Re: [Xen-users] VCPU amount

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

On Thu, Mar 26, 2009 at 8:51 PM, Carsten Schiers <carsten@xxxxxxxxxx> wrote:
> Hi,
>
> I don't want to hijack this thread. But as I lately posted a question which
> mainly was about weight and cap and this fits in perfectly (and also you seem
> to be an expert here), I repeat it below.

I'm just another Xen user :)

>
> In the meantime I saw an academic article Ludmilla Cherkasova et al. from 
> which
> I understood that you have to careful that a Dom0 doesn't get too much weight,
> as otherwise it's I/O hunger will slow down DomU activity.

In normal cirmustances Dom0 handles all physical I/O (disk, network)
including the ones that come from domU, so it should have highest
priority.

In general though, on multicore systems dedicating one core to dom0 is
enough, leaving other cores for domU. In that case dom0's weight is
somewhat irrelevant since it owns a dedicated core anyway.

> currently, I run Xen on a AMD Dualcore CPU. I have

Dual core is somewhat tricky. For best performance I always recommend
giving dom0 a dedicated core. That leaves only one CPU for all domUs,
whose priority you can adjust with cpu_weight.

>
>  Dom0 (clearly)
>  DomU Firewall (Endian)
>  DomU Fileserving (Samba)
>  DomU Mail&Fax (Scalix, Hylafax)
>  DomU Videostraming (Vdr)
>  DomU DMZ (web frontend, file sharing)
>  DomU 64 Bit development
>  DomU 32 Bit development

I don't think your system is suitable to run that many domUs, unless
all of them have very low load. For comparison, my current production
Xen servers range from 2 x dual-core to 4 x 6-core systems with at
least 8 GB memory, all running RHEL5 x86_64.

As usual though, do some testing to find out which setup works best for you.

Regards,

Fajar

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