Re: [Xen-devel] [PATCH 1 of 3] Support of getting scheduler defaults

[George Dunlap <dunlapg@xxxxxxxxx>]

On Tue, May 22, 2012 at 1:58 PM, Juergen Gross
<juergen.gross@xxxxxxxxxxxxxx> wrote:
>>> When setting the parameters from the config file we have no domain yet
>>> which
>>> we would have to specify for XEN_DOMCTL_SCHEDOP_getinfo. I didn't want to
>>> parse part of the config only after domain creation.
>>
>> That seems like another problem with doing this up front instead of
>> doing read-modify-write when we come to set the values for the domain.
>
>
> Do you think it would be okay to parse the scheduler config data _after_
> domain creation? If yes, the read-modify-write approach is simple and always
> correct. If no, you will have to initialize the parameters to something
> scheduler specific, and not domain specific.
>
>
>>> A default should be okay, as this is what we want to modify. :-)
>>> The scheduler should initialize a new domain with this default, of
>>> course.
>>
>> So I can't use this interface to change the current one of the current
>> settings for running a domain, while leaving the others at their current
>> value?
>
>
> I don't understand this sentence :-) ...

I think he means this:  Suppose after the domain is created, you want
to set the weight.  With the proposed interface, you have to manually
read the values and change them yourself.  With an interface that
allows -1 to be default, libxl can do that for you (making programming
easier).

>
>
>> Which interface can I use for that and why isn't it the same as this
>> one?
>
>
> ... making it hard to answer to this one. :-)
> Nevertheless trying: you can change the scheduling parameters of a running
> domain with xl sched-credit/sched-sedf/...

I think Ian has convinced me that just using -1 for default values
would be the best option:
* It doesn't require a new special 'what are the defaults" hypercall
* The same interface can be used to set only a subset of the options
(leaving the other ones alone) *after* the domain is created.

Thanks for doing the work to code it up, though Juergen.

 -George

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