系统结构-ch5-2013.ppt

本章要点 5.1 存储系统的原理 5.1.1 概念 层次化存储系统的基本特征 5.1.2? 存储系统的一般组成 5.1.3 Design principle of Hierarchy Memory System Basic Principles: locality and cost/ performance of memory technology The principle of locality: Programs tend to reuse data and instructions they have used recently. A rule of thumb: a program spends 80% of its execution time in only 20% of the code. Locality An implication of locality is that we can predict with reasonable accuracy what instructions and data a program will use in the near future based on its accesses in the recent past. Temporal locality——Recently accessed items are likely to be accessed in the near future. Spatial locality——Items whose addresses are near one another tend to be referenced close together in time. Design resolution The principle of locality + The smaller Memory is faster led to the hierarchy based on memories of different speeds and sizes. The importance of memory hierarchy has increased with advances in performance of processors. Typical Levels in a Hierarchical Memory The Need for a Memory Hierarchy The levels of the hierarchy usually subset one another; all data in one level is also found in the level below, and all data in that lower level is found in the one below it, and so on until we reach the bottom of the hierarchy. Each level maps addresses from a larger memory to a smaller but faster memory higher in the hierarchy. At the same time of address mapping, address checking and protection schemes for scrutinizing address are taken place too. 18.2 What Makes a Cache Work? Desktop, Drawer, and File Cabinet Analogy Temporal and Spatial Localities 5.1.5? 存储系统性能的三个重要参数 ——容量S、速度T以及价格C Effectiveness of the memory hierarchy CPU execution time = instr. Count ? CPI ? CCtime = (CPU clock cycles + memory stall cycles ) ? CCtime Memory stall cycles = Instr. count ? Memory references per instr. ? Miss rate ? miss penalty Miss rate = cache miss count / total Cache access times Miss penalty : Cost of one cache