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Re: [Xen-devel] xen 4.5.0 rtds scheduler perform poorly with 2vms

On Mon, 2015-11-23 at 07:42 -0800, Yu-An(Victor) Chen wrote:
> Hi all,

> So I was doing some experiments to evaluate RTDS scheduler
> schedubility of real time tasks using 1vm with period of 10000 and
> budget of 10000. The experiment results turn out as expected(perform
> better than xen-credit).
> But when I tried to perform similar experiments with 2 vms (both with
> now period of 10000 and budget of 5000). The schedubility of real
> time tasks turn out really bad. Even if I have one vm idling and the
> other vm running the real time tasks, the schedubility of that vm is
> still really poor(worse than xen-credit). Am I missing some
> configuration I should have set for 2vm cases? Thank you
What is it that you are trying to prove with this setup? This is
despite all Meng is already saying about the non-work conserving nature
of RTDS, and about the LITMUS IPI bug.

In fact, in general, real-time schedulers are really good at isolating
workloads, with precise time guarantees. If you have stuff that needs
to be done in 2 VMs, and you use RTDS for scheduling the 2 VMs, you'll
get good and precisely characterized isolation between them.

But if you put all the stuff in only 1 VM, and then limit its own
utilization, all you are doing is making it hard for the things inside
the VM itself to achieve their target performance, with respect to both
an instance of RTDS where that VM has 100% utilization, as well as with
(almost) any general purpose scheduler.

Then, again, as Meng is saying, if you not only have "stuff" to do
inside the VM, but you are interested in in-guest real-time, then the
scheduling parameters of the VM(s) and the ones of the tasks in the
guest(s), should be set according to a proper real-time hierarchical
scheduling scheme that allows for guarantees to be met.

<<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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