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Re: [Xen-users] dom0 large and in charge or petite?

  • To: xen-users@xxxxxxxxxxxxxxxxxxx
  • From: Henry Pepper <henrypepr@xxxxxxxxx>
  • Date: Fri, 12 Nov 2010 10:29:44 +0100
  • Delivery-date: Fri, 12 Nov 2010 01:31:17 -0800
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  • List-id: Xen user discussion <xen-users.lists.xensource.com>

The Xen scheduler is handling VCPU scheduling.
 As I understand it, the scheduler will by default switch VCPUS every 30ms.
 This includes Dom0.
 So if you have more VCPUs than CPUs some VCPUs will be off CPU in some periods.

As to the Dom0 pinning, I was under the impression that not only will
you have to pin Dom0 to e.g. cpu0, but you will have to ensure that no
other VPCU can run on cpu0. So that would mean that all other domains
will have to be pinned to CPU 1 through 7.

Using the Xen Credit scheduler you can give some domains more CPU
time, than other Domains. Meaning that if one domU1 has 256 credits
and domU2 has 128, then domU1 will, over time, get twice as much CPU
 Note that if domU1 has nothing to do in a period, then domU2 will
get all the CPU time.
Also note that I believe that the domain credit will be split among
the VPCUs in the domain, so if domU1 has 4 VCPUs then each VPCU would
have 64 credits. again if the some of those VCPU doesn't use up their
credit, then the rest of the VCPUs in the domain will get to use the
remaining credits.

Please note that it is only CPU allocation that the scheduler takes
care of, I/O sharing is handled by the Dom0 OS.
 So if the Dom0 OS is up to fairly share disk I/O a single domU could
starve all other domUs.
Also it seems that local Dom0 I/O operations has a tendency to take precedence.

There is also the book: 'The Definitive Guide to the Xen Hypervisor'


On Thu, Nov 11, 2010 at 10:14 PM, Scott Classen <sclassen@xxxxxxx> wrote:
> Hello xen gurus,
> I've recently acquired a new computer for the express purpose of
> exploring xen. Eventually I would like to explore global shared
> filesystems and the ability to migrate running domUs between different
> physical computers, but for now I'm just trying to get a sense of how
> many domUs I can run on a single Xeon and generally exploring the
> capabilities and limitations of xen.
> my test computer:
> Supermico X8SAXS Motherboard.
> A single Xeon L5630 2.13GHz CPU - 4 cores/8 threads
> 12GB RAM
> 9650SE- 2LP 3ware RAID car with two 1TB drives in RAID1 configuration.
> dom0 and all domUs are using the 3ware. domUs are using a disk image
> file (i.e. disk = [ 'tap:aio:/var/lib/xen/images/xen1.img,xvda,w', ] )
> I've installed stock CentOS 5.5 xenified kernel
> (2.6.18-194.26.1.el5xen) and 4 identical domUs with 2VCPUs and 2GB RAM
> each:
> dom0 = 8 VCPU 4GB RAM
> xen1 = 2 VCPU 2GB RAM
> xen2 = 2 VCPU 2GB RAM
> xen3 = 2 VCPU 2GB RAM
> xen4 = 2 VCPU 2GB RAM
> My questions:
> 1. Since I only have 8 threads maybe is this too many domUs? Will they
> be constantly fighting over CPU cycles?
> 2. Should dom0 be the most minimal installation needed to administer
> the domUs? Do the resources allocated to dom0 affect the performance
> of the domUs?
> 3. Should I leave only 1 GB RAM for dom0 and give the rest to the domUs?
> Thanks, for any general advice on setting up xen.
> Scott
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