[Libvir] The problem of the definition of tuning informations

[Daniel Veillard]

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
> > (re-)starts. 
> > 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 ?

Daniel

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