[SystemSafety] Software reliability (or whatever you would prefer to call it)
Peter Bishop
pgb at adelard.com
Tue Mar 10 10:50:10 CET 2015
Now I think I understand your point.
You just object to the term *software* reliability
If the term was *system* reliability in an specified
operational environment, and the system contained software
and the failure was always caused by software
- I take it that would be OK?
A alternative term like *software integrity* or some such would be
needed to describe the property of being correct or wrong on a given input.
(In a lot of mathematical models this is represented as a "score
function" that is either true or false for each possible input)
Peter Bishop
Nick Tudor wrote:
> Now back in the office...for a short while.
>
> Good point David - well put.
>
> I would have responded: There exists a person N who knows a bit about
> mathematics. Person N applies some mathematics and asserts Truth.
> Unfortunately, because of the incorrect application of the mathematics,
> the claims N now makes cannot be relied upon. The maths might well be
> correct, but the application is wrong because - and I have to say it yet
> again - the application misses fails to acknowledge that it is the
> environment that is random rather than the software. Software
> essentially boils down to a string of one's and nought's. Given the same
> inputs (and that always comes from the chaotic environment) then the
> output will always be the same. It therefore makes no sense to talk
> about 'software reliability'.
>
> Nick Tudor
> Tudor Associates Ltd
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> On 9 March 2015 at 12:26, David Haworth <david.haworth at elektrobit.com
> <mailto:david.haworth at elektrobit.com>> wrote:
>
> Peter,
>
> there's nothing wrong with the mathematics, but I've got
> one little nit-pick about its application in the real world.
>
> The mathematics you describe gives two functions f and g,
> one of which is the model, the other is the implementation.
>
> In practice, your implementation runs on a computer and so the
> domain and range are not "the continuum". If your model is mathematical
> (or even runs on a different computer), the output of one will
> necessarily be different from the output of the other. That
> may not be a problem in the discrete sense - you simply specify a
> tolerance t > 0 in the form of:
>
> Corr-f-g(i) = 0 if and only if |f(i)-g(i)| < t
>
> etc.
>
> The problem becomes much larger in the real world of control
> systems where the output influences the next input of the
> sequence. The implementation and the model will tend to drift
> apart. In the worst case what might be nice and stable in the
> model might exhibit unstable behaviour in the implementation.
>
> You're then in the subject of mathematical chaos, where a
> perfectly deterministic system exhibits unstable and unpredictable
> behaviour. However, this email is too small to describe it. :-)
>
> Cheers,
> Dave
>
> On 2015-03-09 11:48:57 +0100, Peter Bernard Ladkin wrote:
> > Nick,
> >
> > Consider a mathematical function, f with domain D and range R.
> Given input i \in D, the output is f(i).
> >
> > Consider another function, g, let us say for simplicity with the
> same input domain D and range R.
> >
> > Define a Boolean function on D, Corr-f-g(i):
> >
> > Corr-f-g(i) = 0 if and only if f(i)=g(i);
> > Corr-f-g(i) = 1 if and only if f(i) NOT-EQUAL g(i)
> >
> > If X is a random variable taking values in D, then f(X), g(X) are
> random variables taking values in
> > R, and Corr-f-g(X) is a random variable taking values in {0,1}.
> >
> > If S is a sequence of values of X, then let Corr-f-g(S) be the
> sequence of values of Corr-f-g
> > corresponding to the sequence S of X-values.
> >
> > Define Min-1(S) to be the least place in Corr-f-g(S) containing a
> 1; and to be 0 if there is no such
> > place.
> >
> > Suppose I construct a collection of sequences S.i, each of length
> 1,000,000,000, by repeated
> > sampling from Distr(X). Suppose there are 100,000,000 sequences I
> construct.
> >
> > I can now construct the average of Min-1(S) over all the
> 1,000,000,000sequences S.i.
> >
> > All these things are mathematically well-defined.
> >
> > Now, suppose I have deterministic software, S. Let f(i) be the
> output of S on input i. Let g(i) be
> > what the specification of S says should be output by S on input
> i. Corr-f-g is the correctness
> > function of S, and Mean(Min-1(S)) will likely be very close to
> the mean time/number-of-demands to
> > failure of S if you believe the Laws of Large Numbers.
> >
> > I have no idea why you want to suggest that all this is
> nonsensical and/or wrong. It is obviously
> > quite legitimate well-defined mathematics.
> >
> > PBL
> >
> > Prof. Peter Bernard Ladkin, Faculty of Technology, University of
> Bielefeld, 33594 Bielefeld, Germany
> > Je suis Charlie
> > Tel+msg +49 (0)521 880 7319 www.rvs.uni-bielefeld.de
> <http://www.rvs.uni-bielefeld.de>
> >
> >
> >
> >
> > _______________________________________________
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>
> --
> David Haworth B.Sc.(Hons.), OS Kernel Developer
> david.haworth at elektrobit.com <mailto:david.haworth at elektrobit.com>
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Peter Bishop
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