Luís Moura Silva

Global checkpointing for distributed programs

A novel algorithm for checkpointing and rollback recovery in distributed systems is presented. Processes belonging to the same program must take periodically a nonblocking coordinated global checkpoint, but only a minimum overhead is imposed during normal computation. Messages can be delivered out of order, and the processes are not required to be deterministic. The nonblocking structure is an important characteristic for avoiding laying a heavy burden on the application programs. The method also includes the damage assessment phase, unlike previous schemes that either assume that an error is detected immediately after it occurs (fail-stop) or simply ignore the damage caused by imperfect detection mechanisms. A possible way to evaluate the error detection latency, which enables one to assess the damage made and avoid the propagation of errors, is presented.<<ETX>>

Singlet and triplet energies of α-oligothiophenes: A spectroscopic, theoretical, and photoacoustic study: Extrapolation to polythiophene

The triplet state energies, ET(0–0), for the α-oligothiophenes (αn’s) with n=1–7, were determined and vary from 18 000 cm−1 (α2) to 13 000 cm−1 (α7). The data were obtained from spectroscopic and calorimetric measurements. It was found that the triplet energies decrease with the increase ring number (n), leveling off beginning with the α-oligothiophene with n=5. A linear relation was obtained for the triplet energy as a function of the reciprocal of n. The data has allowed the extrapolation to polythiophene (n=∞) and the evaluation of its triplet energy (≅10 000 cm−1). The energies (0–0) of the first singlet excited state of n=1–7 were also evaluated and again a linear relation was found for the E[S1(0–0)] as a function of 1/n. The S1−T1 energy splitting, ΔE(S1−T1), was determined and also found to decrease with the increase in the number ring. Semiempirical molecular orbital theoretical calculations reproduce this same trend of decreasing ΔE(S1−T1) with n. Extended FEMO theory was applied to the singlet ...

Sabotage-tolerance and trust management in desktop grid computing

The success of grid computing in open environments like the Internet is highly dependent on the adoption of mechanisms to detect failures and malicious sabotage attempts. It is also required to maintain a trust management system that permits one to distinguish the trustable from the non-trustable participants in a global computation. Without these mechanisms, users with data-critical applications will never rely on desktop grids, and will rather prefer to support higher costs to run their computations in closed and secure computing systems. This paper discusses the topics of sabotage-tolerance and trust management. After reviewing the state-of-the-art, we present two novel techniques: a mechanism for sabotage detection and a protocol for distributed trust management. The proposed techniques are targeted at the paradigm of volunteer-based computing commonly used on desktop grids.

Fault-tolerant execution of mobile agents

In this paper, we will address the list of problems that have to be solved in mobile agent systems and we will present a set of fault-tolerance techniques that can increase the robustness of agent-based applications without introducing a high performance overhead. The framework includes a set of schemes for failure detection, checkpointing and restart, software rejuvenation, a resource-aware atomic migration protocol, a reconfigurable itinerary, a protocol that avoids agents to get caught in node failures and a simple scheme to deal with network partitions. At the end, we will present some performance results that show the effectiveness of these fault-tolerance techniques.

Characterizing result errors in internet desktop grids

Desktop grids use the free resources in Intranet and Internet environments for large-scale computation and storage. While desktop grids offer a high return on investment, one critical issue is the validation of results returned by participating hosts. Several mechanisms for result validation have been previously proposed. However, the characterization of errors is poorly understood. To study error rates, we implemented and deployed a desktop grid application across several thousand hosts distributed over the Internet. We then analyzed the results to give quantitative and empirical characterization of errors stemming from input or output (I/O) failures. We find that in practice, error rates are widespread across hosts but occur relatively infrequently. Moreover, we find that error rates tend to not be stationary over time nor correlated between hosts. In light of these characterization results, we evaluated state-of-the-art error detection mechanisms and describe the trade-offs for using each mechanism.

