Re: RT-Xen on ARM

[Meng Xu <xumengpanda@gmail.com>]

On Wed, Jul 5, 2017 at 4:51 AM, Dario Faggioli
<dario.faggioli@citrix.com> wrote:
> On Mon, 2017-07-03 at 14:42 -0400, Meng Xu wrote:
>> On Mon, Jul 3, 2017 at 10:58 AM, Andrii Anisov <andrii_anisov@epam.co
>> m> wrote:
>> >
>> Once the scheduling policy is determined, you will need to configure
>> the VCPUs' parameters based on the systems' workload.
>> This requires the workload's timing parameters for the CARTS tool to
>> compute the VCPUs' parameters.
>>
> Yes, this is an interesting thing that Meng is mentioning.
>
> RTDS allows you to specify the parameters (budget and period, or,
> depending on how you prefer to see things, utilization and latency) on
> a per-vCPU basis.
>
> This may look cumbersome and overly complicated (and, in fact, if you
> don't need it, you can ignore it :-D), but it may actually be really
> important in a truly RT scenario.
>
> Whether or not it is useful, almost entirely depends on what the VM is
> doing, and how you decide to control and configure things inside it.

Exactly.

Andrii,
If you encountered any question/difficulty in choosing the proper VCPU
parameters for your workload, please don't hesitate to ping me and
Dario.

I'm also trying to make it *easier* for users to *correctly* configure
the VCPUs on RTDS.
The more we understand the real use cases, the more we can help
improve the scheduler and its related tools.

> For example, if you have a particular task in the VM that you need must
> absolutely execute for at least 10ms every 100ms, you can:
> - inside the VM, pin the task to vCPU 0, and give it top priority;
> - at the Xen level, give (with RTDS) to vCPU 0 budget=10ms and
>   period=100ms (or maybe budget of 12ms, to allow for some overhead
>   :-P).

This assumes that the start time of the task's each period is
synchronized with the start time of the VCPU's each period.
If this assumption does not hold, the VCPU need a larger budget to
guarantee the task will always have 10ms in *any* 100ms time interval.
The budget can be computed by CARTS
(https://rtg.cis.upenn.edu/carts/).

>
> This is something that no other scheduler allows you to do. :-)

Exactly.
If you want to reason about the real-time timing properties required
by some safety-critical systems' standard, such as the ISO-26262 for
automotive systems, the above computation and analysis will be
required.


-----------
Meng Xu
PhD Candidate in Computer and Information Science
University of Pennsylvania
http://www.cis.upenn.edu/~mengxu/

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