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Summary of Page Replacement Algorithms

We have now looked at a variety of page replacement algorithms. In this section we will briefly summarize them. The list of algorithms discussed is given in Fig. 4-8.

Figure 4-8. Page replacement algorithms discussed in the text.

The optimal algorithm replaces the page referenced last among the current pages. Unfortunately, there is no way to determine which page will be last, so in practice this algorithm cannot be used. It is useful as a benchmark against which other algorithms can be measured, however.

The NRU algorithm divides pages into four classes depending on the state of the R and M bits. A random page from the lowest numbered class is chosen. This algorithm is easy to implement, but it is very crude. Better ones exist.

FIFO keeps track of the order pages were loaded into memory by keeping them in a linked list. Removing the oldest page then becomes trivial, but that page might still be in use, so FIFO is a bad choice.

Second chance is a modification to FIFO that checks if a page is in use before removing it. If it is, the page is spared. This modification greatly improves the performance. Clock is simply a different implementation of second chance. It has the same performance properties, but takes a little less time to execute the algorithm.

LRU is an excellent algorithm, but it cannot be implemented without special hardware. If this hardware is not available, it cannot be used. NFU is a crude attempt to approximate LRU. It is not very good. However, aging is a much better approximation to LRU and can be implemented efficiently. It is a good choice.

The last two algorithms use the working set. The working set algorithm is reasonable performance, but it is somewhat expensive to implement. WSClock is a variant that not only gives good performance but is also efficient to implement.

All in all, the two best algorithms are aging and WSClock. They are based on LRU and the working set, respectively. Both give good paging performance and can be implemented efficiently. A few other algorithms exist, but these two are probably the most important in practice.

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