Re: [2.4.17/18pre] VM and swap - it's really unusable

yodaiken
2002-01-15T12:35:11Z

On Tue, Jan 15, 2002 at 10:15:11AM +0200, Momchil Velikov wrote:
> >>>>> "yodaiken" == yodaiken <yodaiken@fsmlabs.com> writes:
>
> yodaiken> On Tue, Jan 15, 2002 at 12:34:01AM +0200, Momchil Velikov wrote:
> >> One can consider a non-preemptible kernel as a special kind of
> >> priority inversion, preemptible kernel will eliminate _that_ case of
> >> priority inversion.
>
> yodaiken> The problem here is that priority means something very different in
> yodaiken> a time-shared system than in a hard real-time system. And even in real-time
> yodaiken> systems, as Walpole and colleagues have pointed out, priority doesn't
> yodaiken> really capture much of what is needed for good scheduling.
>
> Well, maybe there are other policies (notably static scheduling ;),
> which may be preferrable in one or other case, anyway, I personally
> tend to think rate-monotonic scheduling is quite adequate in practice.

That's not what we see in real RT applications.

> Of course, anyone is free to prove me wrong by implementing earliest
> deadline or slack time scheduler in the kernel.

My impression is that these are of limited use in applications.

>
> yodaiken> In a general purpose system, priorities are dynamic and "fair".
> yodaiken> The priority of even the lowliest process increases while it waits
> yodaiken> for time. In a raw real-time system, the low priority process can sit
> yodaiken> forever and should wait until no higher priority thread needs the
> yodaiken> processor.
>
> *nod*
>
> yodaiken> So it's absurd to talk of priority inversion in a non RT
> yodaiken> system. When a low priority process is delaying a higher priority task
> yodaiken> for reasons of fairness, increased throughput, or any other valid
> yodaiken> objective, that is not a scheduling error.
>
> What is a non real-time system? E.g. are a desktop playing ogg or a
> server serving multimedia content over ATM, real-time systems? The
> point is that one can view the system as real-time or not depending on
> the dominant _current_ load. A system may have real-time load now and

I don't think so, and certainly the straightforward priority based model
of traditional RT seems grossly wrong. What's the highest priority process -
is it the CD player or is it the niced sendmail that is running in the
background? The fact is we require that _both_ make progress. Therefore
at some time the sendmail process must have effectively higher priority
than the CD player. If we then factor in the many kernel threads that may
be running at any time, the calculation becomes even more problematic.
In RTLinux we have a much simpler environment and we can use the rule:
The highest priority runnable task should be the running task
within some T and the OS gets better as T goes to 0.
If something starves, that's the user problem - our users are programmers
who we want to provide with as much control as possible. We certainly
do not want to have a situation where the stop command to the robot arm
is delayed because the OS needs to run bdflush ...

For Linux, even Linux doing the critical work of playing DVDs, this is
not so simple. The target users are not programmers, liveness and
responsiveness remain important criteria ...
It's clear that much of the problem when people discuss this issue is
lack of clear goals. "Preemption" is a controversial implementation
technique, not a goal in itself - I hope. Even "low latency" doesn't
really capture the intended goal - "low latency" to do what? If it's
simply "low latency" switch to highest priority process, that's not
necesarily anything that is going to be useful in itself. The
"resource reservation" work at CMU seems to me to be way off the point
too. The OGI people have some interesting thoughts about this stuff, but
I don't think they've come close to working out a comprehensive model.

What I think we need is a kind of interval real-time scheduling.
Something like:
The system has a basic timing period of N milliseconds where
N is at lease 500 and probably more.

Over a N millisecond period each process gets a full scheduling quantum
and, if it requests, a full I/O quantum.
For a niced process there is some calculated interval greater than
N. An I/O quantum should correspond somehow to a rate of I/0.

RTLIMIT is used to set the max number of processes allowed to start
and this determines the computation length of "one quantum"

There is this cliche about RT that RT is about deadlines, not speed, but I think
that's only partially true. To say that every task will run during a 1 second period
with today's PC/Server technology is fundamentally different than to say that
every task will run during a 10 millisecond period.

> a non real-time one an hour later, both occasionally intermixed with
> tasks of the other class. It is unreasonable to say "This system is
> non-realtime so use that kernel and when you want to play vidoes,
> please reboot to something else, for example XX-Linux".

Yes, but it's not unreasonable to pop up a window and ask the user if
he/she wants to have the scheduling rates of other applications
stepped down to run the DVD player.
The Oracle logging server is designed to run at 1 second periods
increasing the responsiveness of your current process will
reduce that to a 2 second period.
Email delivery will be delayed up to 1 minute
...
YES NO

and so we can follow the most important OS design technique of all time:
push complexity to user space.

---------------------------------------------------------
Victor Yodaiken
Finite State Machine Labs: The RTLinux Company.
www.fsmlabs.com www.rtlinux.com