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Groups > comp.arch.embedded > #12716

Re: Resource revocation

From Don Y <this@isnotme.com>
Newsgroups comp.arch.embedded, comp.realtime
Subject Re: Resource revocation
Date 2013-07-25 22:12 -0700
Organization Aioe.org NNTP Server
Message-ID <kst0gh$208$2@speranza.aioe.org> (permalink)
References <ksrtvv$kmp$1@speranza.aioe.org> <7xehame7ut.fsf@ruckus.brouhaha.com> <kss75t$dgh$1@speranza.aioe.org> <7xr4emjbaq.fsf@ruckus.brouhaha.com>

Cross-posted to 2 groups.

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Hi Paul,

On 7/25/2013 7:38 PM, Paul Rubin wrote:
> Don Y <this@isnotme.com> writes:
>> I don't see how reliance on a particular language keyword or OS hook
>> would matter when considering the way in which this *functionality* is
>> availed to the developer.
>
> Different languages and OS's lend themselves to different styles.  The
> application's requirements also matter.  E.g. the Erlang approach to
> infrequent failures (which this resource being preempted might amount
> to) is to just let the failing process crash, and restart it through
> supervision.  That might not be ok if you can't tolerate a few
> milliseconds of delay doing that stuff now and then.

On the other hand, it might be acceptable for the task to *think*
it still owns the resource and have it "waste" CPU acting on that
misbelief.

That's why I tried to show different approaches to handling such
a notification (and.or deferring action on it).

>> But, there are noticeable differences (and attendant risk) with each
>> of the following *different* "stylistic" approaches to the problem:
>> ...
>
> Most of those don't seem to handle the issue of the resource being
> preempted while the process in question is trying to use it.

Right!  The way a signal approaches the problem typically
avoids this -- it is a true asynchronous notification that
"interrupts" the task's normal execution sequence.  Though
how the *real* task responds to the handler's actions
(e.g., the handler may simply set a flag that the "real"
algorithm simply examines when convenient)

>>> http://jlouisramblings.blogspot.com/2012/08/getting-25-megalines-of-code-to-behave.html
>
>> I didn't see anything in there germane to my question.  Did I miss
>> something?
>
> I don't know if you missed anything.  The article suggests uses message
> passing for concurrency.  In that scheme, each task typically is
> organized as a synchronous event loop where each pass through the loop
> happens rather quickly.  In that situation, resource preemption can just
> be a matter of sending a message saying the process with the resource
> must be released.

Everything in my system is message based (since "tasks" and
resources and ... can all be located on different physical
processors, message passing makes the most sense).  Signals
are even delivered in this way (sort of like a high priority
message passed to the *scheduler* on a node)

> Most of my stuff is coded in that style these days.  One drawback is
> that it tends to use more threads than a traditional approach would, so
> there is a memory and context switching cost.

Yes.  Everything has the weight of a system call, effectively.
"No free lunch".

> I don't know if that's an
> issue for you.  It does seem easier (at least to me) to keep stuff
> straight with this approach, with fewer ways to go wrong.

Exactly.  And, makes things a sh*tload easier to debug (especially
in a distributed application)

> Or, as you say, you can just kill the target process, in which case it
> helps if the OS or runtime environment automatically reclaims the
> resource you wanted.

Well, the "resource" in question has already been passed along
to someone else.  So, its more a question of do you let the
task continue to consume *other* resources in the mistaken
belief that it still owns the significant resource?  Or, do
you cut your losses now?  (Or, do you let the task figure out
what has happened AT ITS CONVENIENCE and adopt its own strategy
to handling that?)

> Again under Erlang, the runtime has a nice
> framework for restarting killed processes.  You could have a priority
> queue of processes waiting to acquire the resource.  So if the resource
> must be preempted, the preempting process puts in a high priority
> request, then kills the process with the resource, which (on restart)
> tries to get the resource back, but (since it has lower priority) it has
> to wait for the preempting process to finish.

Yes, but this approach assumes there are no associated activities
that must be unwound.  I.e., that everything the task has done
can be discarded and recreated.

