Dynamic sharing of a multiple access channel

In this paper we consider the mutual exclusion problem on a multiple access channel. Mutual exclusion is one of the fundamental problems in distributed computing. In the classic version of this problem, n processes perform a concurrent program which occasionally triggers some of them to use shared resources, such as memory, communication channel, device, etc. The goal is to design a distributed algorithm to control entries and exits to/from the shared resource in such a way that in any time there is at most one process accessing it. We consider both the classic and a slightly weaker version of mutual exclusion, called ep-mutual-exclusion, where for each period of a process staying in the critical section the probability that there is some other process in the critical section is at most ep. We show that there are channel settings, where the classic mutual exclusion is not feasible even for randomized algorithms, while ep-mutual-exclusion is. In more relaxed channel settings, we prove an exponential gap between the makespan complexity of the classic mutual exclusion problem and its weaker ep-exclusion version. We also show how to guarantee fairness of mutual exclusion algorithms, i.e., that each process that wants to enter the critical section will eventually succeed

April 8, 1984

Abstract. Distributed Hash Tables (DHTs) enable fully distributed Peer-to-Peer network construction and maintenance with name-driven routing. There exist very few DHT approaches that consider heterogeneity of nodes inside the construction process or properly serve data of different load. To our best knowledge, there is no construction which smoothly addresses both these issues. We propose a Peer-to-Peer construction that explicitly uses heterogeneity to simplify the routing and maintenance process even in the presence of an adaptive adversary. Using a hypercube and cube connected cycles networks as a backbone, we show how to cope with two types of heterogeneity: one for nodes and one for data

Distributed Alarming in the On-Duty and Off-Duty Models

Decentralized monitoring and alarming systems can be an attractive alternative to centralized architectures. Distributed sensor nodes (e.g., in the smart grid's distribution network) are closer to an observed event than a global and remote observer or controller. This improves the visibility and response time of the system. Moreover, in a distributed system, local problems may also be handled locally and without overloading the communication network. This paper studies alarming from a distributed computing perspective and for two fundamentally different scenarios: on-duty and off-duty. We model the alarming system as a sensor network consisting of a set of distributed nodes performing local measurements to sense events. In order to avoid false alarms, the sensor nodes cooperate and only escalate an event (i.e., raise an alarm) if the number of sensor nodes sensing an event exceeds a certain threshold. In the on-duty scenario, nodes not affected by the event can actively help in the communication process, while in the off-duty scenario, non-event nodes are inactive. We present and analyze algorithms that minimize the reaction time of the monitoring system while avoiding unnecessary message transmissions. We investigate time and message complexity tradeoffs in different settings, and also shed light on the optimality of our algorithms by deriving cost lower bounds for distributed alarming systems.13 page(s

Friction Welding of Aluminium and Aluminium Alloys with Steel

The paper presents our actual knowledge and experience in joining dissimilar materials with the use of friction welding method. The joints of aluminium and aluminium alloys with the different types of steel were studied. The structural effects occurring during the welding process were described. The mechanical properties using, for example, (i) microhardness measurements, (ii) tensile tests, (iii) bending tests, and (iv) shearing tests were determined. In order to obtain high-quality joints the influence of different configurations of the process such as (i) changing the geometry of bonding surface, (ii) using the interlayer, or (iii) heat treatment was analyzed. Finally, the issues related to the selection of optimal parameters of friction welding process were also investigated

Distributed Online and Stochastic Queueing on a Multiple Access Channel

We consider the problems of online and stochastic packet queueing in a distributed system of n nodes with queues, where the communication between the nodes is done via a multiple access channel. In the online setting, in each round, an arbitrary number of packets can be injected to nodes’ queues. Two measures of performance are considered: the total number of packets in all queues, called the total load , and the maximum queue size, called the maximum load . We develop a deterministic distributed algorithm that is asymptotically optimal with respect to both complexity measures, in a competitive way. More precisely, the total load of our algorithm is bigger than the total load of any other algorithm, including centralized online solutions, by only an additive term of O ( n 2 ), whereas the maximum queue size of our algorithm is at most n times bigger than the maximum queue size of any other algorithm, with an extra additive O ( n ). The optimality for both measures is justified by proving the corresponding lower bounds, which also separates nearly exponentially distributed solutions from the centralized ones. Next, we show that our algorithm is also stochastically stable for any expected injection rate smaller or equal to 1. This is the first solution to the stochastic queueing problem on a multiple access channel that achieves such stability for the (highest possible) rate equal to 1. </jats:p

