[SystemSafety] Degraded software performance [diverged from Fault, Failure and Reliability Again]

[DREW Rae]

Michael,
I need to give more than one example, because the point is general, rather
than specific to the individual causes. In each case the cumulative
probability of software failure increases over time.

1) Damage to the instruction set
e.g. the physical record of the instructions on a storage medium changes
very specific e.g. bit flip on a magnetic storage device holding the
executable files

2) Increased unreliability of the physical execution environment
e.g. an increased rate of processor errors
very specific e.g. dust accumulates on part of the processor card, making
it run hot and produce calculation errors

3) Increased unreliability of input hardware
e.g. software is required to detect and respond correctly to an increased
rate and variety of sensor failure combinations
Note: This is the one that challenges "but we're running the software in
exactly the same hardware environment". Hardware environments change as
they get older.

4) Software accumulates information during runtime
e.g. a count of elapsed time
e.g. increasing volume of stored data
e.g. memory leak

NB1: In all of these cases I've heard arguments "that's not the software,
that's X". Those arguments are only relevant if you can control for X when
collecting data for software reliability calculation. Software without an
execution environment is a design. It "never fails" in the way that _no_
design fails. When it does fail, it is subject to the same degredation over
time as any physical implementation

NB2: I'm not claiming that failure due to physical degredation is
significant compared to failure due to errors in the original instructions.
I'm saying that we don't know, and that not knowing becomes a big issue
once we've tested to the point of not finding errors in the original
instructions. At that point, absent evidence to the contrary, we should be
assuming that physical degredation is signficant.

Drew

On 4 March 2015 at 12:27, Michael J. Pont <M.Pont at safetty.net> wrote:

> Drew,
>
> “The underlying point holds, that software _can_ exhibit degraded
> performance over time.”
>
> Can you please give me a simple example of what you mean by this.
>
> Thanks,
>
> Michael.
>
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