Process algebra for performance evaluation

This paper surveys the theoretical developments in the field of stochastic process algebras, process algebras where action occurrences may be subject to a delay that is determined by a random variable. A huge class of resource-sharing systems – like large-scale computers, client–server architectures, networks – can accurately be described using such stochastic specification formalisms. The main emphasis of this paper is the treatment of operational semantics, notions of equivalence, and (sound and complete) axiomatisations of these equivalences for different types of Markovian process algebras, where delays are governed by exponential distributions. Starting from a simple actionless algebra for describing time-homogeneous continuous-time Markov chains, we consider the integration of actions and random delays both as a single entity (like in known Markovian process algebras like TIPP, PEPA and EMPA) and as separate entities (like in the timed process algebras timed CSP and TCCS). In total we consider four related calculi and investigate their relationship to existing Markovian process algebras. We also briefly indicate how one can profit from the separation of time and actions when incorporating more general, non-Markovian distributions

Modular Verification for Almost-Sure Termination of Probabilistic Programs

International audienceIn this work, we consider the almost-sure termination problem for probabilistic programs that asks whether agiven probabilistic program terminates with probability 1. Scalable approaches for program analysis oftenrely on modularity as their theoretical basis. In non-probabilistic programs, the classical variant rule (V-rule)of Floyd-Hoare logic provides the foundation for modular analysis. Extension of this rule to almost-suretermination of probabilistic programs is quite tricky, and a probabilistic variant was proposed in [Fioriti andHermanns 2015]. While the proposed probabilistic variant cautiously addresses the key issue of integrability,we show that the proposed modular rule is still not sound for almost-sure termination of probabilistic programs.Besides establishing unsoundness of the previous rule, our contributions are as follows: First, we present asound modular rule for almost-sure termination of probabilistic programs. Our approach is based on a novelnotion of descent supermartingales. Second, for algorithmic approaches, we consider descent supermartingalesthat are linear and show that they can be synthesized in polynomial time. Finally, we present experimentalresults on a variety of benchmarks and several natural examples that model various types of nested whileloops in probabilistic programs and demonstrate that our approach is able to efficiently prove their almost-suretermination property

StocHy: automated verification and synthesis of stochastic processes

StocHy is a software tool for the quantitative analysis of discrete-time stochastic hybrid systems (SHS). StocHy accepts a high-level description of stochastic models and constructs an equivalent SHS model. The tool allows to (i) simulate the SHS evolution over a given time horizon; and to automatically construct formal abstractions of the SHS. Abstractions are then employed for (ii) formal verification or (iii) control (policy, strategy) synthesis. StocHy allows for modular modelling, and has separate simulation, verification and synthesis engines, which are implemented as independent libraries. This allows for libraries to be easily used and for extensions to be easily built. The tool is implemented in C++ and employs manipulations based on vector calculus, the use of sparse matrices, the symbolic construction of probabilistic kernels, and multi-threading. Experiments show StocHy's markedly improved performance when compared to existing abstraction-based approaches: in particular, StocHy beats state-of-the-art tools in terms of precision (abstraction error) and computational effort, and finally attains scalability to large-sized models (12 continuous dimensions). StocHy is available at www.gitlab.com/natchi92/StocHy

Composition of Stochastic Transition Systems Based on Spans and Couplings

Conventional approaches for parallel composition of stochastic systems relate probability measures of the individual components in terms of product measures. Such approaches rely on the assumption that components interact stochastically independent, which might be too rigid for modeling real world systems. In this paper, we introduce a parallel-composition operator for stochastic transition systems that is based on couplings of probability measures and does not impose any stochastic assumptions. When composing systems within our framework, the intended dependencies between components can be determined by providing so-called spans and span couplings. We present a congruence result for our operator with respect to a standard notion of bisimilarity and develop a general theory for spans, exploiting deep results from descriptive set theory. As an application of our general approach, we propose a model for stochastic hybrid systems called stochastic hybrid motion automata

Eco‑evo‑devo and iterated learning : towards an integrated approach in the light of niche construction

In this paper we argue that ecological evolutionary developmental biology (ecoevo-devo) accounts of cognitive modernity are compatible with cultural evolution theories of language built upon iterated learning models. Cultural evolution models show that the emergence of near universal properties of language do not require the preexistence of strong specific constraints. Instead, the development of general abilities, unrelated to informational specificity, like the copying of complex signals and sharing of communicative intentions is required for cultural evolution to yield specific properties, such as language structure. We argue that eco-evo-devo provides the appropriate conceptual background to ground an account for the many interconnected genetic, environmental and developmental factors that facilitated the emergence of an organic system able to develop language through the iterated transmission of information. We use the concept of niche construction to connect evolutionary developmental accounts for sensory guided motor capacities and cultural evolution guided by iterated learning models. This integrated theoretical model aims to build bridges between biological and cultural approaches

Verificare: a platform for composable verification with application to SDN-Enabled systems

Software-Defined Networking (SDN) has become increasing prevalent in both the academic and industrial communities. A new class of system built on SDNs, which we refer to as SDN-Enabled, provide programmatic interfaces between the SDN controller and the larger distributed system. Existing tools for SDN verification and analysis are insufficiently expressive to capture this composition of a network and a larger distributed system. Generic verification systems are an infeasible solution, due to their monolithic approach to modeling and rapid state-space explosion. In this thesis we present a new compositional approach to system modeling and verification that is particularly appropriate for SDN-Enabled systems. Compositional models may have sub-components (such as switches and end-hosts) modified, added, or removed with only minimal, isolated changes. Furthermore, invariants may be defined over the composed system that restrict its behavior, allowing assumptions to be added or removed and for components to be abstracted away into the service guarantee that they provide (such as guaranteed packet arrival). Finally, compositional modeling can minimize the size of the state space to be verified by taking advantage of known model structure. We also present the Verificare platform, a tool chain for building compositional models in our modeling language and automatically compiling them to multiple off-the-shelf verification tools. The compiler outputs a minimal, calculus-oblivious formalism, which is accessed by plugins via a translation API. This enables a wide variety of requirements to be verified. As new tools become available, the translator can easily be extended with plugins to support them