Tag Archives: performance engineering

11/06/2014 – Talk by Ivan Stojic

Title: Optimisation of servers with different quality of services
Time: 14:00
Location: Meeting room
Type: Research Result
Speaker: Ivan Stojic

In many applications involving client/server or equivalent architecture, it is possible to vary the quality of services provided by the servers. Dynamic control of the quality of services is an effective mechanism for control of the performance of the system in conditions of varying system load. One of the basic problems involved in applying this mechanism is optimisation of the controlling policy that determines the level of quality of services based on the state of the system. In this seminar, after a brief introduction to Markov chains and basic queueing models, a Markov chain based model of a multiserver queue with processor-sharing discipline and different quality of services is presented. Calculation of the performance indices from the model and the prohibitive complexity of the exact solution are discussed next, motivating the introduction of a simpler model that heuristically approximates the original model and allows for more efficient model solution and optimisation of the controlling policy.

17.07.2013 – Talk by Gian-Luca dei Rossi

Title: On the solution of cooperating stochastic models
Time: 14:00
Location: Meeting room
Type: Research Result
Speaker: Gian-Luca dei Rossi

Stochastic models are widely used in the performance evaluation community. In particular, Markov processes, and more precisely, Continuous Time Markov Chains (CTMCs), often serve as underlying stochastic processes for models written in higher-level formalisms, such as Queueing Networks, Stochastic Petri Nets and Stochastic Process Algebras.. While compositionality, i.e., the ability to express a complex model as a combination of simpler components, is a key feature of most of those formalisms, CTMCs, by themselves, don’t allow for mechanisms to express the interaction with other CTMCs. In order to mitigate this problem various lower-level formalisms have been proposed in literature, e.g., Stochastic Automata Networks (SANs), Communicating Markov Processes, Interactive Markov chains and the labelled transition systems derived from PEPA models.
However, while the compositionality of those formalism is a useful property which makes the modelling phase easier, exploiting it to get solutions more efficiently is a non-trivial task. Ideally one should be able to either detect a product-form solution and analyse the components in isolation or, if a product form cannot be detected, use other techniques to reduce the complexity of the solution, e.g., reducing the state space of either the single components or the joint process. Both tasks raised considerable interest in the literature, e.g., the RCAT theorem for the product-form detection or the Strong Equivalence relation of PEPA to aggregate states in a component-wise fashion.This talk deals with the aforementioned problem of efficiently solving complex Markovian models expressed in term of multiple components. We restrict our analysis to models in which components interact using an active-passive semantics. The main contributions rely on automatic product-forms detection, in components-wise lumping of forward and reversed processes and in showing that those two problems are indeed related, introducing the concept of conditional product-forms.

Open postdoc fellowship

We are seeking to recruit a post-doc fellow to conduct integrative theoretical and experimental research in modelling principles and analysis techniques for distributed and pervasive systems.

Requirements: Applicants must hold a Ph.D degree in Computer Science or related subject before the application deadline. The successful candidate will submit a scientifically excellent research proposal,  relevant to the topics of the call, and display:

  • Scientific knowledge across the relevant subjects,
  • Technical competence and a strong publication record,
  • Team working skills,
  • Leadership experience,
  • Ability to organize her/him self and others.

Funding: is for one year, renewable, at approximately 24,000 Euro gross, including mobility allowances (that corresponds to around 21,000 Euro net). The fellowship is expected to start in May / June 2013

See this page for full detalis.