[certi-dev] Re: CERTI-Devel Digest, Vol 55, Issue 11

[Christian Stenzel]

Hello,

Pierre and Eric are experts here, but I suppose to know, that CERTI uses a 
null-message synchronization scheme.

What you observed is known throughout the literature as lookahead creep.
You can learn more about this phenomena on these slides:

http://www.cc.gatech.edu/classes/AY2006/cs4230_fall/Lectures/06-Lookahead%20(9-14-05).ppt

In short: To synchronize different federates or so-called logical processes you 
can use null message synchronization (also known as chandy, misra, bryant 
algorithm). A nullmessage is an explicit guarantee that a federate which is 
able to schedule events for another federate, will never create events with a 
timestamp smaller than its own time plus it's own lookahead.

In a classical distributed discrete event simulation, a federate (or logical 
process) would send a nullmessage after each time advance. The receiver of that 
nullmessage can advance its local time to the timestamp of the received 
nullmessage. If the lookahead is relatively small the whole distributed 
simulation advances it's time only in small portions. When uisng nullmessage 
for synchronization you can not advance to the time of the next event directly.

So, I think that everything is okay. No failure at your side.

Christian
-------- Original-Nachricht --------
> Datum: Fri, 29 Oct 2010 12:01:08 -0400
> Von: address@hidden
> An: address@hidden
> Betreff: CERTI-Devel Digest, Vol 55, Issue 11

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> Today's Topics:
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>    1.  Question about NextEventRequest Service (Roth, Christoph)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Fri, 29 Oct 2010 16:21:23 +0200
> From: "Roth, Christoph" <address@hidden>
> Subject: [certi-dev] Question about NextEventRequest Service
> To: <address@hidden>
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> 
> Hi,
> 
>  
> 
> I have got a question concerning synchronization. My scenario is the
> following:
> 
>  
> 
> I have two federates f1 and f2. Both are event based and use the
> NextEventRequest (NERQ) Service to advance time. The simulation loop in
> both federates looks like this:
> 
>  
> 
> ...
> 
> rtiAmb->updateAttributeValues();
> 
>  
> 
> rtiAmb->nextEventRequest(newTime);
> 
> while(isAdvancing) {
> 
>                 rtiAmb->tick2();
> 
> } 
> 
> ...
> 
>  
> 
> Both federates have a lookahead of 10. Assume both federates are at the
> same time (=0). Now, if both federates request for the same new time
> with very high value like newTime=1000000 the tick2()-call lasts very
> long (several seconds) until the simulation proceeds. Have you any idea
> about the reason for this long delay? It seems as if the delay depends
> on the low lookahead, because if I increase the lookahead up to the
> newTime value simulation is much faster.
> 
>  
> 
> In my opinion the low lookahead should have no influence on execution
> speed since both federates guarantee to not send any message with a
> lower timestamp than newTime+lookahead, so the simulation should be able
> to directly advance (This is what I could read in the IEEE 1516.1.2000
> standard page139), or did I make a misstake?
> 
>  
> 
> My setup:
> 
> -          Ubuntu linux
> 
> -          Local execution
> 
> -          Certi 3.4.0
> 
>  
> 
>  
> 
> Thank you very much and regards,
> 
> Christoph
> 
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