[Ns-developers] ***SPAM*** Re: [ns-3] proposed RNG changes [PMX:####]

[George Riley]

Hi Michelle,
See my comments below.

> Hi all,
>
> After discussions with Tom and Mathieu, here are some proposed  
> changes to the way the RNG and RandomVariable classes work in ns-3.   
> Please feel free to comment.
>
> * no time-based seed initialization
>
> We decided to remove the default behavior of setting the global RNG  
> seed by timeofday.  Instead, the default will be to have the seed  
> set to some constant value.  This will result in ns-3 producing the  
> same results each time it is run, unless the user changes the global  
> seed or changes the run number.
This was discussed at length some time ago (about a year I believe)  
and the current implementation was the decision.
I don't think we should be re-visiting ad nauseam prior decisions.
>
>
> The reason for this change is to enforce good simulation practices.   
> If the global seed is set by timeofday, the user needs to carefully  
> record the exact seed used in order to re-run the same experiment.   
> More importantly, if the user plans on running several experiments  
> to obtain statistics like an average with confidence intervals,  
> using timeofday to set the seed does not guarantee that the multiple  
> runs will be independent.
It is easy enough to print out the timeofday seeds on standard out  
along with other statistics; I don't see the problem
>
>
> The L'Ecuyer RNG default in ns-3 provides a very easy way to ensure  
> that multiple runs are guaranteed to be independent. This is by  
> setting a global seed once and changing only the run number for each  
> replication of the experiment.  The resulting numbers are guaranteed  
> to be random and independent.
I agree completely with this. I expect the "good simulation practice"  
would be to set a global seed manually, and then change the run number
to get the desired independence and randomness.
>
>
> In addition, with the L'Ecuyer RNG, there is no notion of 'bad'  
> seeds or 'good' seeds.  Any number used as a seed is valid. The  
> range of possible seeds is 1 to MAXINT.
You know much more about this than I do, but there is a subroutine  
provided by L'Ecuyer called "CheckSeeds".  See the call to it in  
random-variable.cc.
I'm not sure I understand why he would provide such a subroutine if  
there were indeed no "bad" seeds.
>
>
> The RNG also provides a large stream of random numbers so that for  
> each experiment, up to 2^51 (or, 2.3 x 10^15) independent  
> replications are possible.  This means that the maximum run number  
> allowed is 2^51 (or less, depending on the datatype used to store  
> the run number).
>
> * change the interface of setting the RNG seed to 1 value instead of  
> 6 values
This is fine as long as you don't remove the original one; If I want  
to specify the full 6 seed values I should be able to do it.  We don'  
t need
to "force users" to our way of thinking how this should be done
>
>
> This is just a simplification of the interface.  The actual seed in  
> the L'Ecuyer RNG is a 6-tuple, which will be set all to the same  
> value provided by the user.
>
> * remove the GetSingleValue function from RandomVariable classes
>
> The GetSingleValue function allows for potential non-independence as  
> it draws values from a single RNG stream for different random  
> variables.  Each random variable should have its own RNG stream.
Agree;
>
>
> * provide Stream API
Agree;

George
>
>
> We will provide a Stream API to allow the user to set the seed and  
> run number of the global RNG.  This Stream API will also allow  
> access to the first RNG stream (not associated with any random  
> variable) and allow the user to draw numbers from a uniform  
> distribution with min and max as parameters.
> Ex: double PickUniform (double min, double max);
> This is different from the random variable Uniform because once the  
> RandomVariable object is created, min and max cannot be changed.
>
> -Michele
>
> --
> Michele Weigle
> Assistant Professor
> Department of Computer Science
> Old Dominion University
> Norfolk, VA 23539
> mweigle at cs.odu.edu
> http://www.cs.odu.edu/~mweigle
> (757) 683-6001 ext. 5050
>
>