Re: [RFD] sched/deadline: EDF dynamic quota design

[Luca Abeni]

Hi,

first of all, sorry for the ultra-delayed reply: I've been busy,
and I did not notice this email... Anyway, some comments are below

On 05/16/2014 09:11 AM, Henrik Austad wrote:
[...]
> > > > This can also be implemented in user-space (without modifying the
> > > > scheduler)
> > > > by having a daemon that monitors the SCHED_DEADLINE tasks and changes
> > > > their
> > > > runtimes based on some kind of feedback (for example, difference
> > > > between the
> > > > scheduling deadline of a task and its actual deadline - if this
> > > > information
> > > > is made available by the scheduler). I developed a similar
> > > > implementation in
> > > > the past (not based on SCHED_DEADLINE, but on some previous
> > > > implementation
> > > > of the CBS algorithm).
>
> This sounds like a very slow approach. What if the extra BW given by T2 was
> for one period only? There's no way you could create a userspace daemon to
> handle that kind of budget-tweaking.
Right. With "This can also be implemented in user-space..." I was referring
to the feedback scheduling (adaptive reservation) approach, which is designed
to "auto-tune" the reservation budget following slower variations (it is like
a low-pass filter, which can set the budget to something between the average
used budget and the largest one). Basically, it works on a larger time scale.
If you want a "per-period" runtime donation, you need a reclaiming mechanism
like GRUB, CASH, or similar, which needs to be implemented in the kernel.


> Also, it sounds like a *really* dangerous idea to let some random (tm)
> userspace daemon adjust the deadline-budget for other tasks in the system
> based on an observation of spent budget for the last X seconds. It's not
> military funding we're concerned with here.
>
> When you state your WCET, it is not because you need -exactly- that budget,
> it is because you should *never* exceed that kind of rquired
> computational time.
Exact. But the idea of feedback scheduling was that sometimes you do not know
the WCET... You can guess it, or measure it over a large number of runs (but
the Murphy law ensures that you will miss the worst case anyway ;-).
And there are situations in which you do not need to respect all of the
deadlines... The daemon I was talking about just monitors the difference between
the scheduling deadline and the "real job deadline" for some tasks, in order to
understand if the runtime they have been assigned is enough or not... If some
task is not receiving enough runtime (that is, if the difference between its
scheduling deadline and the "real deadline" becomes too large), the daemon
tries to increase its runtime. When the system is overloaded (that is, everyone
of the monitored tasks wants too much runtime, and the admission control fails),
the daemon decreases the runtimes according to the weight of the tasks...
Of course, the daemon does not have to monitor all of the SCHED_DEADLINE tasks
in the system, but only the ones for which adaptive reservations are useful
(tasks for which the WCET is not known for sure, and that can tollerate some
missed deadlines). The other SCHED_DEADLINE tasks can stay with their fixed
runtimes unchanged.


> Blindly reducing allocated runtime is defeating that whole purpose.
Of course, there could be a minimum guaranteed runtime per task.

> Granted, if you use EDF for BW-control only, it could be done - but then
> the thread itself should do that. Real-time is not about being fair. Heck,
> it's not even about being optimal, it's about being predictable and
> "dynamically adjusting" is not!
Well, this could lead to a long discussion, in which everyone is right and
everyone is wrong... Let's say that it depends on the applications requirements
and constraints.

[...]
> > > Will EDF has dynamic quota in future?
> >
> > Well, as Luca said, you can already use SCHED_DEADLINE as the backend
> > for feedback scheduling (that pertain mostly to user-space).
> >
> > And yes, there are already thoughts to modify it a bit to go towards
> > Lipari's et al. GRUB algorithm. That would be probably helpful in
> > situations like yours. But I can't give you any timing for it.
>
> Need to read up on GRUB before involving myself in this part of the
> discussion, but I'm not sure how much I enjoy the idea of some part of
> userspace (more or less) blindly adjusting deadline-params for other tasks.
No, GRUB does not involve the user-space adjusting any scheduling parameter.
GRUB is a reclaiming algorithm, which works in a different way respect to
the feedback scheduling approach I described, and requires modifications in
the scheduler.
The basic ideas are (warning! This is an over-simplification of the algorithm! :)
- You assign runtime and period to each SCHED_DEADLINE task as usual
- Each task is guaranteed to receive its runtime every period
- You can also define a maximum fraction Umax of the CPU time that the
  SCHED_DEADLINE tasks can use. Note that Umax _must_ be larger or equal
  than sum_i runtime_i / period_i
  (note: in the original GRUB paper, only one CPU is considered, and Umax is
  set equal to 1)
- If the tasks are consuming less than Umax, then the scheduling algorithm
  allows them to use more runtime (but not less than the guaranteed runtime_i)
  in order to use up to Umax.
  This is achieved by modifying the rule used to decrease the runtime: in
  SCHED_DEADLINE, if a task executes for a time delta, its runtime is decreased
  by delta; using GRUB, it would be decreased by a smaller amount of time
  (computed based on Umax, on the active SCHED_DEADLINE tasks, etc...).
  This requires to implement some kind of state machine (the details are in
  the GRUB paper)

I also had an implementation of the GRUB algorithm (based on a modification
of my old CBS scheduler for Linux), but the computational complexity of the
algorithm was too high. That's why I never proposed to merge it in SCHED_DEADLINE.
But maybe there can be some trade-off between the "exact compliance with the
GRUB algorithm" and implementation efficiency that can make it acceptable...


> I think we need to be very careful about whether or not we're talking about
> EDF as a "CPU BW"-enforcer or a "meet a deadline for a periodic
> task"-enforcer. Those 2 will react quite differently from having their
> runtime adjusted on the fly.
Well, IMHO the nice thing about SCHED_DEADLINE is that it can be both :)
It depends on how you configure the scheduling parameters.


			Luca
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