Imagine if the task is machining a piece of metal and the resource
that it wanted has been withdrawn.  A timely notification of this
would allow the task to possibly recover and attempt to reacquire
the resource -- instead of having to discard the "work in process"
(a slab of metal).

Or, the task could checkpoint its progress ON DEMAND (instead of
continually) so that it can return to the point at which it was
operating when the resource vanished.

The "advantage" to an asynchronous notification *like* a signal is
it gives the developer as much rope as he wants in deciding how
to best recover from that potential problem.

The *disadvantage* is he can quickly hang himself by doing
something (or failing to do something) wrong.

The problem lies in the nature of asynchronous events and how
we think of them as developers.  While they *can* occur at
any time, we tend to only *think* of them occurring at "key
points" in our code.  E.g., after a multibyte object has
been completely updated; not *during* it's update, etc.

The other half of this is that it is REALLY HARD to test for
these things.  You can't just inject an asynchronous event at
every possible place where it could occur and verify proper
operation.  :(

Then, their are the timeliness aspects of its assertion and
its handling...

<frown>

I.e., there is a strong (personal) inclination to make the
manner in which these situations are handled fit some mold
so they are all handled in a similar fashion that *minimizes*
problems.  But, I really dislike telling people they "can't
run with scissors" -- since those folks who are adept at
doing so shouldn't be penalized by the "clumsiness" of those
who can't!

