13-Main_Memory_2spp

13-Main_Memory_2spp - CSE 421/521 Operating Systems Fall...

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1 CSE 421/521 - Operating Systems Fall 2011 Tevfik Ko ş ar University at Buffalo October 13 th , 2011 Lecture - XIII Main Memory Management - II 2 Roadmap • Main Memory Management • Fragmentation • Address Binding • HW Address Protection • Paging
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30 Dynamic Storage-Allocation Problem First-fit : Allocate the first hole that is big enough Best-fit : Allocate the smallest hole that is big enough; must search entire list, unless ordered by size. Produces the smallest leftover hole. Worst-fit : Allocate the largest hole; must also search entire list. Produces the largest leftover hole. How to satisfy a request of size n from a list of free holes First-±t is faster. Best-±t is better in terms of storage utilization. Worst-±t may lead less fragmentation. 32 Fragmentation External Fragmentation – total memory space exists to satisfy a request, but it is not contiguous (in average ~50% lost) Internal Fragmentation – allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used Reduce external fragmentation by compaction Shuffle memory contents to place all free memory
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13-Main_Memory_2spp - CSE 421/521 Operating Systems Fall...

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