Mark Williamson wrote:
>> This feature is very important for quality of service aspects. How
>> many physical CPU do I have? How many CPU time can I use?
I'm not exactly sure whether a domU needs to know this? or do you mean dom0
Actually, the domU itself shouldn't know this, but maybe an application
running in domU. Imagine the following situation:
Three party setup:
1) Software vendor
2) Hosting provider
3) Customer (using Software from 1) and a virtual machine from 2))
In case the hosting provider changes something in the setup, the
performance decreases and Customer complains at the software vendor the
really bad performance of the software sold. Having no chance to get at
least number of physical cpu's assigned to domU makes problem solving
impossible. This theoretical setup would need the interaction of a third
party, the Hosting provider.
If there would be a possibility for the hosting provider to provide this
information (physical CPU assigned) to a domU, reduces the costs and
time spend for interaction getting a third party involved in a support
call. The information which physical CPU's are assigned to a domU does
not affect security or encapsulation, because the domU does _not_ know
how many physical CPU's dom0 has at all or how many of them are assigned
to other VM's.
The number of physical CPUs the domain's VCPUs are being scheduled across
might change dynamically. You can, however, change the number of VCPUs a
domain is allocated as an indication of how many CPUs it should try to
That it changes dynamically is not a problem at all. We would collect
the data, how many physical CPU's are assigned on a 5 minute base in our
application to keep track of it. We then can check if response times
correlate with number of physical CPU's.
There's also /sys/hypervisor which contains some relevant
Yes, that for the hint. Unfortunately is data is not enough :(
dom0 is permitted to access more detailed information about the host machine
through a hypercall (use "xm info" to see what's available).
Other important > information would the real memory and the hostname of the
I'm not sure why a domU would need to know the hostname of the host machine?
Also, this could change during migration, so it's not a stable value.
In case nothing like the physical CPU assignment will be implemented,
domU must know, which CIM server to query to get this kind of
information. I know that this value will change, and because of this we
need the information. If the domO would stay the same all the time, I
could set a parameter in my software to query the CIM server directly,
which wouldn't work after a migration.
The third possibility is to setup a dedicated VM which interacts between
all available dom0's and all available domU's. But this is the last
possible solution I'd prefer, because a third instance to manage is a
third source of errors.
Basically all this information is provided by other virtualization
technologies as read only values. Having a huge hosting solution with
enterprise software running in virtual machines depends on these quality
of service aspects. Having a black box makes debugging of performance
issues impossible and thus the whole solution is not usable.
Is it possible to provide such a solution with Xen (of course only if
enabled in dom0 due to security reasons!) over the /proc or /sysfs?
* Hannes Kuehnemund
* SAP Linuxlab
* SAP AG
* Dietmar Hopp Allee 16
* 69190 Walldorf, Germany
T +49 6227 7-40615
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Registergericht/Commercial Register Mannheim No HRB 350269
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