Edmundo de Souza e Silva

Calculating availability and performability measures of repairable computer systems using randomization

Repairable computer systems are considered, the availability behavior of which can be modeled as a homogeneous Markov process. The randomization method is used to calculate various measures over a finite observation period related to availability modeling of these systems. These measures include the distribution of the number of events of a certain type, the distribution of the length of time in a set of states, and the probability of a near-coincident fault. The method is then extended to calculate performability distributions. The method relies on coloring subintervals of the finite observation period based on the particular application, and then calculating the measure of interest using these colored intervals.

State space exploration in Markov models

Performance and dependability analysis is usually based on Markov models. One of the main problems faced by the analyst is the large state space cardinality of the Markov chain associated with the model, which precludes not only the model solution, but also the generation of the transition rate matrix. However, in many real system models, most of the probability mass is concentrated in a small number of states in comparison with the whole state space. Therefore, performability measures may be accurately evaluated from these “high probable” states. In this paper, we present an algorithm to generate the most probable states that is more efficient than previous algorithms in the literature. We also address the problem of calculating measures of interest and show how bounds on some measures can be efficiently calculated.

Performability analysis of computer systems: from model specification to solution

Abstract Computer systems reliability/availability modeling deals with the representation of changes in the structure of the system being modeled, which are generally due to faults, and how such changes affect the availability of the system. On the other hand, performance modeling involves representing the probabilistic nature of user demands and predicting the system capacity to perform useful work, under the assumption that the system structure remains constant. With the advent of degradable systems, the system may be restructured in response to faults and may continue to perform useful work, even though operating at lower capacity. Performability modeling considers the effect of structural changes and their impact on the overall performance of the system. The complexity of current computer systems and the variety of different problems to be analyzed, including the simultaneous evaluation of performance and availability, demonstrate the need for sophisticated tools that allow the specification of general classes of problems while incorporating powerful analytic and/or simulation techniques. Concerning model specification, a recently proposed object oriented modeling paradigm that accommodates a wide variety of applications is discussed and compared with other approaches. With respect to solution methods, a brief overview of past work on performability evaluation of Markov models is presented. Then it is shown that many performability related measures can be calculated using the uniformization or randomization technique by coloring distinguished states and/or transitions of the Markov model of the system being studied. Finally, the state space explosion problem is addressed and several techniques for dealing with the problem are discussed.

An algorithm to calculate transient distributions of cumulative rate and impulse based reward

Markov reward models have been used to solve a wide variety of problems. In these models, reward rates are associated to the states of a continuous time Markov chain, and impulse rewards are associated to transitions of the chain. Rate based rewards are gained per unit time in the associated state, while impulse rewards are gained instantaneously each time certain transitions occur. We develop an efficient algorithm to calculate the distribution of the total accumulated reward over a given interval of time when both rate and impulse rewards are present. As special cases, we obtain an algorithm to handle models for which only rate rewards occur and another algorithm for the case when only impulse rewards are present. The development is based purely on probabilistic arguments, and the recursions obtained are simple and have a low computational cost.

Transient Solutions for Markov Chains

Much of the theory developed for solving Markov chain models is devoted to obtaining steady state measures, that is, measures for which the observation interval (0, t) is “sufficiently large” (t → ∞). These measures are indeed approximations of the behavior of the system for a finite, but long, time interval, where long means with respect to the interval of time between occurrences of events in the system. However, an increasing number of applications requires the calculation of measures during a relatively “short” period of time. These are the so-called transient measures. In these cases the steady state measures are not good approximations for the transient, and one has to resort to different techniques to obtain the desired quantities.

Quality assessment of interactive voice applications

The conversational quality of a VoIP communication is dependent on several factors such as the coding process used, the network conditions and the type of error correction or concealment employed. Furthermore, the quality perceived by the users is also dependent on the characteristics of the conversation itself. Assessing this kind of communication is a very difficult problem, and most of the studies available in the literature simplify the issue by restricting the analysis to only one or two parameters. However, the number of potentially affecting factors is typically higher, and their joint effect on quality is complex. In this paper we study the combined effects of bit rate, forward error correction, loss rate, loss distribution, delay and jitter on the perceived conversational quality. In order to achieve this we use the pseudo-subjective quality assessment (PSQA) technique, which allows us to obtain accurate, subjective-like assessments, in real time if necessary. Our contributions are thus twofold: firstly, we offer a detailed analysis of the impact of these parameters and their interactions on the perceived conversational quality. Secondly, we show how the PSQA methodology can be used to provide accurate conversational quality estimations.

Assessing the research and education quality of the top Brazilian Computer Science graduate programs

This article reports about a study conducted to assess the quality of the top Brazilian Computer Science graduate programs. The study is based on data from DBLP and considers the scientific production of these programs in the triennial 2004--2006. A comparison of the scientific production of the Brazilian programs against that of reputable programs in North America and Europe indicates that the former compares well with these programs, both in terms of publication rate and number of graduates. The study also shows that the Brazilian programs follow international publication ratios of more than two conference papers per journal article. These results are a clear indication that the Computer Science field has reached maturity in Brazil.

Calculating transient distributions of cumulative reward

Markov reward models have been employed to obtain performability measures of computer and communication systems. In these models, a continuous time Markov chain is used to represent changes in the system structure, usually caused by faults and repairs of its components, and reward rates are assigned to states of the model to indicate some measure of accomplishment at each structure. A procedure to calculate numerically the distribution of the reward accumulated over a finite observation period is presented. The development is based solely on probabilistic arguments, and the final recursion is quite simple. The algorithm has a low computational cost in terms of model parameters. In fact, the number of operations is linear in a parameter that is smaller than the number of rewards, while the storage required is independent of the number of rewards. We also consider the calculation of the distribution of cumulative reward for models in which impulse based rewards are associated with transitions.

Estimating self-sustainability in peer-to-peer swarming systems

Peer-to-peer swarming is one of the \emph{de facto} solutions for distributed content dissemination in todays Internet. By leveraging resources provided by clients, swarming systems reduce the load on and costs to publishers. However, there is a limit to how much cost savings can be gained from swarming; for example, for unpopular content peers will always depend on the publisher in order to complete their downloads. In this paper, we investigate this dependence. For this purpose, we propose a new metric, namely \emph{swarm self-sustainability}. A swarm is referred to as self-sustaining if all its blocks are collectively held by peers; the self-sustainability of a swarm is the fraction of time in which the swarm is self-sustaining. We pose the following question: how does the self-sustainability of a swarm vary as a function of content popularity, the service capacity of the users, and the size of the file? We present a model to answer the posed question. We then propose efficient solution methods to compute self-sustainability. The accuracy of our estimates is validated against simulation. Finally, we also provide closed-form expressions for the fraction of time that a given number of blocks is collectively held by peers.

Efficient mechanisms for recovering voice packets in the Internet

Multimedia applications, such as voice transmission, have increasingly been used over the Internet. However, there are still many issues under investigation, related to the quality of audio delivered, such as the reduction of the jitter and the loss of audio packets. Concerning audio packet loss, many recovery mechanisms have been proposed to improve the audio quality. However, the efficiency of these mechanisms is largely dependent on the loss process in the network. In this paper we study the packet loss process of audio streams aiming at comparing different recovery mechanisms. We also propose a new packet recovery mechanism and show that its efficiency can be significantly better than others proposed in the literature.