--don

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Thread

Resource revocation Don Y <this@isnotme.com> - 2013-07-25 12:23 -0700
  Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-25 12:51 -0700
    Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-25 15:00 -0700
      Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-25 19:38 -0700
        Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-25 22:12 -0700
          Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-25 23:37 -0700
            Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-26 01:13 -0700
              Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-26 02:48 -0700
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-26 04:19 -0700
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-26 09:46 -0700
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-26 10:21 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-26 19:30 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-26 11:56 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-26 20:08 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-26 13:11 -0700
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-26 12:31 -0700
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-27 11:08 -0700
                Re: Resource revocation Rob Gaddi <rgaddi@technologyhighland.invalid> - 2013-07-29 09:16 -0700
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-29 10:22 -0700
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-29 11:22 -0700
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-27 08:56 -0700
                Re: Resource revocation Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2013-07-26 19:25 +0200
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-26 10:51 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-26 19:21 +0100
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-26 11:50 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-26 19:42 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-26 12:43 -0700
                Does the Buddha have a real time nature? ;)  (was Resource revocation) Roberto Waltman <usenet@rwaltman.com> - 2013-07-26 17:06 -0400
                Re: Does the Buddha have a real time nature? ;)  (was Resource revocation) Don Y <this@isnotme.com> - 2013-07-26 17:12 -0700
                Re: Does the Buddha have a real time nature? ;)  (was Resource revocation) Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-27 09:44 +0100
                Re: Does the Buddha have a real time nature? ;)  (was Resource revocation) Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-27 09:45 +0100
                Re: Does the Buddha have a real time nature? ;)  (was Resource revocation) Don Y <this@isnotme.com> - 2013-07-27 12:28 -0700
                Re: Does the Buddha have a real time nature? ;)  (was Resource revocation) Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2013-07-27 14:11 +0200
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-26 12:40 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-26 21:29 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-26 13:52 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-26 22:55 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-26 17:22 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-27 10:02 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-27 09:29 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-27 18:20 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-27 12:48 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-27 22:57 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-27 16:18 -0700
                Re: Resource revocation Roberto Waltman <usenet@rwaltman.com> - 2013-07-31 16:33 -0400
                Re: Resource revocation upsidedown@downunder.com - 2013-07-27 22:53 +0300
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-27 22:42 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-27 16:41 -0700
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-27 16:49 -0700
                Re: Resource revocation upsidedown@downunder.com - 2013-07-28 08:39 +0300
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-27 23:11 -0700
                Re: Resource revocation Roberto Waltman <usenet@rwaltman.com> - 2013-07-31 16:15 -0400
                Re: Resource revocation upsidedown@downunder.com - 2013-08-01 00:13 +0300
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-27 15:16 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-27 23:05 +0100
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-27 15:37 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-27 23:38 +0100
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-27 16:08 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-28 01:16 +0100
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-27 18:43 -0700
                Re: Resource revocation upsidedown@downunder.com - 2013-07-28 09:01 +0300
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-27 15:21 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-27 23:07 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-26 12:00 -0700
                Re: Resource revocation stephenXXX@mpeforth.com (Stephen Pelc) - 2013-07-27 16:49 +0000
                Re: Resource revocation Richard Damon <Richard@Damon-Family.org> - 2013-07-28 18:31 -0400
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-28 15:51 -0700
                Re: Resource revocation Richard Damon <Richard@Damon-Family.org> - 2013-07-28 20:12 -0400
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-28 18:35 -0700
                Re: Resource revocation Richard Damon <Richard@Damon-Family.org> - 2013-07-29 00:10 -0400
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-29 00:23 -0700
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-29 01:05 -0700
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-29 12:07 -0700
                Re: Resource revocation upsidedown@downunder.com - 2013-07-30 01:11 +0300
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-29 15:20 -0700
                Re: Resource revocation Rob Gaddi <rgaddi@technologyhighland.invalid> - 2013-07-29 15:42 -0700
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-29 16:41 -0700
                Re: Resource revocation upsidedown@downunder.com - 2013-07-30 08:51 +0300
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-30 09:09 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-30 01:30 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-30 10:04 +0100
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-30 02:55 -0700
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-30 07:12 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-30 16:59 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-30 13:17 -0700
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-30 10:12 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-30 18:48 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-30 20:18 -0700
                Re: Resource revocation upsidedown@downunder.com - 2013-07-31 09:27 +0300
                Re: Resource revocation [long] Don Y <this@isnotme.com> - 2013-07-31 00:47 -0700
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-30 02:50 -0700
                Re: Resource revocation upsidedown@downunder.com - 2013-07-30 13:22 +0300
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-30 07:24 -0700
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-30 01:17 -0700
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-29 19:28 -0700
                Re: Resource revocation upsidedown@downunder.com - 2013-07-29 11:40 +0300
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-29 11:44 -0700
                Re: Resource revocation Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-07-29 20:56 +0100
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-29 13:16 -0700
                Re: Resource revocation Richard Damon <Richard@Damon-Family.org> - 2013-07-30 00:08 -0400
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-30 23:41 -0700
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-31 00:58 -0700
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-31 10:18 -0700
                Re: Resource revocation George Neuner <gneuner2@comcast.net> - 2013-08-02 06:19 -0400
                Re: Resource revocation Richard Damon <Richard@Damon-Family.org> - 2013-08-03 02:26 -0400
                Re: Resource revocation Paul Rubin <no.email@nospam.invalid> - 2013-07-28 19:31 -0700
                Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-29 00:44 -0700
  Re: Resource revocation Richard Damon <Richard@Damon-Family.org> - 2013-07-25 22:42 -0400
    Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-25 22:12 -0700
  Re: Resource revocation "Boudewijn Dijkstra" <sp4mtr4p.boudewijn@indes.com> - 2013-07-31 13:41 +0200
    Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-31 07:51 -0700
      Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-31 08:07 -0700
        Re: Resource revocation Anders.Montonen@kapsi.spam.stop.fi.invalid - 2013-07-31 15:55 +0000
          Re: Resource revocation Don Y <this@isnotme.com> - 2013-07-31 10:18 -0700
      Re: Resource revocation "Boudewijn Dijkstra" <sp4mtr4p.boudewijn@indes.com> - 2013-08-01 14:42 +0200
        Re: Resource revocation Don Y <this@isnotme.com> - 2013-08-01 14:34 -0700
      Re: Resource revocation upsidedown@downunder.com - 2013-08-02 09:08 +0300
        Re: Resource revocation Don Y <this@isnotme.com> - 2013-08-02 07:26 -0700

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