31,846 research outputs found
Review of the school workload advisory panel: final report
The School Workload Advisory Panel (SWAP) was established as an independent body in 2004 by the Welsh Assembly Government with a remit that was aimed at reducing bureaucratic burdens on schools. This paper includes the review’s main recommendations, followed by a more detailed account of the review findings and recommendations
Large Distance Continuous Variables Communication with Concatenated Swaps
The radiation-pressure interaction between electromagnetic fields and
mechanical resonators can be used to efficiently entangle two light fields
which couple to a single mechanical mode. We analyze the performance of this
process under realistic optomechanical conditions, and we determine the
effectiveness of the resulting entanglement as a resource for quantum
teleportation of continuous-variable light signals, over large distances,
mediated by concatenated swap operations. We study the sensitiveness of the
protocol to the quality factor of the mechanical systems, and its performance
in non-ideal situations in which losses and reduced detection efficiencies are
taken into account.Comment: 13 pages, 10 figure
Wait-Free Global Virtual Time Computation in Shared Memory Time-Warp Systems
Global Virtual Time (GVT) is a powerful abstraction used to discriminate what events belong (and what do not belong) to the past history of a parallel/distributed computation. For high performance simulation systems based on the Time Warp synchronization protocol, where concurrent simulation objects are allowed to process their events speculatively and causal consistency is achieved via rollback/recovery techniques, GVT is used to determine which portion of the simulation can be considered as committed. Hence it is the base for actuating memory recovery (e.g. of obsolete logs that were taken in order to support state recoverability) and nonrevocable operations (e.g. I/O). For shared memory implementations of simulation platforms based on the Time Warp protocol, the reference GVT algorithm is the one presented by Fujimoto and Hybinette [1]. However, this algorithm relies on critical sections that make it non-wait-free, and which can hamper scalability. In this article we present a waitfree shared memory GVT algorithm that requires no critical section. Rather, correct coordination across the processes while computing the GVT value is achieved via memory atomic operations, namely compare-and-swap. The price paid by our proposal is an increase in the number of GVT computation phases, as opposed to the single phase required by the proposal in [1]. However, as we show via the results of an experimental study, the wait-free nature of the phases carried out in our GVT algorithm pays-off in reducing the actual cost incurred by the proposal in [1]
The Lock-free -LSM Relaxed Priority Queue
Priority queues are data structures which store keys in an ordered fashion to
allow efficient access to the minimal (maximal) key. Priority queues are
essential for many applications, e.g., Dijkstra's single-source shortest path
algorithm, branch-and-bound algorithms, and prioritized schedulers.
Efficient multiprocessor computing requires implementations of basic data
structures that can be used concurrently and scale to large numbers of threads
and cores. Lock-free data structures promise superior scalability by avoiding
blocking synchronization primitives, but the \emph{delete-min} operation is an
inherent scalability bottleneck in concurrent priority queues. Recent work has
focused on alleviating this obstacle either by batching operations, or by
relaxing the requirements to the \emph{delete-min} operation.
We present a new, lock-free priority queue that relaxes the \emph{delete-min}
operation so that it is allowed to delete \emph{any} of the smallest
keys, where is a runtime configurable parameter. Additionally, the
behavior is identical to a non-relaxed priority queue for items added and
removed by the same thread. The priority queue is built from a logarithmic
number of sorted arrays in a way similar to log-structured merge-trees. We
experimentally compare our priority queue to recent state-of-the-art lock-free
priority queues, both with relaxed and non-relaxed semantics, showing high
performance and good scalability of our approach.Comment: Short version as ACM PPoPP'15 poste
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