A Practical Introduction to Data Structures and Algorithm Analysis

292 Chap. 8 File Processing and External Sortingreading blocks stored on slower, secondary memory (such as on the disk drive).The disk stores the complete contents of the virtual memory. Blocks are read intomain memory as demanded by memory accesses. Naturally, programs using virtualmemory techniques are slower than programs whose data are stored completely inmain memory. The advantage is reduced programmer effort because a good virtualmemory system provides the appearance of larger main memory without modifyingthe program.Example 8.2 Consider a virtual memory whose size is ten sectors, andwhich has a buffer pool of five buffers associated with it. We will use a LRUreplacement scheme. The following series of memory requests occurs.9017668135171After the first five requests, the buffer pool will store the sectors in the order6, 7, 1, 0, 9. Because Sector 6 is already at the front, the next request can beanswered without reading new data from disk or reordering the buffers. Therequest to Sector 8 requires emptying the contents of the least recently usedbuffer, which contains Sector 9. The request to Sector 1 brings the bufferholding Sector 1’s contents back to the front. Processing the remainingrequests results in the buffer pool as shown in Figure 8.5.Example 8.3 Figure 8.5 illustrates a buffer pool of five blocks mediatinga virtual memory of ten blocks. At any given moment, up to five sectors ofinformation can be in main memory. Assume that Sectors 1, 7, 5, 3, and 8are currently in the buffer pool, stored in this order, and that we use theLRU buffer replacement strategy. If a request for Sector 9 is then received,then one sector currently in the buffer pool must be replaced. Because thebuffer containing Sector 8 is the least recently used buffer, its contents willbe copied back to disk at Sector 8. The contents of Sector 9 are then copiedinto this buffer, and it is moved to the front of the buffer pool (leaving thebuffer containing Sector 3 as the new least-recently used buffer). If the nextmemory request were to Sector 5, no data would need to be read from disk.Instead, the buffer containing Sector 5 would be moved to the front of thebuffer pool.When implementing buffer pools, there are two basic approaches that can betaken regarding the transfer of information between the user of the buffer pool and