Re: RT-Xen on ARM

[Meng Xu <xumengpanda@gmail.com>]

Hi Andrii,

I'm sorry for replying to this thread late. I was busy with a paper
deadline until last Saturday morning.

I saw Dario's thorough answer which explains the high-level idea of
the real-time analysis that is the theoretical foundation of the
analysis tool, e.g., CARTs.
Hopefully, he answered your question.
If not, please feel free to ask.

I just added some very quick comment about your questions/comments below.

On Thu, Sep 28, 2017 at 5:18 AM, Andrii Anisov <andrii_anisov@epam.com> wrote:
> Hello,
>
>
> On 27.09.17 22:57, Meng Xu wrote:
>>
>> Note that:
>> When you use gEDF scheduler in VM or VMM (i.e., Xen), you should use
>> MPR2 model
>
> I guess you mean DMPR in CARTS terms.
>
>>   to compute the resource interface (i.e., VCPU parameters).
>> When you use pEDF scheduler, you should use PRM model to compute.
>>>
>>>      - Could you please provide an example input xml for CARTS described
>>> a
>>> system with 2 RT domains with 2 VCPUs each, running on a 2PCPUs, with
>>> gEDF
>>> scheduling at VMM level (for XEN based setup).
>>
>> Hmm, if you use the gEDF scheduling algorithm, this may not be
>> possible. Let me explain why.
>> In the MPR2 model, it computes the interface with the minimum number
>> of cores. To get 2 VCPUs for a VM, the total utilization (i.e., budget
>> / period) of these two VCPUs must be larger than 1.0. Since you ask
>> for 2 domains, the total utilization of these 4 VCPUs will be larger
>> than 2.0, which are definitely not schedulable on two cores.
>
> Well, if we are speaking about test-cases similar to described in [1], where
> the whole real time tasks set utilization is taken from 1.1...(PCPU*1)-0.1,
> there is no problem with having VCPU number greater than PCPUs. For sure if
> we take number of domains  more that 1.

The number of VCPUs can be larger than the number of PCPUs.

>
>> If you are considering VCPUs with very low utilization, you may use
>> PRM model to compute each VCPU's parameters; after that, you can treat
>> these VCPUs as tasks, create another xml file, and ask CARTS to
>> compute the resource interface for these VCPUs.
>
> Sounds terrible for getting it scripted :(

If you use python to parse the xml file, it should not be very
difficuly. Python has api to parse the xml. :)

>>
>> (Unfortunately, the current CARTS implementation does not support
>> mixing MPR model in one XML file, although it is supported in theory.
>> This can be worked around by using the above approach.)
>>
>>> For pEDF at both VMM and
>>> domain level, my understanding is that the os_scheduler represents XEN,
>>> and
>>> VCPUs are represented by components with tasks running on them.
>>
>> Yes, if you analyze the entire system that uses one type of scheduler
>> with only one type of model (i.e., PRM or MPR2).
>>
>> If you mixed the scheduling algorithm or the interface model, you can
>> compute each VM or VCPU's parameters first. Then you treat VCPUs as
>> tasks and create another XML which will be used to compute the number
>> of cores to schedule all these VCPUs.
>>
>>>      - I did not get a concept of min_period/max_period for a
>>> component/os_scheduler in CARTS description files. If I have them
>>> different,
>>> CARTS gives calculation for all periods in between, but did not provide
>>> the
>>> best period to get system schedulable.
>>
>> You should set them to the same value.
>
> Ok, how to chose the value for some taskset in a component?

Tasks' periods and execution time depends on the tasks' requirement.
As Dario mentioned, if a sensor needs to process every 100ms, the
sensor task's period is 100ms. Its execution time is the worst-case
execution time of the sensor task.

As to the component (or VM)'s period, it's better to be smaller than
its tasks' periods. Usually, I may want to set to a value divisible by
its tasks' periods.
You may try different values for components' periods, because the
VCPU's bandwidth (budget/period) will be different for different
components' periods.
You can choose the component's period that produces a smaller VCPU's
bandwidth, which may help make VCPUs easiler to be scheduled on PCPUs.

Best,

Meng


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

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