Visualizing Concurrent Computations

This paper describes the underlying model for a visualization environment concerned with exploring, monitoring, and presenting concurrent computations. The model is declarative in the sense that visualization is treated as the composition of several mappings which, as a whole, map computational states into full-color images of a 3-D geometric world. The mappings defining the visualizations are specified using a rule-based notation. The visualization methodology is proof-based, i.e., it captures abstract formal properties of programs (e.g. safety and progress) rather than operational details. An algorithm for termination detection in diffusing computations is used to illustrate the specification method and to demonstrate its conceptual elegance and flexibility

Enabling Correct Interest Forwarding and Retransmissions in a Content Centric Network

We show that the mechanisms used in the name data networking (NDN) and the original content centric networking (CCN) architectures may not detect Interest loops, even if the network in which they operate is static and no faults occur. Furthermore, we show that no correct Interest forwarding strategy can be defined that allows Interest aggregation and attempts to detect Interest looping by identifying Interests uniquely. We introduce SIFAH (Strategy for Interest Forwarding and Aggregation with Hop-Counts), the first Interest forwarding strategy shown to be correct under any operational conditions of a content centric network. SIFAH operates by having forwarding information bases (FIBs) store the next hops and number of hops to named content, and by having each Interest state the name of the requested content and the hop count from the router forwarding an Interest to the content. We present the results of simulation experiments using the ndnSIM simulator comparing CCN and NDN with SIFAH. The results of these experiments illustrate the negative impact of undetected Interest looping when Interests are aggregated in CCN and NDN, and the performance advantages of using SIFAH

Preserving Stabilization while Practically Bounding State Space

Stabilization is a key dependability property for dealing with unanticipated transient faults, as it guarantees that even in the presence of such faults, the system will recover to states where it satisfies its specification. One of the desirable attributes of stabilization is the use of bounded space for each variable. In this paper, we present an algorithm that transforms a stabilizing program that uses variables with unbounded domain into a stabilizing program that uses bounded variables and (practically bounded) physical time. While non-stabilizing programs (that do not handle transient faults) can deal with unbounded variables by assigning large enough but bounded space, stabilizing programs that need to deal with arbitrary transient faults cannot do the same since a transient fault may corrupt the variable to its maximum value. We show that our transformation algorithm is applicable to several problems including logical clocks, vector clocks, mutual exclusion, leader election, diffusing computations, Paxos based consensus, and so on. Moreover, our approach can also be used to bound counters used in an earlier work by Katz and Perry for adding stabilization to a non-stabilizing program. By combining our algorithm with that earlier work by Katz and Perry, it would be possible to provide stabilization for a rich class of problems, by assigning large enough but bounded space for variables.Comment: Moved some content from the Appendix to the main paper, added some details to the transformation algorithm and to its descriptio

A quick termination detection protocol by reducing overload for mobile ad hoc networks

An ad hoc network is characterized by the fact that there is no fixed topology due to the mobility of nodes, interference, multipath propagation and path loss. Execution of applications in such kind of networks typically consists of a number of successive phases such as network reprogramming, localization, power monitoring, and parameter updates. Termination detection of a phase is therefore a critical operation to safely execute a new phase on some or all of the network nodes. In resource constrained network environment the overhead should be minimum in order to increase throughput and minimize delay. This paper studies the existing solutions for termination detection by analyzing their effectiveness. Moreover, in this paper, we propose an efficient algorithmic solution to encounter termination detection by minimizing the network overloads

Formal verification of distributed deadlock detection algorithms

The problem of distributed deadlock detection has undergone extensive study. Formal verification of deadlock detection algorithms in distributed systems is an area of research that has largely been ignored. Instead, most proposed distributed deadlock detection algorithms have used informal or intuitive arguments, simulation or just neglect the entire aspect of verification of correctness; As a consequence, many of these algorithms have been shown incorrect. This research will abstract the notion of deadlock in terms of a temporal logic of actions and discuss the invariant and eventuality properties. The contributions of this research are the development of a distributed deadlock detection algorithm and the formal verification of this algorithm

System Description for a Scalable, Fault-Tolerant, Distributed Garbage Collector

We describe an efficient and fault-tolerant algorithm for distributed cyclic garbage collection. The algorithm imposes few requirements on the local machines and allows for flexibility in the choice of local collector and distributed acyclic garbage collector to use with it. We have emphasized reducing the number and size of network messages without sacrificing the promptness of collection throughout the algorithm. Our proposed collector is a variant of back tracing to avoid extensive synchronization between machines. We have added an explicit forward tracing stage to the standard back tracing stage and designed a tuned heuristic to reduce the total amount of work done by the collector. Of particular note is the development of fault-tolerant cooperation between traces and a heuristic that aggressively reduces the set of suspect objects.Comment: 47 pages, LaTe

A Survey and analysis of algorithms for the detection of termination in a distributed system

This paper looks at algorithms for the detection of termination in a distributed system and analyzes them for effectiveness and efficiency. A survey is done of the published algorithms for distributed termination and each is evaluated. Both centralized distributed systems and fully distributed systems are reviewed. The algorithms are analyzed for the overhead and conclusions are made about the situations in which they can be used, i.e. an operating system, a real-time system, or a user application. An original algorithm is presented for the asynchronous case with first-in-first-out message ordering. It allows any process to initiate detection of termination and makes use of multiple tokens