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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: comp.arch.embedded
Subject: Re: Dealing with "past" events
Date: Sat, 23 Nov 2024 20:40:24 -0700
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On 11/23/2024 7:14 PM, Waldek Hebisch wrote:
> Don Y <blockedofcourse@foo.invalid> wrote:
>>> That is confusing formulation of the problem.  First, what is
>>> "the OS"?
>>
>> Doesn't matter.  The question is about how one should EXPECT an OS
>> to handle such things.
> 
> It matters.  OS kernel should implement something simple, but enough
> to support higher layers.  At higher layers you may have something
> more fancy.

You asked:  "What OS?"  The OS doesn't matter.  Rather, how
the mechanism is modeled.

As you've said, the OS "should implement something simple".
Something easily conceptualized, even if it doesn't address
all possible use cases.

If, for example, the OS assumes that "past" events are no longer of
interest and ignores the request, then any higher layers have to
take that into account to COMPENSATE for the POLICY decision that
the OS has enforced.

>>> If by "the OS" you mean operating system kernel, then you want
>>> it as simple as possible and move various functions to upper
>>> layers.  In particular, in general purpose OS you may have
>>> timers which activate a task some time after scheduled time
>>> (hopefully close to scheduled time, but there is no warranty).
>>> Maybe you are thinking of such a timer.
>>
>> I am thinking one layer above that, in terms of abstraction.
>> Imagine some concurrent agency "watching the clock" and
>> taking action on your request to make something happen at a
>> particular time.  *YOU* may be busy with something else.
> 
> I take this and what you write later that you think about
> library interface for "applications".

A library function or a *service*.

>>> However, there may be higher level "scheduler".  For example
>>> Unix 'cron' and 'at' are userspace programs, but are considered
>>> part of operating system. 'cron' and 'at' are rather crude,
>>> if you may need more.  In particular, an automations system
>>> typically must schedule various activities with specific time
>>> constraints.
>>>
>>> Anyway, your system probably is intended to do something useful
>>> and that will lead to some requirements.  Once you have more
>>> concerte requirements, you can think how to best satisfy them.
>>> If you have only handful of time-dependent tasks and each
>>> differs a lot from other, then putting all higher level
>>> time handling in the tasks may be reasonable.  But usually
>>> it pays to have common handling.  As George mentioned there
>>> can be interdependencies between task, so it is likely that
>>> you may need a higher level scheduler to coordinate them.
>>
>> My contention is that said service can't possibly know of all
>> of the interdependencies that could exist (examples provided
>> in other posts).  So, let the service focus on a single criteria:
>> the time at which you desire the action to occur.
> 
> You can not solve _all_ scheduling problems.  It does not

Correct.

> mean that you should not solve some.  Windows style interface

You should only solve those that are commonplace -- and, only
as a convenience for your clients.

If nothing is common (unlike windows where applications may want
to know if the host is running on batteries, just came out of
hibernation, etc), then anything additional that you do in
the kernel just adds to the kernel's complexity without any
real reward.

> where you have a lot of specific cases and can not do anything
> else than provided choices does not look attractive to me.
> OTOH 'constraint language' to specify various constraints
> (which may be combined in arbitrary ways) could go quite
> far.  There is also time-honored approach of allowing "user"
> extentions (possibly up to replacing "standard" scheduler).

The "constraints" would then have to be available/accessible
to the scheduler.  This leads to monolithic designs -- the scheduler
has to be able to know ANYTHING that may be of importance.

Instead, it should only have to know what is ESSENTIAL for it to perform
its action.

"Schedule this at 1:00AM but only if it did not rain, today"

Why should the scheduler have to know about local precipitation?
Instead, the task that thought that was a significant criteria
likely already knows about it; why not let *it* condition the
scheduler's actions?

>> Consider, however, that the service may be displaced by some higher
>> priority tasks executing on its hardware.  So, it can't guarantee
>> that it ever *sees* the specific time you've specified; other
>> tasks may have preempted it.  So, expecting to hit a specific
>> time is wishful thinking.
> 
> Again, this is question of design of your OS and your requirements.
> If you have badly behaving third party code, then it may be hard to
> give any warranty.

The code doesn't even have to be malicious.  I'm sure everyone
has had their workstation grind to a halt because something is
working as designed, but the set of applications aren't working
as EXPECTED.

(E.g., my browser routinely seems to run out of memory... or sockets...
or <whatever> meaning I can't open another page until I close it
and reopen it.  Even if all the same tabs are reopened with it!)

> But if you have need and apropriate resurces
> then there are ways to get real-time behaviour (that is hitting
> specified time window).  Even if you do not have real-time
> motivation, I think that time windows (as opposed to discrete
> events) appear frequently.  For example, you want to turn on
> a device during night tariff.

Yes.  Or, the developer may have "noticed" (or expected)
certain behaviors that it can exploit.  E.g., at this time,
the system load is low so it would be a good use of resources
(without incurring conflicts) for me to perform this resource
intensive task.

> Anyway, if you can not give reasonable warranties for time
> when your scheduler runs, then it means that you do not care.

No.  It means you care MORE about other things.

E.g., if power fails, I start shedding computational
loads so I can power down processors.  The first to
go are those that are "unessential" or that can easily
be restarted to resume their state.

As I don't know how long a power outage will last
(but I *do* know how long my backup supply will support
a particular load set), I can't guarantee that "your"
task will be running anywhere when its deadline or
event times come to pass.

Similarly, if some high priority task manages to continually
preempt you, then you can't predict when you will next
"run" on the processor.

You've been preempted because your stated timing requirements
have been deemed insufficient to compete for the CURRENT
set of resources, given the other candidates vying for them.

The consequences of you not running may be serious.  Or,
insignificant.  Only YOU can determine that.

If, instead, you make a proclamation to the system
regarding your "importance"/timeliness, then it's easy
for foreign code to abuse that mechanism and always
claim to be "very important" and with "immediate"
timing criteria.

[To counter such abuse, if the workload manager sees
that it can't meet your requirements, it simply refuses
to run your application:  "Why bother?  You've already
CLAIMED to have these requirements and I *know* that
I can't meet them, so why forfeit resources trying to
meet your goals -- at the expense of other tasks -- only
to KNOW it will fail?"]

> By extention, you do not care when scheduled tasks run.
> Which may be fine for some purposes and in such case simplistic
> handling which you propose may be good enough.
> 
> When you wrote your original post, I had someting different in
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