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Re: [Libvir] The problem of the definition of tuning informations



beth kon wrote:

Daniel Veillard wrote:

On Thu, Nov 08, 2007 at 02:00:10PM -0600, Ryan Harper wrote:
* Daniel Veillard <veillard 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 ?
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.

Daniel and I have been discussing this a bit on IRC, so I will dump that information on the list... (correct me if I misstate something here, Daniel :-)

Daniel wants to have the xml contain all parameters that must be specified at domain creation in order to achieve proper function, and cannot be tuned later. I agree this is a reasonable definition. In this case, cpuset would need to be in the xml.

My concern is that currently Xen will fail a domain create request if the cpu is out of range with the error "invalid argument", so the user will not have enough information to correct the problem in the xml and try again. We can pursue getting a more explicit error message from Xen. Or Xen could ignore the cpuset and start the domain, perhaps with a warning message.

My thinking was that ideally it might be good to have libvirt provide 2 start methods - minimal and tuned, but Daniel thinks it is not worth the complexity. It should be up to the user to correct issues in the xml and try again.


Daniel





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


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