Subject: Re: SDL-News: How is Nondeterminism in SDL practically useful?
From: Rick Reed TSE (rickreed#tseng.co.uk)
Date: Fri Jun 30 2000 - 16:11:40 GMT
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-----From Rick Reed TSE <rickreed#tseng.co.uk> to sdlnews -----
Kamal.Hashmi#icl.com at Kamal.Hashmi#icl.com wrote on 26/06/2000 14:27:
> It is also considered useful for test specifications,
> but actually it is the pseudo-random number generator which
> usually underlies many executable models of non-determinism which
> is useful - true non-determinism would be non-repeatable and hence
> useless in a test specification.
I disagree about the usefulness of a truly non-deterministic models for
A truly non-deterministic test generator could be useful for so called
"flood" testing. This would enable a large number of possibilities to be
tested. Of course, this would be indistinguishable from a good pseudo-random
number generator in many cases. However, there is no real need to repeat a
random sequence as long as the sequence is recorded. In the absence of
errors, it would be possible to analyse how random the sequence actually
was. If errors are found, then the record of the sequence would allow the
source of the error to be traced.
If noisy hardware (such as a marginal radio channel) is involved it is not
possible to make sequences pseudo-random - they are usually truly random in
some aspect. A simulation of any such component or many sources of traffic
should probably be made as non-deterministic as possible.
In my opinion, if the real source is random, the test data should not be the
same pseudo-random number generator with the same seed every time. This
could mask an error. For example, for a random set of integers the system
mail fail only if the first integer is a multiple of 3. Using the same
sequence every time would have a probability of only one in three of finding
-- Rick Reed - rickreed#tseng.co.uk Tel:+44 1455 55 96 55 Fax:+44 1455 55 96 58 Mob.:+44 7970 50 96 50
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