César Sánchez

LOLA: runtime monitoring of synchronous systems

We present a specification language and algorithms for the online and offline monitoring of synchronous systems including circuits and embedded systems. Such monitoring is useful not only for testing, but also under actual deployment. The specification language is simple and expressive; it can describe both correctness/failure assertions along with interesting statistical measures that are useful for system profiling and coverage analysis. The algorithm for online monitoring of queries in this language follows a partial evaluation strategy: it incrementally constructs output streams from input streams, while maintaining a store of partially evaluated expressions for forward references. We identify a class of specifications, characterized syntactically, for which the algorithms memory requirement is independent of the length of the input streams. Being able to bound memory requirements is especially important in online monitoring of large input streams. We extend the concepts used in the online algorithm to construct an efficient offline monitoring algorithm for large traces. We have implemented our algorithm and applied it to two industrial systems, the PCI bus protocol and a memory controller. The results demonstrate that our algorithms are practical and that our specification language is sufficiently expressive to handle specifications of interest to industry.

Temporal Logics for Hyperproperties

Hyperproperties, as introduced by Clarkson and Schneider, characterize the correctness of a computer program as a condition on its set of computation paths. Standard temporal logics can only refer to a single path at a time, and therefore cannot express many hyperproperties of interest, including noninterference and other important properties in security and coding theory. In this paper, we investigate an extension of temporal logic with explicit path variables. We show that the quantification over paths naturally subsumes other extensions of temporal logic with operators for information flow and knowledge. The model checking problem for temporal logic with path quantification is decidable. For alternation depth 1, the complexity is PSPACE in the length of the formula and NLOGSPACE in the size of the system, as for linear-time temporal logic.

Regular linear temporal logic

We present regular linear temporal logic (RLTL), a logic that generalizes linear temporal logic with the ability to use regular expressions arbitrarily as sub-expressions. Every LTL operator can be defined as a context in regular linear temporal logic. This implies that there is a (linear) translation from LTL to RLTL. Unlike LTL, regular linear temporal logic can define all ω-regular languages, while still keeping the satisfiability problem in PSPACE. Unlike the extended temporal logics ETL*, RLTL is defined with an algebraic signature. In contrast to the linear time µ-calculus, RLTL does not depend on fix-points in its syntax.

Event Correlation: Language and Semantics

Event correlation is a service provided by middleware platforms that allows components in a publish/subscribe architecture to subscribe to patterns of events rather than individual events. Event correlation improves the scalability and performance of distributed systems, increases their analyzability, while reducing their complexity by moving functionality to the middleware. To ensure that event correlation is provided as a standard and reliable service, it must possess well-defined and unambiguous semantics.

Packet wavelet decomposition: An approach for atrial activity extraction

Detection of atrial activity (AA) is quite important in the study and monitoring of atrial rhythms, in particular atrial flutter and atrial fibrillation (FA). An efficient noninvasive study of the AA needs the ventricular activity cancellation. The Discrete Packet Wavelet Transform (DPWT) allows the decomposition of the original ECG in a set of coefficients with different temporal and spectral features, showing that it is possible to obtain the AA with a finite set of this blocks and the inverse transform. The principal advantage of the DPWT analysis is that it does not require several leads of the same ECG register so it should be applicable to the detection of different arrhythmias in Holter registers, where the number of leads is reduced.

Algorithms for Model Checking HyperLTL and HyperCTL

We present an automata-based algorithm for checking finite state systems for hyperproperties specified in HyperLTL and HyperCTL\(^*\). For the alternation-free fragments of HyperLTL and HyperCTL\(^*\) the automaton construction allows us to leverage existing model checking technology. Along several case studies, we demonstrate that the approach enables the verification of real hardware designs for properties that could not be checked before. We study information flow properties of an I2C bus master, the symmetric access to a shared resource in a mutual exclusion protocol, and the functional correctness of encoders and decoders for error resistant codes. Open image in new window

Abstracting runtime heaps for program understanding

Modern programming environments provide extensive support for inspecting, analyzing, and testing programs based on the algorithmic structure of a program. Unfortunately, support for inspecting and understanding runtime data structures during execution is typically much more limited. This paper provides a general purpose technique for abstracting and summarizing entire runtime heaps. We describe the abstract heap model and the associated algorithms for transforming a concrete heap dump into the corresponding abstract model as well as algorithms for merging, comparing, and computing changes between abstract models. The abstract model is designed to emphasize high-level concepts about heap-based data structures, such as shape and size, as well as relationships between heap structures, such as sharing and connectivity. We demonstrate the utility and computational tractability of the abstract heap model by building a memory profiler. We use this tool to identify, pinpoint, and correct sources of memory bloat for programs from DaCapo.

Convolutive Blind Source Separation Algorithms Applied to the Electrocardiogram of Atrial Fibrillation: Study of Performance

The analysis of the surface electrocardiogram (ECG) is the most extended noninvasive technique in medical diagnosis of atrial fibrillation (AF). In order to use the ECG as a tool for the analysis of AF, we need to separate the atrial activity (AA) from other cardioelectric signals. In this matter, statistical signal processing techniques, like blind source separation (BSS), are able to perform a multilead statistical analysis with the aim to obtain the AA. Linear BSS techniques can be divided in two groups depending on the mixing model: algorithms where instantaneous mixing of sources is assumed, and convolutive BSS (CBSS) algorithms. In this work, a comparison of performance between one relevant CBSS algorithm, namely Infomax, and one of the most effective independent component analysis (ICA) algorithms, namely FastICA, is developed. To carry out the study, pseudoreal AF ECGs have been synthesized by adding fibrillation activity to normal sinus rhythm. The algorithm performances are expressed by two indexes: the signal to interference ratio (SIRAA) and the cross-correlation (RAA) between the original and the estimated AA. Results empirically prove that the instantaneous mixing model is the one that obtains the best results in the AA extraction, given that the mean SIRAA obtained by the FastICA algorithm (37.6 plusmn 17.0 dB) is higher than the main SIRAA obtained by Infomax (28.5 plusmn 14.2 dB). Also the RAA obtained by FastICA (0.92 plusmn 0.13) is higher than the one obtained by Infomax (0.78 plusmn 0.16).

Reusable models for timing and liveness analysis of middleware for distributed real-time and embedded systems

Distributed real-time and embedded (DRE) systems have stringent constraints on timeliness and other properties whose assurance is crucial to correct system behavior. Formal tools and techniques play a key role in verifying and validating system properties. However, many DRE systems are built using middleware frameworks that have grown increasingly complex to address the diverse requirements of a wide range of applications. How to apply formal tools and techniques effectively to these systems, given the range of middleware configuration options available, is therefore an important research problem.This paper makes three contributions to research on formal verification and validation of middleware-based DRE systems. First, it presents a reusable library of formal models we have developed to capture essential timing and concurrency semantics of foundational middleware building blocks provided by the ACE framework. Second, it describes domain-specific techniques to reduce the cost of checking those models while ensuring they remain valid with respect to the semantics of the middleware itself. Third, it presents a verification and validation case study involving a gateway service, using our models.

Atrial fibrillation analysis based on ICA including statistical and temporal source information

The analysis and characterization of atrial fibrillation requires the prior extraction of the atrial activity from the electrocardiogram, where the independent atrial and ventricular activities are combined in addition to noise. An independent component analysis method is proposed where additional knowledge about the time and statistical structure of the sources is incorporated. Finally, a combined method based on maximum likelihood and second order blind identification is obtained and validated with results that improve those obtained with traditional ICA algorithms.