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IMPLEMENTATION ISSUES
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IMPLEMENTATION ISSUES

Implementers of virtual memory systems have to make selections among the major theoretical algorithms, such as second chance versus aging, local versus global page allocation, and demand paging versus prepaging. But they also have to be aware of a number of practical

Cleaning Policy / Virtual Memory Interface
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Cleaning Policy / Virtual Memory Interface

Paging works best when there are lots of free page frames that can be claimed as page faults take place. If every page frame is full, and moreover customized, before a new page can be brought in, an old page must first be written to disk. To make sure a plentiful supply of free

Shared Libraries / Mapped Files
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Shared Libraries / Mapped Files

Sharing can be executed at other granularities than individual pages. If a program is started up twice, most operating systems will automatically share all the text pages so that only one copy is in memory. Text pages are always read only, so there is no problem here.

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

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

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

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

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.

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

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