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Programming languages that provide multidimensional arrays and a flat linear model of memory must implement a mapping between these two domains to order array elements in memory. This layout function is fixed at language definition time and constitutes an invisible, non-programmable array attribute. In reality, modern memory systems are architecturally hierarchical rather than flat, with substantial differences in performance among different levels of the hierarchy. This mismatch between the model and the true architecture of memory systems can result in low locality of reference and poor performance. Some of this loss in performance can be recovered by re-ordering computations using transformations such as loop tiling. We explore nonlinear array layout functions as an additional means of improving locality of reference. For a benchmark suite composed of dense matrix kernels, we show by timing and simulation that two specific layouts (4D and Morton) have low implementation costs (2-5% of total running time) and high performance benefits (reducing execution time by factors of 1.1-2.5); that they have smooth performance curves, both across a wide range of problem sizes and over representative cache architectures; and that recursion-based control structures may be needed to fully exploit their potential.

TM (transactional memory) is a concurrency control paradigm that provides atomic and isolated execution for regions of code. TM is considered by many researchers to be one of the most promising solutions to address the problem of programming multicore processors. Its most appealing feature is that most programmers only need to reason locally about shared data accesses, mark the code region to be executed transactionally, and let the underlying system ensure the correct concurrent execution. This model promises to provide the scalability of fine-grain locking, while avoiding common pitfalls of lock composition such as deadlock. In this article we explore the performance of a highly optimized STM and observe that the overall performance of TM is significantly worse at low levels of parallelism, which is likely to limit the adoption of this programming paradigm.

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We present a class of solutions for a heat conducting fluid sphere, which radiates energy during collapse without the appearance of horizon at the boundary at any stage of the collapse. A simple model shows that there is no accumulation of energy due to collapse since it radiates out at the same rate as it is being generated.