[Libvir] The problem of the definition of tuning informations

beth kon eak at us.ibm.com
Tue Nov 13 14:00:02 UTC 2007

Daniel Veillard wrote:

>On Thu, Nov 08, 2007 at 02:00:10PM -0600, Ryan Harper wrote:
>>* Daniel Veillard <veillard at redhat.com> [2007-11-08 10:08]:
>>>  I promised that mail for the beginning of the week but I still have
>>>I think tuning informations are that set of parameters associated
>>>to a domain or a host, which are not stricly needed to get the 
>>>domain(s) working but improve their runtime behaviour.
>>>To me this includes:
>>>   - scheduling parameters the scope may be host/hypervisor/domain
>>>   - vcpu affinity i.e. to which set of physical CPU each of the
>>>     vcpu may be bound
>>>   - and possibly others ...
>>>The problem:
>>>People would like to associate those to the XML domain informations,
>>>the goal being to be able to restore those informations when a domain
>>>I have been objecting it so far because, I think those informations
>>>don't have the same lifetime and scope as the other domain informations
>>>saved in the XML. Since they are not needed to start the domain, and
>>>that once the domain is started the existing domain API can be used
>>>to change those informations, it is better to keep them separate.
>>For at least (maybe only) Xen NUMA systems, the application of "tuning"
>>information after a domain is started does not achieve the same affect
>>as including the information during the initial construction of the
>>domain.  In particular, Xen needs to know which physical cpus are being
>>used to determine which cpus it from which numanode it will allocate
>>memory.  Adjusting affinity after the domain has allocated memory
>>doesn't allow libvirt or any management app to control from which node
>>domains pull memory.
>  yes, I understand and that's why I agreed to add the cpuset information
>at that point it's more than tunning because it may be irreversible for the
>lifetime of the domain, so this really should be in the XML. I'm not
>suggesting to go back about 'cpu affinity' i.e. to which physical CPUs
>a domain should be bound, but 'vcpu affinity' i.e. then how the virtual
>CPUs of the domain are mapped onto that cpu set, that can change
>dynamically without (serious) performance penalty. 
>>I don't have any objection to separating "tuning" information as long as
>>we have the ability to merge permanent domain parameters with its
>>"tuning" information prior to domain construction.
>  My point is that you don't need the tuning informations to create the
>domain, if you need them it's not tuning. When you say you want to
>merge them, do you want this to create the domain ? It should not
>be necessary (or I take a counter example that would help me), right ?
It seems to me that the only reason cpuset information is being treated 
as more than tuning is due to an artifact of Xen (i.e., it must be 
specified at domain creation). For KVM, for example, I believe this can 
be specified after domain creation.

 From a libvirt perspective, I think the XML config/tuning split should 
be hypervisor-neutral, and based solely on what is required to get a 
domain running (ignoring performance):

1) XML contains arguments absolutely needed to start a domain in any 
hypervisor. This could be thought of as the minimum requirements for 
starting a domin.

2) Tuning information contains arguments that affect performance, and 
may be changed.

When a domain is started, the caller can specify a minimal start (XML 
only) or a tuned start (XML plus tuning). Lower level libvirt code would 
understand the specifics of the hypervisor well enough to know whether 
it had to include some of the tuning information at domain creation time.


Elizabeth Kon (Beth)
IBM Linux Technology Center
Open Hypervisor Team
email: eak at us.ibm.com

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