Re: RT-Xen on ARM

[Meng Xu <xumengpanda@gmail.com>]

On Wed, Jul 5, 2017 at 4:29 AM, Dario Faggioli
<dario.faggioli@citrix.com> wrote:
> On Tue, 2017-07-04 at 11:12 -0400, Meng Xu wrote:
>> On Tue, Jul 4, 2017 at 8:28 AM, Andrii Anisov <andrii_anisov@epam.com
>> > wrote:
>> >
>> > So you are suggesting to introduce more RT schedulers with
>> > different algorithms. Did I get you right?
>>
>> The EDF scheduling cares about the overall system's RT performance.
>> If
>> you want to guarantee the *soft* real-time performance of the IVI
>> domains and allow the IVI domain to delay the two RT domains in some
>> scheduling periods, the EDF scheduling is better than the RM
>> scheduling. Note that we need to reserve enough CPU resources to make
>> sure the delay from the IVI domain to the two RT domains won't cause
>> the deadline miss of the two RT domains.
>>
> This is technically correct, but, at the same time, I don't think it is
> the best way to describe why and how one should use the RTDS scheduler.

Thank you very much, Dario, for giving a better explanation in the
user's perspectives.

>
> In fact, what scheduling and prioritization strategy is used,
> internally in the scheduler, is (for now) not exposed to the user, and
> it hence should not have an impact in deciding whether or not to adopt
> the scheduler... Unless we've done things in a very wrong way! :-P
>
> What I'd say, as a description of what RTDS can give, to people
> interested in using it, would be as follows.
>
> RTDS gives you the chance to provide your VMs, guarantees of CPU
> utilization that is precise, and has a well defined and strictly
> enforced granularity. In fact, by using RTDS, it's possible to specify
> two things:
> - that a VM should at least be able to execute for a certain U% of
>   total CPU time
> - that a VM will be able to exploit this 'reservation' with a time
>   granularity of P milliseconds.
>
> U, in fact, is expressed as U=B/P, P (called period) is how frequently
> a VM is given a chance to run, while B (called budget) is for how long
> it will be able to run, on every time interval of length P.
>
> So, if, as an example, a VM has a budget of 10 milliseconds and a
> period of 100 milliseconds, this means:
> - the VM will be granted 10% CPU execution time;
> - if an event for the VM arrives at time t1, the VM itself will be
>   able to start processing process it no later than t2=t1+2*P-2*B
>
> That's why, IMO, the period matters (a lot!). If one "just" knows that
> a VM will roughly need, say, 40% CPU time, then it does not matter if
> the scheduling parameters are B/P=4/10, or B/P=40/100, or
> B/P=40000/100000.
> OTOH, if one also cares about the latency, doing the math and setting
> the period properly.
>
> In fact, this capability of specifying the granularity of a
> reservation, is one of the main differences between RTDS (and, in
> general, or real time scheduling algorithms) and other general purpose
> algorithm. In fact, it is possible with general purpose algorithms too
> (for example, using weights, in Credit1 and Credit2, or using `nice' in
> Linux's CFS) to specify a certain utilization of a VM (task). But, in
> those algorithms, it's impossible to specify precisely, and on a per-VM
> basis, the granularity of such reservation.
>
> The caveat is that, unfortunately, the guarantee does not extend to
> letting you exploit the full capacity. What I mean is that, while on
> uniprocessor systems all that I have said above stays true, with the
> only constraint of not giving, to the various VMs cumulatively, more
> than 100% utilization, on multiprocessors, that is not true. Therefore,
> if you have 4 pCPUs, and you assign the parameters to the various VMs
> in such a way that the sum of B/P of all of them is <= 400%, it's not
> guaranteed that _all_ of them will actually get their B, in every
> interval of length P.
>
> Knowing what the upper bound is, for a given number of pCPU, is not
> easy. A necessary and sufficient limit has (to the best of my
> knowledge, which may not be updated to the current state of the art of
> RT academic literature) yet to be found. There are various limits, and
> various ways of computing them, none of which is suitable to be
> implemented inside an hypervisor... so Xen won't tell you whether or
> not your overall set of parameters is feasible or not. :-(
>
> (Perhaps we could, at least, keep track of the total utilization and at
> least warn the user when we overcome full capacity. Say, if with 4
> pCPUs, we go over 400%, we can well print a warning saying that
> deadlines will be missed. Meng?)

The total utilization can help answer if the VCPU parameters are
feasible or not.
But I'm thinking there may exist a better (yet optimal) approach to
answer the question: If all VCPUs on K cores are globally scheduled or
completely partitioned onto each of the K cores, we can use
Utilization Bound of the EDF scheduling algorithm for checking if the
VCPU's performance can be safely provided.
This requires the VCPUs' parameters (which also computes the total
utilization), which are easy to get.

Another thing is where this schedulability check should be provided:
in Xen kernel, in Xen toolstack, or as a separate utility tool?
In my opinion, a separate utility tool seems to be better than the
other tool approaches?

Best,

Meng

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

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