We consider several basic communication problems in a hypercube network of processors. These include the problem of simultaneous broadcast of the same packet from every processor to all other processors and the problem of simultaneous exchange of different packets between every pair of processors. The algorithms proposed for these problems are optimal in terms of execution time and communication resource requirements, that is, they require the minimum possible number of time steps and packet transmissions. This is a particularly strong form of optimality, which has not been considered in earlier investigations.

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Peer-to-peer are popular environments for exchanging services. A reputation mechanism is a proper means of discovering low-performing peers that fail to provide their services. In this paper, we present an in-depth and innovative study of how reputation can be exploited so that the right incentives for high performance are provided to peers. Such incentives do not arise if peers exploit reputation only when selecting the best providing peer; this approach may lead high-performing peers to receive unfairly low value from the system. We argue and justify experimentally that the calculation of reputation values has to be complemented by proper reputation-based policies that determine the pairs of peers eligible to interact with each other. We introduce two different dimensions of reputation-based policies, namely "provider selection" and "contention resolution", as well as specific policies for each dimension. We perform extensive comparative assessment of a wide variety of policy pairs and identify the most effective ones by means of simulations of dynamically varying peer-to-peer environments. We show that both dimensions have considerable impact on both the incentives for peers and the efficiency attained. In particular, when peers follow fixed strategies, certain policy pairs differentiate the value received by different types of peers in accordance to the value offered to the system per peer of each type. Moreover, when peers follow dynamic strategies, incentive compatibility applies under certain pairs of reputation-based policies: each peer is provided with the incentive to improve her performance in order to receive a higher value. Finally, we show experimentally that reputation values can be computed quickly and accurately by aggregating only a small randomly selected subset of the ratings' feedback provided by the peers, thus reducing the associated communications' overhead.

Abstract. This paper discusses the rationales for a Grid market and, in particular, the introduction of a market place for trading commoditized computing resources. The market place proposed makes computing resources from different providers substitutable through virtualization. This includes the definition of a spot and future market as well as the parameters that a market mechanism for computing resources should consider. The above market place is complemented by a set of value-added services (e.g. insurance against resource failures, capacity planning, resource quality assurance, stable price offering) that ensure quality for Grid users over time. The market place technology for all of the above services has been designed by the GridEcon project, contributing to a broader adoption of Grid technology and enabling a service-oriented knowledge utility environment.