This paper demonstrates a practical approach to the study of the failure behavior of computer systems. Particular attention is devoted to the analysis of permanent failures. A number of important techniques, which may have general applicability in both failure and workload analysis, are brought together in this presentation. These include: smeared averaging of the workload data, clustering of like failures, and joint analysis of workload and failures. Approximately 17 percent of all failures affecting the CPU were estimated to be permanent. The manifestation of a permanent failure was found to be strongly correlated with the level and type of workload prior to the failure. Although, in strict terms, the results only relate to the manifestation of permanent failures and not to their occurrence, there are strong indications that permanent failures are both caused and discovered by increased activity. More measurements and experiments are necessary to determine their respective contributions to the measured workload/failure relationship.
Software-based active replication is expensive in terms of performance overhead. Multithreading can help improve performance; however, thread scheduling is a source of nondeterminism in replica behavior. This paper presents a Preemptive Deterministic Scheduling (PDS) algorithm for ensuring deterministic replica behavior while preserving concurrency. Threads are synchronized only on updates to the shared state. A replica execution is broken into a sequence of rounds, and in a round each thread can acquire up to two mutexes. When a new round fires, all threads' mutex requests are known; thus, it is possible to form a deterministic scheduling of mutex acquisitions in the round. No inter-replica communication is required. The algorithm is formally specified, and the proposed formalism is used to prove its correctness.Failure behavior and performance of a PDS algorithm's implementation are evaluated in a triplicated system and compared with two existing solutions: nonpreemptive deterministic schedulers and the Loose Synchronization Algorithm (LSA) proposed by the authors in an earlier paper. The results show that PDS outperforms nonpreemptive deterministic schedulers. Compared with LSA, PDS has lower throughput; however, it provides additional benefits in terms of system dependability and, hence, can be considered as a trade-off between performance and dependability. These characteristics are investigated with fault injection.
The wide area cellular networks that offer voice and data services to the end users represent some of the most widely deployed and used networks of today. In the USA, these networks use 825. MHz cellular bands or 1.85-1.91 GHz, 1.930-1.99 GHz frequency bands that are licensed to service providers on a long-term basis. As new technologies such as multi-carrier EV-DO [10], HSDPA [4] show promise of broadband wide area wireless access, increased amount of spectrum will be needed.Abstract-Majority of research in Dynamic Spectrum Access (DSA) networks has focused on free-for-all, opportunistic spectrum access for peer-to-peer ad-hoc communication, typically targeting military applications. In this paper, we propose that a simple form of DSA called Coordinated DSA that relies on a regional spectrum broker to control spectrum access can bring benefits to current statically provisioned cellular networks. We argue that two concepts, namely a dynamically sharable spectrum band called Coordinated Access Band (CAB) band and Statistically Multiplexed Access (SMA) to spectrum that relies on demand aggregation can achieve better spectrum utilization. We describe in detail our spectrum measurements in existing cellular networks to conclusively demonstrate the feasibility of these concepts.
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