Software Aging and Rejuvenation in a SOAP-based Server

Web-services and service-oriented architectures are gaining momentum in the area of distributed systems and Internet applications. However, as we increase the abstraction level of the applications we are also increasing the complexity of the underlying middleware. In this paper, we present a dependability benchmarking study to evaluate and compare the robustness of some of the most popular SOAP-RPC implementations that are intensively used in the industry. The study was focused on Apache Axis where we have observed a high susceptibility of software aging. Building on these results we propose a new SLA-oriented software rejuvenation technique that proved to be a simple way to increase the dependability of the SOAP-server, the degree of self-healing and to maintain a sustained level of performance in the applications

Using machine learning for non-intrusive modeling and prediction of software aging

The wide-spread phenomenon of software (running image) aging is known to cause performance degradation, transient failures or even crashes of applications. In this work we describe first a method for monitoring and modeling of performance degradation in SOA applications, particularly application servers. This method works for a large class of the aging processes caused by resource depletion (e.g. memory leaks). It can be deployed non-intrusively in a production environment, under arbitrary service request distributions. Based on this schema we investigate in the second part of the paper how machine learning (classification) algorithms can be used for proactive detection of performance degradation or sudden drops caused by aging. We leverage the predictive power of these algorithms with several techniques to make the measurement-based aging models more adaptive and more robust against transient failures. We evaluate several state-of-the-art classification methods for their accuracy and computational efficiency in this scenario. The studies are performed on a data set generated by a TPC-W benchmark instrumented with a memory leak injector. The results show that the probing method yields accurate aging models with low overhead and the machine learning approach gives statistically significant short-term predictions of degrading application performance. Both approaches can be used directly to fight aging via adaptive software rejuvenation (restart of the application), for operator alerting, or for short-term capacity planning.

JAMES: A Platform of Mobile Agents for the Management of Telecommunication Networks

This paper presents an overview of JAMES, a Java-based platform of mobile agents that is mainly oriented for the management of data and telecommunication networks. This platform has been developed on behalf of a Eureka Project (Σ!1921) and the project partners are Siemens SA, University of Coimbra and Siemens AG. We describe the main architecture of the platform giving more emphasis to the most important features. To show the effectiveness of some of the techniques that have been implemented we will present some performance results that compare the JAMES platform with the Aglets Workbench. The main target of our platform is network management and telecommunication applications. In this line, we have done a Java-based implementation of SNMP that has been integrated within the platform. The industrial partners of our project (i.e. Siemens S.A.) have developed a prototype application for TMN performance management. Although it is still a prototype it is being used to validate the technological advantages of using mobile agents in the management of telecommunication networks.

Resource usage of Windows computer laboratories

Studies focusing on Unix have shown that the vast majority of workstations and desktop computers remain idle for most of the time. In this paper we quantify the usage of main resources (CPU, main memory, disk space and network bandwidth) of Windows 2000 machines from classroom laboratories. For that purpose, 169 machines of 11 classroom laboratories were monitored over 77 consecutive days. Samples were collected from all machines every 15 minutes for a total of 583653 samples. Besides evaluating availability of machines (uptime and downtime) and usage habits of users, the paper assesses usage of main resources, focusing on the impact of interactive login sessions over resource consumptions. Also, resorting to Self Monitoring Analysis and Reporting Technology (SMART) parameters of hard disks, the study estimates the average uptime per hard drive power cycle for the whole life of monitored computers. Our results show that resources idleness in classroom computers is very high, with an average CPU idleness of 97.9%, unused memory averaging 42.1% and unused disk space of the order of gigabytes per machine. Moreover, this study confirms the 2:1 equivalence rule found out by similar works, with N non-dedicated resources delivering an average CPU computing power roughly similar to N/2 dedicated machines. These results confirm the potentiality of these systems for resource harvesting, especially for grid desktop computing schemes.

Photophysical and theoretical studies of naphthalene-substituted oligothiophenes

A comprehensive photophysical characterization of a new class of naphthalene derivatives of the important α-oligothiophenes (αn’s) has been undertaken in solution at room (293 K) and low (77 K) temperature. This includes absorption and fluorescence spectra, fluorescence quantum yields (φF), and lifetimes (τF), as a function of temperature and solvent. Triplet–triplet absorption spectra and triplet formation quantum yields (φT) were also determined. From the above, all the rate constants for the radiative (kF) and radiationless (kIC and kISC) have been calculated. It is shown that the lowest singlet excited state is an allowed π,π* state in all solvents. The results show that although the behavior of the oligomers is similar to their parent compounds (αn’s), significant differences are observed. By comparison of the extinction coefficients of the naph(αn’s) and the (αn’s), a good correlation was found between naph(αn) and α(n+1). On the basis of this proposed pairing, a consistent blue-shift was observed i...