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Operating Systems

by: Mrs. Carolyne Abbott

Operating Systems CS 4414

Mrs. Carolyne Abbott
GPA 3.71

Sang Son

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Sang Son
Class Notes
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This 3 page Class Notes was uploaded by Mrs. Carolyne Abbott on Monday September 21, 2015. The Class Notes belongs to CS 4414 at University of Virginia taught by Sang Son in Fall. Since its upload, it has received 9 views. For similar materials see /class/209669/cs-4414-university-of-virginia in ComputerScienence at University of Virginia.

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Date Created: 09/21/15
CS 414 Operating Systems UNIVERSITY OF VIRGINIA Department of Computer Science Spring 2008 Topic 13 Sharing Main Memory Paging a Readings for this topic Ch8 84 amp 85 a Paging goal is to make allocation and swapping easier 0 Make all chunks of memory the same size call them pages Typical sizes range from 5128k bytes 0 For each process a page table de nes the base address of each of that process pages along with readonly and existence bits Translation process page number always comes directly from the address Since page size is a power of two no comparison or addition is necessary Just do table lookup and bit substitution 0 Easy to allocate keep a free list of available pages and grab the rst one Easy to swap since everything is the same size which is usually the same size as disk blocks to and from which pages are swapped 0 Problems ofpaging 0 Internal fragmentation The larger the page the worse this is a Ef ciency of access even small page tables are generally too large to load into fast memory in the relocation box Instead page tables are kept in main memory and the re location box only has the page table s base address It thus takes one overhead reference for every real memory reference a Table space Page tables are big How big Consider a 32bit addresss space with 4k pages as in Windows XP 71317 a Paging with segmentation use two levels of mapping to make tables manageable a Each segment contains one or more pages a Segments correspond to logical units code data stack Segments vary in size and are often large Pages are for the use of the OS they are xedsize to make it easy to manage memory 0 Going from paging to PS is like going from single segment to multiple segments ex cept at a higher level Instead of having a single page table have many page tables with a base and bound for each Call the stuff associated with each page table a segment a We can share at two levels single page or single segment whole page table Pages eliminate external fragmentation and make it possible for segments to grow without any reshuf ing a If page size is small compared to most segments then internal fragmentation is not too bad a The user is not given access to the paging tables Problem with segmentation and paging extra memory references to access translation tables can slow programs down by a factor of two or three Too many entries in translation tables to keep them all loaded in fast processor memory Page table structures a Multilevel hierarchical paging paging the page table a For big address space twolevel paging may not be enough a Hashed page tables page number is the virtual address is hashed into the hash table a Each entry in the hash table contains linked list of elements consisting of l virtu al page number 2 physical page number 3 pointer to the next element in the link list a Inverted page tables page table contains one netry for each physical page frame each entry consists of process id and virtual page number 0 Logical address contains the process id virtual page number and offset 71327 The notion of locality at any given time a process is only using a few pages or segments Idea Translation Lookaside Buffer TLB A translation buffer is used to store a few of the translation table entries It s very fast but only remembers a small number of entries On each memory reference a First ask TB if it knows about the page If so the reference proceeds fast a If TB has no info for page must go through page and segment tables to get info Refer ence takes a long time but give the info for this page to TB so it will know it for next reference TB must forget one of its current entries in order to record new one TB Organization Virtual page number goes in physical page location comes out Similar to a cache usually directmapped TB is just a memory with some comparators Typical size of memory 2k entries Each entry holds a virtual page number and the corresponding physical page number How can memory be organized to nd an entry quickly a One possibility search whole table from start on every reference a A better possibility restrict the info for any given virtual page to fall in exactly one lo cation in the memory Then only need to check that one location Eg use the low order bits of the virtual page number as the index into the memory This is the way real TB s work Why loworder instead of highorder bits Disadvantage of TB scheme if two pages use the same entry of the memory only one of them can be remembered at once If process is referencing both pages at same time TB doesn t work very well Example TB with 64 entries Suppose the following virtual pages are referenced octal 621 2145 621 2145 321 2145 321 621 In practice TB s have been extremely successful 98 hit ratio is typical for 128 entries 7 133 i


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