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

[Ian Campbell <Ian.Campbell@citrix.com>]

On Tue, 2012-05-22 at 14:05 +0100, George Dunlap wrote:
> On Tue, May 22, 2012 at 1:58 PM, Juergen Gross
> <juergen.gross@ts.fujitsu.com> 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?

No, I don't think there's any sensible way to make this work at the
libxl interface, you'd need to call back out to xl half way through
domain build to ask it to parse the sched params for you.

>  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).

Right. And furthermore with this patch the interface for setting the
initial values is different to the one for updating them, since this
patch gets you the defaults which you change.

> 
> >
> >
> >> 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/...

Right, my question above was -- why I can't I use the same libxl
interface as these commands at build time (perhaps internally inside
libxl)

> 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