Re: [PATCH v1 3/4] libxl: add rt scheduler

[Dario Faggioli <dario.faggioli@citrix.com>]

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On gio, 2014-09-04 at 11:07 -0400, Meng Xu wrote:

> 2014-09-04 10:51 GMT-04:00 George Dunlap
> <george.dunlap@eu.citrix.com>:
>         On 09/04/2014 03:47 PM, Meng Xu wrote:
>         
>         > Hi George,
>         > 
>         > 
>         > 2014-09-04 10:27 GMT-04:00 George Dunlap
>         > <George.Dunlap@eu.citrix.com>:
>         >         On Wed, Sep 3, 2014 at 4:33 PM, George Dunlap
>         >         <George.Dunlap@eu.citrix.com> wrote:
>         >         >
>         >         > While the domctl interface is not stable, the
>         >         libxl interface *is*
>         >         > stable, so we definitely need to think carefully
>         >         about what we want
>         >         > this to look like.
>         >         >
>         >         > Let me give that a think. :-)
>         >         
>         >         
>         >         OK, so we had a chat about this at our team meeting
>         >         today, and here is
>         >         what we came up with.
>         >         
>         >         The feature freeze for 4.5 is next Wednesday.
>         >         
>         >         The core scheduler is in good enough shape to be
>         >         checked in as an
>         >         "experimental" mode, so it would be really nice to
>         >         be able to get this
>         >         checked in.
>         >         
>         >         The DOMCTL interface isn't stable so we can change
>         >         that if we need to;
>         >         however, the libxl interface *is* stable.
>         >         
>         >         The current libxl scheduler parameter interface
>         >         assumes one set of
>         >         parameters per domain; it's not yet setup for
>         >         per-vcpu parameters.  It
>         >         is unlikely that we would be able to converge on a
>         >         new interface by
>         >         next week.
>         >         
>         >         So the suggestion was this: For the moment, use the
>         >         existing libxl
>         >         interface on a per-domain basis.  Internally, this
>         >         will set all vcpus
>         >         to the same values.  This will allow us to check in
>         >         a useable version
>         >         of the scheduler for people to test and improve.
>         >         Then for 4.6 we can
>         >         start working on a suitable libxl interface for
>         >         setting per-vcpu
>         >         scheduling parameters.
>         >         
>         >         Dario / Ian, did I miss anything?
>         >         
>         >         Meng / &c, does that sound reasonable?
>         > 
>         > 
>         > I have a question as to the user interface.
>         > For 4.5, we only allow users to set all vcpus to the same
>         > values (I'm totally fine with it.); 
>         > But how about the get function? When users issue the command
>         > "xl sched-rt", how should we display the parameters of
>         > vcpus? We just give the "period", "budget" and "#VCPU" for a
>         > domain? I'm fine with this display for 4.5.
>         > 
>         > 
>         > However ,my concerns is: In 4.6, when we allow vcpus to have
>         > different parameters and need to display every vcpu's
>         > parameters, how should we display when users use command "xl
>         > sched-rt"? When vcpus have different period and budget, we
>         > cannot display like what we did in 4.5 then. :-(
>         > 
>         > 
>         > It's just my thought, just in case we neglect it. :-)
>         
>         
>         I think the xl interface doesn't have quite the same
>         consistency guarantees as the libxl interface.  For now, I
>         think just make it print one budget / period for the domain;
>         and we can change it later.
>         
>         
> 
> 
> ​I see. I'm totally ​ok with the decision! :-)
> ​So I will only use the existing libxl interface without adding an
> array to it, to set/get the vcpus' parameters of a domain. Am I right?
>
Yep, no array. You just add a 'period' and a 'budget' fields _inside_
libxl_domain_sched_params, without putting them inside any wrapping
struct, union or array.

Then, in the implementation, you just take those twos, an apply them to
all the domain's vcpus (by using the array based interface we agreed
upon for the hypervisor).

Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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