Re: [SystemSafety] Statistical Assessment of SW ......

From: Daniel Kästner < >
Date: Fri, 23 Jan 2015 10:28:57 +0100 (CET)


>In particular, if you don't think that the execution of SW can have a
stochastic nature, such as
>Jean-Louis, you are thereby committed to the view that IEC 61508 and its
derivates are inherently
>incoherent. It must be a difficult world to live in ......
>

I think it's worth making a difference between extrinsic stochastic behavior and intrinsic stochastic behavior. If the software reacts deterministically to some probabilistic input the execution of the SW-based system can be considered to have a stochastic behavior. But for one specific input still the reactions of the SW are predictable. This is very different to having some non-deterministic SW which may react in different ways to the same input. The interesting question is what the consequences are for the verification process: Providing statistical evidence (e.g. by black-box testing) to show SW correctness may not be able to detect the difference. If you consider timing behavior and just run some measurements on the SW then you don't know where the variations in time you'll be observing come from and which SW paths have been exerted at all. In other words there is a degree of uncertainty in the system coming from outside the SW, and then there is - or not - another level of uncertainty from within, and the second one could be avoided. Ideally there is a formal reasoning demonstrating that for a given set of scenarios a SW fault can happen or not and I think that's what Jean-Louis is pointing out.
Dr.-Ing. Daniel Kaestner ----------------------------------------------
AbsInt Angewandte Informatik GmbH      Email: kaestner_at_xxxxxx
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> -----Ursprüngliche Nachricht-----
> Von: systemsafety-bounces_at_xxxxxx [mailto:systemsafety-bounces_at_xxxxxx von
> Peter Bernard Ladkin
> Gesendet: Freitag, 23. Januar 2015 07:43
> An: systemsafety_at_xxxxxx > Betreff: Re: [SystemSafety] Statistical Assessment of SW ......
>
> On 2015-01-21 14:15 , jean-louis Boulanger wrote:
> > For software it's not possible to have statistical evidence.
> > the failure is 1 (yes the software have fault and failure appear)
>
> This argument came up again yesterday in a standards-committee meeting.
It is usually attributed to
> third party "engineers with whom I work", because nobody quite seems to
claim they hold the view
> themselves when I'm in the room :-) ....
>
> So it might be worthwhile to adduce the proof - again. It's real short.
>
> Suppose you have a piece of SW S which is deterministic. And S is also
not perfect, so it outputs
> right answers on some inputs and wrong answers on others. And S reverts
to an initial state with no
> memory of its previous behavior each time it produces its output.
>
> Suppose the distribution of inputs to S has a stochastic character. That
is, the input I is a random
> variable. Then the output outS(I), which is a function of the input I,
also has stochastic
> character. A deterministic transformation of a random variable is itself
a random variable.
>
> Let us transform outS(I) further, deterministically. Define
> CorrS(I) = 1 if outS(I) is correct
> CorrS(I) = 0 if outS(I) is incorrect
>
> Then again CorrS(I) has also a stochastic nature and is a random
variable.
>
> Thus, if the input to a piece of SW has stochastic nature, then so does
the correctness behavior of
> the SW.
>
> QED.
>
> The only reasonable objection to this argument which I have heard is to
dispute whether inputs have
> a stochastic nature.
>
> So, say you build a railway locomotive control system. The piece of
track the locomotive runs on has
> a fixed architecture, so the argument would run that the behavior of the
locomotive is more or less
> determined within certain parameters (whether signal X is red or green)
and does not have a
> stochastic nature. But various parameters such as the condition of the
track, the nature of the load
> on the locomotive, and other environmental conditions such as wind speed
and weather (icy track, or
> dry track, and when icy where the ice is) make it practically all but
impossible to predict the
> inputs to the control system. Besides, at design time the design does
not involve designing to the
> specific route the locomotive will run on. The designer is ignorant of
the application. So the
> inputs to the control system as known at design time have a stochastic
nature if you are a Bayesian.
>
> I would like to remark here, again, on a couple of incoherences in IEC
61508 and "derivative"
> standards.
>
> Something which executes a safety function must consist of both HW and
SW, because SW alone cannot
> take action. A HW-SW element which executes a safety function is
assigned a reliability goal, which
> is mostly encapsulated in the SIL. These reliability goals are the
safety requirements. A
> reliability goal is expressed in terms of probability of function
failure per demand, or per unit
> time. Suppose that the correct functioning of the HW-SW element E is
functionally dependent on the
> correct functioning of its SW S (which for most actuators it is). The
standard requires one
> demonstrates that the reliability is attained (that the safety
requirement is fulfilled).
>
> How this is actually done must be something like the following.
>
> We assume as above that the element E deterministically transforms its
inputs. We define the
> function CorrE as above. Given a distribution of inputs Distr(I), then
the probability that E
> functions correctly is given by
> (Integral over Distr(I) of the function CorrE(I)) divided by (Integral
over Distr(I) of the constant
> 1).
>
> Notice that the probability of correct functioning, the safety
requirement as laid down by IEC
> 61508, is dependent on Distr(I). Change Distr(I) and one can usually
expect the probability to
> change. (For example, let Distr(I) be the Dirac Delta function on one
incorrect input. Then the
> probability that E functions correctly is 0.)
>
> Yet in IEC 61508, and everywhere else, Distr(I) is not mentioned. Not
once.
>
> This is incoherent.
>
> One could fix it, maybe, by just assuming the uniform distribution on
all inputs, by default. Or the
> normal distribution. There may be reasons for this, but it is worth
pointing out that Distr(I) in
> real applications is almost never uniform or normal. If there is a
distribution D for which it can
> be argued that the real-world input distribution "almost always
approximates D" then one could
> choose D as the default instead.
>
> The second incoherence is as follows. If the SW does not attain the
safety requirement, then E does
> not attain the safety requirement, under a certain plausible assumption,
namely that if CorrS(I) =
> 0, then CorrE(I) is almost always 0. (That is, the HW may sometimes
fortuitously compensate for
> incorrect SW behavior, but mostly not.) Then in order for E to fulfil
the safety requirement, it
> must be the case that
>
> (Integral over Distr(I) of the function CorrS(I)) divided by (Integral
over Distr(I) of the constant
> 1) GEQ (Integral over Distr(I) of the function CorrE(I)) divided by
(Integral over Distr(I) of the
> constant 1)- epsilon
>
> (epsilon is there to instantiate the "almost" part of the assumption).
>
> So, since the safety requirement on E has a probabilistic calculation as
a component, so must the
> inherited safety requirement on S.
>
> Yet there is no requirement in IEC 61508 to substantiate that inherited
safety requirement on S. The
> only condition on software safety requirements is the techniques which
are recommended to be used
> during development of S.
>
> In particular, if you don't think that the execution of SW can have a
stochastic nature, such as
> Jean-Louis, you are thereby committed to the view that IEC 61508 and its
derivates are inherently
> incoherent. It must be a difficult world to live in ......
>
> 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
>
>
>
>
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