memory management - part 4 (10)

memory management - part 4 (10) - COP 4600 Summer 2011...

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COP 4600: Intro To OS (Memory Management – Part 4) Page 1 © Dr. Mark Llewellyn COP 4600 – Summer 2011 Introduction To Operating Systems Memory Management – Part 4 Department of Electrical Engineering and Computer Science Computer Science Division University of Central Florida Instructor : Dr. Mark Llewellyn [email protected] HEC 236, 407-823-2790 http://www.cs.ucf.edu/courses/cop4600/sum2011
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COP 4600: Intro To OS (Memory Management – Part 4) Page 2 © Dr. Mark Llewellyn Operating System Software Algorithms employed for various aspects of the memory management unit. Fetch Policy Demand Pre-paging Resident Set Management Resident set size Fixed Variable Replacement Scope Global Local Placement Policy Cleaning Policy Demand Pre-cleaning Replacement Policy Basic Algorithms Optimal LRU FIFO Clock Page Buffering Load Control Degree of Multi-programming
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COP 4600: Intro To OS (Memory Management – Part 4) Page 3 © Dr. Mark Llewellyn Design Considerations – More Details The primary issue is one of performance: we need to minimize the rate at which page faults occur due to the overhead involved with handling a page fault. At a minimum the overhead includes deciding which resident page or pages to replace, and the I/O of exchanging the pages. Also the OS must schedule another process to run during the page I/O, causing a process switch. Accordingly, we’d like to arrange matter so that, during the time that a process is executing, the probability of referencing a word on a missing page is minimized. There will be no overall best policy that will cover all occasions.
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COP 4600: Intro To OS (Memory Management – Part 4) Page 4 © Dr. Mark Llewellyn Design Considerations – More Details The task of memory management in a paging environment is fiendishly complex. Furthermore, the performance of any particular set of policies depends on main memory size, the relative speed of main and secondary memory, the size and number of processes competing for resources, and the execution behavior of individual programs. Smaller OS designers should pick a set of policies that will provide “good” behavior over a wide range of conditions. Larger OS should be equipped with monitoring and control tools to tune the OS based on site conditions.
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COP 4600: Intro To OS (Memory Management – Part 4) Page 5 © Dr. Mark Llewellyn Fetch Policy The fetch policy determines when a page should be brought into main memory. The two common alternatives are demand paging and prepaging. With demand paging , a page is brought into memory only when a reference is made to a location on that page. With prepaging , pages other than the one demanded by a page fault are brought into main memory. This technique attempts to take advantage of secondary memory characteristics. For example, if the pages of a process are stored contiguously in secondary memory, then it is more efficient to bring in a number of contiguous pages at one time as opposed to bringing them in one at a time over an extended period.
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COP 4600: Intro To OS
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