Re: Not to incite a language war but apparently the Corona lockdown was based on 13 year old undocumented C-Code

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 2020-05-13 0:27, Jeffrey R. Carter wrote:
> On 5/12/20 9:16 PM, Niklas Holsti wrote:
>>>
>>> It seems clear that the code that was executed to create the 
>>> predictions was not deterministic on a single core.
>>
>> The first of the two cases of single-core "non-determinism" described 
>> and bewailed in the "review" is that the time evolution of the 
>> predicted epidemic is different depending on the value of a program 
>> option -- whether the "network" is created anew, or reused -- that 
>> should not affect the result. No actual non-determinism is claimed in 
>> the review, just that the epidemic reaches large infection rates 
>> sooner for one option value than for the other. It seems that the code 
>> was using the PRN generator slightly differently in the two cases, 
>> resulting in different PRN sequences.
> 
> The single-core non-determinism that I gathered from the referenced 
> interpretation of the review, which I clearly have not studied as 
> carefully as you, was that the program gave different results on the 
> same computer for identical inputs: same initial conditions and same RNG 
> seed(s).


That would indeed be non-determinism, also to me.

> Perhaps I misunderstood, but that was what I got from it, and 
> the only part that seemed troubling.


I agree that it would be troubling. If you could find that statement, it 
would interest me.

I did not read all the bug reports and correction reports on github, so 
I can't claim to be sure there was no such real non-determinism, but I 
did not find examples in the review. The review claims that there were 
"many" bugs, but I don't trust the review on that. The review also says 
that the program's authors defended and seemed to accept "minor cases of 
non-determinism", but it was not clear what those cases are, if they are 
not the two examples discussed in the review.

> This is different from the kind of non-determinism of simulations
> that are intended to be run many times with different RNG seeds.


Yes indeed, and I would not call that non-determinism of the code. The 
reviewer also condemned that kind of stochastic exploration and seemed 
to confuse it with real non-determinism. It is true that Monte Carlo is 
often used when the system being simulated is chaotic in some way so 
that its behaviour is essentially unpredictable, and can be so sensitive 
that no practical floating-point precision can give a truly reliable end 
result in any given simulation.

> Possibly that would be caused by accessing uninitialized variables.

Yes.

-- 
Niklas Holsti
niklas holsti tidorum fi
       .      @       .