Burnout, neurotic symptoms and coping strategies in medical students

the early stages of a medical career - as early as in medical college. Medical studies are considered one of the most stressful majors, leading to early burnout and other related symptoms such as neurotic symptoms. Our aim was to examine this topic by assessing burnout and neurotic symptoms as well as strategies of coping with stress experienced during each year of studies. Method: We used a web-based questionnaire, consisting of the Maslach Burnout Inventory-Student Survey (MBI-SS), Coping Inventory for Stressful Situations (CISS) and Symptom Checklist S-III, and invited medical students at various stages of a 6-year medical course to fill it in online. Questionnaire was filled by 781 students in total. Results: Statistical analysis revealed an interesting pattern of symptoms severity in students, with highest scores at the beginning and at the end of the medical course and the lowest score during the 3rd year of studies. This pattern was clearly visible for MBI-SS Exhaustion, and somewhat less pronounced for MBI-SS Cynicism and S-III scores, where only the decrease of symptoms was significant. Coping strategies seemed to be similar for all medical students with a higher score for the Distraction scale among the 3rd - year students compared with the 2nd-year students. Discussion: These results, however unexpected, seem to be consistent with available literature, emphasizing higher levels of stress experienced during great changes regarding expectations in students at the beginning of their course and in soon-to-be doctors. Conclusions: The results prompt to reflect on ways of countering emerging symptoms of burnout not only in experienced students, but also among those starting medical college

Dynamic page migration under brownian motion

Abstract. We consider Dynamic Page Migration (DPM) problem, one of the fundamental subproblems of data management in dynamically changing networks. We investigate a hybrid scenario, where access patterns to the shared object are dictated by an adversary, and each processor performs a random walk in X. We extend the previous results of [4]: we develop algorithms for the case where X is a ring, and prove that with high probability they achieve a competitive ratio of Õ(min { 4 √ D, n}), where D is the size of the shared object and n is the number of nodes in the network. These results hold also for any d-dimensional torus or mesh with diameter at least ˜ Ω ( √ D).

of Dynamic Intelligent Systems

In a (randomized) oblivious routing scheme the path chosen for a request between a source s and a target t is independent from the current traffic in the network. Hence, such a scheme consists of probability distributions over s − t paths for every source-target pair s, t in the network. In a recent result [11] it was shown that for any undirected network there is an oblivious routing scheme that achieves a polylogarithmic competitive ratio with respect to congestion. Subsequently, Azar et al. [4] gave a polynomial time algorithm that for a given network constructs the best oblivious routing scheme, i.e. the scheme that guarantees the best possible competitive ratio. Unfortunately, the latter result is based on the Ellipsoid algorithm; hence it is unpractical for large networks. In this paper we present a combinatorial algorithm for constructing an oblivious routing scheme that guarantees a competitive ratio of O(log 4 n) for undirected networks. Furthermore, our approach yields a proof for the existence of an oblivious routing scheme with competitive ratio O(log 3 n), which is much simpler than the original proof from [11]. Partially supported by DFG-Sonderforschungsbereich 37

Improved algorithms for dynamic page migration

Abstract The dynamic page migration problem [4] is defined in a distributed network of n mobile nodes sharing one indivisible memory page of size D. During runtime, the nodes can both access a unit of data from the page and move with a constant speed, thus changing the costs of communication. The problem is to compute online a schedule of page movements to minimize the total communication cost. In this paper we construct and analyze the first deterministic algorithm for this problem. We prove that it achieves an (up to a constant factor) optimal competitive ratio O(n · √ D). We show that the randomization of this algorithm improves this ratio to O ( √ D · log n) (against an oblivious adversary). This substantially improves an O(n · √ D) upper bound from [4]. We also give an almost matching lower bound of Ω ( √ D · √ log n) for this problem.