MEMORY MANAGEMENT

The Not Recently Used Page Replacement Algorithm
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The Not Recently Used Page Replacement Algorithm

In order to allow the operating system to collect useful page usage statistics, most computers with virtual memory have two status bits associated with each page. R is set whenever the page is referenced (read or written). M is set when the page is written to (i.e., modified).

The Least Recently Used (LRU) Page Replacement Algorithm
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The Least Recently Used (LRU) Page Replacement Algorithm

A good estimation to the best algorithm is based on the observation that pages that have been heavily used in the last few instructions will possibly be heavily used again in the next few. On the other hand, pages that have not been used for ages will possibly remain unused for a

Simulating LRU in Software
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Simulating LRU in Software

Although both of the previous LRU algorithms are (in principle) realizable, few, if any, machines have the required hardware. Instead, a solution that can be implemented in software is required One possibility is called the NFU (Not Frequently Used) algorithm. It requires a

The Working Set Page Replacement Algorithm
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The Working Set Page Replacement Algorithm

In the purest form of paging, processes are started up with none of their pages in memory. As soon as the CPU tries to get the first instruction, it gets a page fault, causing the operating system to bring in the page containing the first instruction. Other page faults for global

The WSClock Page Replacement Algorithm
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The WSClock Page Replacement Algorithm

The main working set algorithm is unwieldy, since the entire page table has to be scanned at each page fault until a suitable candidate is located. An improved algorithm, that is based on the clock algorithm but also uses the working set information, is called WSClock (Carr and

Summary of Page Replacement Algorithms
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Summary of Page Replacement Algorithms

We have now considered a variety of page replacement algorithms. In this section we will briefly summarize them. The list of algorithms discussed is given in Figure 1.

DESIGN ISSUES FOR PAGING SYSTEMS
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DESIGN ISSUES FOR PAGING SYSTEMS

In the preceding sections we have described in detail how paging works and have given a few of the fundamental page replacement algorithms and shown how to model them. But knowing the bare mechanics is not sufficient To design a system, you have to know a lot more

Load Control
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Load Control

Even with the best page replacement algorithm and optimal global allocation of page frames to processes, it can come about that the system thrashes. Actually, whenever the combined working sets of all processes exceed the capacity of memory, thrashing can be expected. One

Page Size
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Page Size

The page size is sometimes a parameter that can be chosen by the operating system. Even if the hardware has been designed with, for instance, 512-byte pages, the operating system can easily regard page pairs 0 and 1, 2 and 3, 4 and 5, and so on, as 1-KB pages by always

Separate Instruction and Data Spaces / Shared Pages
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Separate Instruction and Data Spaces / Shared Pages

The majority of computers have a single address space that holds both programs and data, as shown in Figure 1(a). If this address space is large enough, everything works fine. On the other hand, it is often too small, forcing programmers to stand on their heads to fit everything

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