[Timber] bug (?) in ASYNC implementation

Johan Nordlander johan.nordlander at ltu.se
Mon Oct 26 09:13:26 EDT 2009

Timing in Timber relies on two principles:

A: the baseline of an action is independent of the time when the  
action was called.
B: the timing of any computation is correct as long as it occurs  
within its designated execution window.

However, when the desired execution window for a called action  
overlaps with the execution window of the caller these principles  
clash, as some executions that are perfectly valid according to  
principle B might nevertheless lead to timing constraints for the  
callee that are impossible to met according to A.

The clash can be resolved by giving principle B priority over A (the  
current Timber choice) or giving A priority over B (the choice  
described in "Programming with Time-Constrained Reactions").  Per and  
Andrey are right in describing the issue as not finally settled, but  
there are a few good arguments in favor of giving priority to  
principle B:

- It leads to systems that are less rigid in the presence of dynamic  
- It gives the programmer an option to actually express non- 
deterministic timing if desired.
- It still allows completely deterministic timing whenever the  
execution windows of caller and callee don't overlap, and the  
programmer is always free to ensure this is the case.

Hope this clarifies the reasons for the current design.  Still,  
examples where a different design would seem more appropriate are most  

-- Johan

> Hi Ivan
> You put the finger on a very interesting and import design feature,  
> timing without drift.
> The permissible excecution window of a reaction is denoted by its  
> absolute baseline and deadline.
> As long as the after constructs denotes specific points in time that  
> are later (outside the permissible window), we will never suffer  
> drift, as Andrey mentioned.
> If set inside the excecution window, it can actually be posted after  
> its baseline is due, right.
> So what should the baseline be in that case, we have the choice to  
> define the timing semantics such that the baseline and deadline to  
> be inherited from the sender (and hence a deadline offset would  
> extend the execution window beyond that of the senders. This is  
> actually the "original" timing semantics. But this does not go well  
> with the idea of the before construct as giving a relative offset to  
> the release of the event.
> Anyways Ivan, your observation is perfectly valid, and I'm not sure  
> we have seen the end of this "debate".
> In any case, as long as you keep after constructs outside your  
> current execution window everything is fine and dandy.
> Best regards,
>  Per
> On Oct 26, 2009, at 10:17 AM, Ivan Tarasov wrote:
>> Andrey,
>> thanks for pointing out that this is documented. Unfortunately I  
>> don't
>> understand the reasons for introducing that special case.
>> Particularly, from my understanding of the following:
>>> Because the baseline offset 50 ms is measured relative the current  
>>> baseline at each invocation
>>> (and not the actual time of the recursive call), the  
>>> implementation does not suffer from accumulating drift.
>> (from Johan Nordlander, Mark P. Jones, Magnus Carlsson, and Jan
>> Jonsson,  Programming with Time-Constrained Reactions. Technical
>> report, Luleå University of Technology, 2005.)
>> this special case breaks the requirement of not causing the newly
>> scheduled actions to drift.
>> Is there some other problem which this special case is intended to
>> fix, which I don't recognize?
>> Regards,
>> Ivan
>> 2009/10/26 Andrey Kruglyak <kruglyak at mac.com>:
>>> From timber-lang.org >> Language summary >> Time constructs:
>>> Time windows of reactions are assigned as follows:
>>> The time window of a reaction to an external event has as baseline  
>>> the time
>>> instant when the event occurs, and as deadline an idealised instant
>>> infinitely far into the future.
>>> In particular, the start action of a program gets as baseline the  
>>> time
>>> instant when program execution begins.
>>> When a message without time constraints is sent (i.e., an plain  
>>> action is
>>> called) from a method with current baseline bl and deadline dl,  
>>> the reaction
>>> to the message inherits both bl and dl.
>>> The rule in the previous item can be changed by explicit program  
>>> constructs:
>>> The expression after t act sets the effective baseline for act to  
>>> the
>>> current baseline plus t.
>>> The expression before t act sets the effective deadline for act to  
>>> its
>>> effective baseline plust.
>>> In both cases, act can be any expression of type Action, which  
>>> includes the
>>> time-annotated constructs just introduced. Nested baseline offsets  
>>> are
>>> composed using addition, while multiple deadline offsets are  
>>> resolved by
>>> selecting the minimal value. The relative order
>>> of after and before constructs is not relevant.
>>> Special case: if the baseline denoted by an after construct is an  
>>> already
>>> passed time instant, the effective baseline of the reaction is  
>>> rounded off
>>> to the actual time of the call.
>>> Regards,
>>> Andrey
>>> On Oct 26, 2009, at 10:01, Ivan Tarasov wrote:
>>> I'm trying to understand what constitutes an RTS and how a Timber
>>> program translated into C is actually executed. So, I'm reading the
>>> RTS implementations' code.
>>> In rtsARM/rts.c, in ASYNC implementation, lines 439-442 we can see  
>>> the
>>> following code:
>>>           if (LESS(m->baseline, now)) {
>>>               m->baseline = now;
>>>               // debug("^");
>>>           }
>>> Same code is present in ASYNC implementation in rtsPOSIX/rts.c,  
>>> lines
>>> 354-355
>>> That means that if the absolute time value for the baseline for the
>>> action to be scheduled is in the past, we set the baseline to the
>>> current time. However, since all the time calculations are  
>>> relative to
>>> the baseline, that would mean that all the following actions to be
>>> scheduled from the one being scheduled at the moment would  
>>> "drift". Is
>>> this an intended behavior? I'm not sure that's the right decision.
>>> Ivan
>>> _______________________________________________
>>> Timber mailing list
>>> Timber at haskell.org
>>> http://www.haskell.org/mailman/listinfo/timber
>>> ------------------------------------------------------------------------
>>> Andrey Kruglyak
>>> PhD student
>>> http://www.andreykruglyak.com
>>> Dept. of Computer Science and Electrical Engineering
>>> Lulea University of Technology, Lulea, Sweden
>>> T +46 920 49 23 57 (office)
>>> T +46 706 16 76 03 (mobile)
>>> Office A2314 (A-huset)
>>> _______________________________________________
>>> Timber mailing list
>>> Timber at haskell.org
>>> http://www.haskell.org/mailman/listinfo/timber
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