10 research outputs found
Behavioural Preorders on Stochastic Systems - Logical, Topological, and Computational Aspects
Computer systems can be found everywhere: in space, in our homes, in our
cars, in our pockets, and sometimes even in our own bodies. For concerns of
safety, economy, and convenience, it is important that such systems work
correctly. However, it is a notoriously difficult task to ensure that the
software running on computers behaves correctly.
One approach to ease this task is that of model checking, where a model of
the system is made using some mathematical formalism. Requirements expressed in
a formal language can then be verified against the model in order to give
guarantees that the model satisfies the requirements.
For many computer systems, time is an important factor. As such, we need our
formalisms and requirement languages to be able to incorporate real time.
We therefore develop formalisms and algorithms that allow us to compare and
express properties about real-time systems. We first introduce a logical
formalism for reasoning about upper and lower bounds on time, and study the
properties of this formalism, including axiomatisation and algorithms for
checking when a formula is satisfied.
We then consider the question of when a system is faster than another system.
We show that this is a difficult question which can not be answered in general,
but we identify special cases where this question can be answered. We also show
that under this notion of faster-than, a local increase in speed may lead to a
global decrease in speed, and we take step towards avoiding this.
Finally, we consider how to compare the real-time behaviour of systems not
just qualitatively, but also quantitatively. Thus, we are interested in knowing
how much one system is faster or slower than another system. This is done by
introducing a distance between systems. We show how to compute this distance
and that it behaves well with respect to certain properties.Comment: PhD dissertation from Aalborg Universit
A Faster-Than Relation for Semi-Markov Decision Processes
When modeling concurrent or cyber-physical systems, non-functional
requirements such as time are important to consider. In order to improve the
timing aspects of a model, it is necessary to have some notion of what it means
for a process to be faster than another, which can guide the stepwise
refinement of the model. To this end we study a faster-than relation for
semi-Markov decision processes and compare it to standard notions for relating
systems. We consider the compositional aspects of this relation, and show that
the faster-than relation is not a precongruence with respect to parallel
composition, hence giving rise to so-called parallel timing anomalies. We take
the first steps toward understanding this problem by identifying decidable
conditions sufficient to avoid parallel timing anomalies in the absence of
non-determinism.Comment: In Proceedings QAPL 2019, arXiv:2001.0616
Timed Comparisons of Semi-Markov Processes
Semi-Markov processes are Markovian processes in which the firing time of the
transitions is modelled by probabilistic distributions over positive reals
interpreted as the probability of firing a transition at a certain moment in
time. In this paper we consider the trace-based semantics of semi-Markov
processes, and investigate the question of how to compare two semi-Markov
processes with respect to their time-dependent behaviour. To this end, we
introduce the relation of being "faster than" between processes and study its
algorithmic complexity. Through a connection to probabilistic automata we
obtain hardness results showing in particular that this relation is
undecidable. However, we present an additive approximation algorithm for a
time-bounded variant of the faster-than problem over semi-Markov processes with
slow residence-time functions, and a coNP algorithm for the exact faster-than
problem over unambiguous semi-Markov processes
Development of GUIs for CERN devices
This report documents the work that was done during a summer student internship in the CERN BE-BI-SW group in the summer of 2013. The original project proposal was to produce a java GUI to simplify the setting up of a CERN beam instrument. The end result of the project is two desktop applications for monitoring two different beam instruments as well as an Android app meant as a prototype for monitoring arbitrary beam instruments on a smartphone
Reasoning About Bounds in Weighted Transition Systems
We propose a way of reasoning about minimal and maximal values of the weights
of transitions in a weighted transition system (WTS). This perspective induces
a notion of bisimulation that is coarser than the classic bisimulation: it
relates states that exhibit transitions to bisimulation classes with the
weights within the same boundaries. We propose a customized modal logic that
expresses these numeric boundaries for transition weights by means of
particular modalities. We prove that our logic is invariant under the proposed
notion of bisimulation. We show that the logic enjoys the finite model property
and we identify a complete axiomatization for the logic. Last but not least, we
use a tableau method to show that the satisfiability problem for the logic is
decidable
On the axiomatisability of priority III: Priority strikes again
Aceto et al., proved that, over the process algebra BCCSP with the priority operator of Baeten, Bergstra and Klop, the equational theory of order-insensitive bisimilarity is not finitely based. However, it was noticed that by substituting the action prefixing operator of BCCSP with BPA's sequential composition, the infinite family of equations used to show that non-finite axiomatisability result could be proved by a finite collection of sound equations. That observation left as an open question the existence of a finite axiomatisation for order-insensitive bisimilarity over BPA with the priority operator. In this paper we provide a negative answer to this question. We prove that, in the presence of at least two actions, order-insensitive bisimilarity is not finitely based over BPA with priority