about dom0 cpu. It's generally best to assign at least one
physical core/cpu exclusive for dom0 usage. What I did is use this on
grub.conf

        kernel /xen.gz-3.4.1 com2=115200,8n1 console=vga,com2 dom0_mem=768M
dom0_vcpus_pin

and this on xend-config.sxp

(dom0-cpus 1)

and set domUs to use cpus other than cpu0, with a config line like "cpus=1-7"

The difference between this approach and the one you're trying to use:
- I hard-limit dom0 memory. This is useful because dom0 won't have to
balloon-down memory for domU usage, plus it doesn't have to maintain
necessary memory metadata to support full physical available memory
(thus more efficient memory usage)
- I don't limit dom0 cpu on xen.gz line, but I do it on
xend-config.sxp. What this means is that dom0 will start initially
with all available cpu, but when xend starts it changes to only 1 cpu.
The part "dom_vcpus_pin" ensures that the active cpu for dom0 will be
cpu0, thus you can assign other cpus to domUs. The benefit of this
approach is that it allows me to later add more cpus to dom0 when
necessary (for example, if I want to compile a new kernel on dom0).

If I understand correctly, dom0_max_vcpus=1 means that you hardlimit
dom0's cpu to 1, thus you'll be unable to add more cpu to dom0 without
reboot. This might be the reason why novell guys recommended against
it. You'll have to ask them for the details though. Note that
"unsupported" does not allways mean it won't work :D

-- 
Fajar

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