Re: [PATCH v1 1/4] xen: add real time scheduler rt

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

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On gio, 2014-09-04 at 11:30 -0400, Meng Xu wrote:
> 
> 
> 2014-09-04 10:27 GMT-04:00 Dario Faggioli <dario.faggioli@citrix.com>:

>         > > For instance, I can put, in an SMP guest, two real-time
>         applications
>         > > with different timing requirements, and pin each one to a
>         different
>         > > (v)cpu (I mean pin *inside* the guest). At this point, I'd
>         like for each
>         > > vcpu to have a set of RT scheduling parameters, at the Xen
>         level, that
>         > > matches the timing requirements of what's running inside.
>         > >
>         > > This may not look so typical in a server/cloud
>         environment, but can
>         > > happen (at least in my experience) in a mobile/embedded
>         env.
>         >
>         > But to play devil's advocate for a minute here:
>         >
>         
>         Hehe, please, be my guest! :-D :-D
>         
>         > couldn't you just put
>         > them in two different single-vcpu VMs then?
>         >
>         
> 
> 
> ​Well, let me give a simpler example:
> Suppose we have three tasks in one VM, each task has period 4ms and
> budget ​6ms (its utilization is 2/3).
>
You mean budget=4ms and period=6ms, don't you? :-)

>  If all these three tasks starts execution at the same time, we can
> use two full-capacity vcpus (200% capacity cpu resource) to schedule
> these three tasks. 
> However if you want to get two VMs, each of which has a full capacity
> vcpu (100% capacity cpu), we cannot schedule these three tasks,
> because one tasks cannot (well, at least very hard) migrate from one
> VM to another. 
> 
But... In this case, in the former configuration (1 VM with 2 vcpus),
each vcpu would (or at least can) have the same bandwidth of 100%, i.e.,
the same parameters... or am I missing something?

What we're tying to assess here, is the usefulness of the possibility of
setting _different_ parameters (and hence different pcpu bandwidth) for
each vcpu.

Also, it looks like you're assuming to have a real-time scheduler inside
the VM, which may or may not be the case.

> ​This is just a simple example, we could of course have an example
> like this but the vcpus are not full-capacity vcpus. :-)
> 
Yeah, well, perhaps it's a bit too simple. :-D

Don't get me wrong, I continue thinking per-vcpu params is something we
really want, it's just the example I'm not sure I'm getting/liking.

I still think the example of multiple, concurrent and strictly related
activities having different timing requirements to be a really sensible
one. In fact, in that case, especially if one does not have a real-time
scheduler inside the guest, mapping those requirements on the Xen
scheduler is the easier (only?) way to port the app from baremetal to
virtual machine!

Regards,
Dario

PS. BTW, Meng, can you use plain text emails when sending to the list?

-- 
<<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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