Re: Dealing with "past" events

Don Y <blockedofcourse@foo.invalid> wrote:

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.

I asked: what is "the OS"?  In particular where you put the
line between OS and applications.

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*.

You provide an interface.  In modern times that is usually
in form of a function call.  "Applications" need not care
if the actual implementation is library code or in kernel
or as a separate task.

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.

As I wrote, IMO correct place for such a functionality is inside OS,
but outside the kernel.

The "nothing is common" part looks strange: it is pretty common
to want triggering on edges and also time windows.

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?

In extendible scheduler that would be just abstract bit of information
and _application_ would tell its value to the scheduler.  If there
are dependencies something have to know about them.  In particular,
if you have dependence between independently developed application
it is natural to have scheduling as a third party.

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.

I did not write "malicious".  Rather, something like polling in
a busy loop instead of using blocking system call.  Or Windows
graphic drivers which did not check that hardware is ready to
accept the data.  Microsoft wrote that this was common because
it slightly improved average throughput of the driver.  But
it could stall PCI bus, effectively freezing the whole computer
for say some miliseconds (IIUC now such access fails rather
quickly, but it used to be a problem).

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.

"Do not care", "do not care enough" or "care more about other
things" mean essentially the same here.

E.g., if power fails, I start shedding computational
loads so I can power down processors.

I wrote about "reasonable warranties".  There is notion of
"force majeste" and clearly power failure counts as such.

 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.

PLC-s have vendor provided I/O, so vendor knows how much time
I/O will take.  They also have "logic", which is stright-line
code so vendor knows maximal execution time.  Consequently,
if PLC task is given sufficiently high priority it will run
with predicable frequency.  You may consider it unneeded,
you may think that other features are more important.
But in the end, either you care enough to provide something
like this or you do not care enough.  In later case there
is no much point in thinking a lot about missing deadlines.

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
mind.  Not so long ago I looked at one of home automation
system.  I searched for some specific info and one of first
things that I found was a discussion about scheduling interface
in the system.  A user wanted simply looking thing "if washing
machine is idle for some specified time, then turn its power off".
System allowed specifying conditions which caused some actions.
But there were limitations on what could be put as a condition,
and in effect correctly specifingt needed thing was rather
awkward.

 
Exactly.  Baking that into the OS is wrong.  It should
implement MECHANISMS (i.e., the ability to load an application
into memory and ensure it's required resources are available)
and not POLICY (the criteria that an application may deem important)

I should have been clearer.  The "home automation system" consisted
of devices (which ATM are of no interest) and user interface/scheduler
working as a normal application on standard OS.  User interface
was supposed to be easy but allows users to define various
actions.  The point is that in making it "easy" (and probably
simple to implement) user interface got crippled so that resonable
thing was hard to do.

--
                              Waldek Hebisch