We describe the software architecture, technical features, and performance of TICK (Transparent Incremental Checkpointer at Kernel level), a system-level checkpointer implemented as a kernel thread, specifically designed to provide fault tolerance in Linux clusters. This implementation, based on the 2.6.11 Linux kernel, provides the essential functionality for transparent, highly responsive, and efficient fault tolerance based on full or incremental checkpointing at system level. TICK is completely user-transparent and does not require any changes to user code or system libraries; it is highly responsive: an interrupt, such as a timer interrupt, can trigger a checkpoint in as little as 2.5µs; and it supports incremental and full checkpoints with minimal overhead-less than 6% with full checkpointing to disk performed as frequently as once per minute.

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Networks of workstations (NOWs) are being considered as a costeffective alternative to parallel computers. Many NOWs are arranged as a switchbased network with irregular topology, which makes routing and deadlock avoidance quite complicated. Current proposals use the up * /down * routing algorithm to remove cyclic dependencies between channels and avoid deadlock. Recently, a simple and effective methodology to compute up * /down * routing tables has been proposed by us. The resulting up * /down * routing scheme makes use of a different link direction assignment to compute routing tables. Assignment of link direction is based on generating an underlying acyclic connected graph from the network graph. In this paper, we propose and evaluate new heuristic rules to compute the underlying graph. Moreover, we propose a traffic balancing algorithm to obtain more efficient up * /down * routing tables when source routing is used. Evaluation results show that the routing algorithm based on the new methodology increases throughput by a factor of up to 2.8 in large networks, also reducing latency significantly.

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Los Alamos National Laboratory, an affirmative actionlequal opportunity employer, is operated by the University of California for the US. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the US. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for US. Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the US. Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researcher's right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness.

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