Xen users - Jun 2009 - present domU with 1 CPU with multiple cores

Hi all,

I have a machine with 2 physical CPUs with each 2 cores and as such when I do
cat /proc/cpuinfo on Dom0 it reports 4 CPUs. I want to run a couple of DomU
instances on this machine. However to make sure that Dom0 will always respond
even when the DomU instances have a high load, I want to keep CPU 0 available
only for Dom0. Therefore in the DomU config files I''m using the
following CPU definition: cpus="1-3". Does it makes sense to do this?
Or would Xen handle this better on its own?

Anyway, so one of the DomU instances runs software that I have bought a 1CPU
license for. Consequently I have to set ''vcpus=1'' otherwise it
will tell me that I do not have a multiple CPU license. However it is ok to have
1 CPU with multiple cores. Therefore to speed it up I was wondering how I can
tell Xen to assign say 2 cores to this DomU instance, but present it as 1 CPU
with 2 cores.

Also how do I see on which physical CPU a DomU instance is running? When I set
cpus="1-3" and vcpus="1" , then ''xm top''
will show that the instance is running on vCPU 0 (which I assume is CPU 0 of
that instance, not physical CPU0 since I deliberately pinned it to CPUs 1-3).
Also ''xm list'' will show how many vCPUs that DomU runs on, but
not on which physical CPU.

Thanks!

Best wishes,

Jonathan Salomon
IT Manager
Panaya Inc. - Cut 50% of your SAP upgrade cost with automation software.
Tel: +972 (0) 9 761 8007
Cell: +972 (0) 52 7335999
Fax: +972 (0) 9 746 7901
E-mail: jonathan@panayainc.com<mailto:jonathan@panayainc.com>
Web: www.panayainc.com<http://www.panayainc.com>



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> From: xen-users-bounces@lists.xensource.com
[mailto:xen-users-bounces@lists.xensource.com] On Behalf Of Jonathan
Salomon
> Sent: Tuesday, June 23, 2009 4:30 AM
> To: xen-users@lists.xensource.com
> Subject: [Xen-users] present domU with 1 CPU with multiple cores
> However to make sure that Dom0 will always respond even when
> the DomU instances have a high load, I want to keep CPU 0 available
> only for Dom0. Therefore in the DomU config files I''m using the
following
> CPU definition: cpus="1-3". Does it makes sense to do this?
> Or would Xen handle this better on its own?
I think it''s a good idea.  If your domU load is CPU-intensive, dom0 may
not need to run often.  However that is often not the case, and each
domU may frequently need to perform either disk or network I/O.  Dom0
needs to be scheduled on an available CPU to complete that I/O, since it
hosts the driver backend.
> Also how do I see on which physical CPU a DomU instance is running?
Take a look at "xm vcpu-list".

-Jeff



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Jeff,

Thanks for your reply! Regarding the "xm vcpu-list": that''s
what I was looking for (RTFM). However I still have no luck with the cpuid. One
of the example config files contains the following section for a one socket 8
core cpu:

#   Expose to the guest multi-core cpu instead of multiple processors
# Example for intel, expose a 8-core processor :
#cpuid=[''1:edx=xxx1xxxxxxxxxxxxxxxxxxxxxxxxxxxx,
#          ebx=xxxxxxxx00010000xxxxxxxxxxxxxxxx'',
#     ''4,0:eax=001111xxxxxxxxxxxxxxxxxxxxxxxxxx'']
#  - CPUID.1[EDX][HT] : Enable HT
#  - CPUID.1[EBX] : Number of vcpus * 2
#  - CPUID.4,0[EAX] : Number of vcpus * 2 - 1
#vcpus=8


Therefore I figured that for a single CPU with 2 cores I needed to change this
as follows:

cpuid=[''1:edx=xxx1xxxxxxxxxxxxxxxxxxxxxxxxxxxx,ebx=xxxxxxxx00000100xxxxxxxxxxxxxxxx'',''4,0:eax=000011xxxxxxxxxxxxxxxxxxxxxxxxxx'']
vcpus=2

However my application still complains that ''The server has 2 CPUs, but
this product is licensed for 1''. What am I doing wrong? I have a
m1.xlarge instance on Amazon EC2, which exposes a single CPU with 4 cores to the
OS and the application eats that perfectly. So it should be possible...

Also I cannot find any documentation on the cpuid syntax (e.g. none in the
xmdomain.cfg manpages). Could anyone point me to that or just explain it to me?

Thank you very much in advance!

Best wishes,

Jonathan Salomon
IT Manager
Panaya Inc. - Cut 50% of your SAP upgrade cost with automation software.
Tel: +972 (0) 9 761 8007
Cell: +972 (0) 52 7335999
Fax: +972 (0) 9 746 7901
E-mail: jonathan@panayainc.com
Web: www.panayainc.com



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