Operating System Concepts and Techniques Lecture 8 Memory Management-1 M. Naghibzadeh Reference M. Naghibzadeh, Operating System Concepts and Techniques, First ed., iUniverse Inc., 2011. To order: www.iUniverse.com, www.barnesandnoble.com, or www.amazon.com

Memory Management (MM) Memory management policies and techniques have tremendously changed throughout the years The simplest being single contiguous partition memory management The most complex being multilevel-page table virtual memory with cache A Brief list follows

A Brief list of MM policies Old policies Single contiguous partition MM Static partition MM Dynamic partition MM Multiple partition MM Middle age policies Segmentation MM Relocatable partition MM Page (non-virtual memory) MM Virtual memory policies Page-based virtual memory MM Multilevel page table page-based virtual memory MM

Single contiguous partition MM MM is divided into two parts One part for operating system The other is called user memory So simple that there is no MM subsystem, loader takes care of it Make sure user program does not destroy OS Many disadvantages Supports only single-programming System utilization is very low Waste of memory for small programs A program larger than the user’s main memory will not be able to run

Static Partition MM More than one user partition with fixed sizes Sizes are different to reduce memory waste and accept larger programs Make sure user programs do not interfere and do not destroy OS Number of partitions determines degree of multiprogramming Advantages Simple to implement; a table for partitions information Multiprogramming is possible the size of runable programs limited to the largest partition size Memory waste A program larger than users main memory will not be able to run

Dynamic Partition MM More than one user partition with variable sizes Sizes are different to reduce memory waste and accept larger programs Make sure user programs do not interfere and do not destroy OS Number of partitions determines the degree of multiprogramming At start there is only one big free partition When new programs are accepted new partitions are created Upon program completion free neighbor partitions can join Advantages Simple to implement; two tables, free and allocated partitions Multiprogramming is possible The size of a runable programs limited to the size of user memory A program larger than users main memory will not be able to run

Dynamic Partition Memory waste There is a smallest memory allocation unit, usually 1K bytes A program which is 1025 bytes will be given 2K bytes, 1023 bytes are wasted in the form of internal fragmentation We are not allowed to relocate programs External fragmentation occurs when all free memory partitions combined are large enough to load the coming program into, but these spaces do not form a contiguous partition and each one cannot accept the coming program

Dynamic Partition Memory waste… The 50% rule: Without other information, on the average, the number of free partitions is half the number of allocated partitions Remember: adjacent free partitions join, but adjacent occupied partitions house different programs Let C = (average free partition size)/(average program size), then external fragmentation fraction=

Partition allocation First-Fit Next-Fit Best-Fit Worst-Fit Look at the free partitions data structure (not the main memory itself) and select the first which is as large as the program Next-Fit From partitions data structure, from where the pointer points, select the first which is as large as the program Best-Fit pick a qualified partition whose size is closest to the size of the program Worst-Fit Pick the largest free partition

Partition allocation- Buddy syatem The size of a partition is 2i*m A partition size of 2i*K could split into two size 2i-1*K which are called buddies Remember that any two partition of size 2i*m are not buddies even if they are adjacent A complete partition is given to a program Remember partitions cannot start from any arbitrary address A program the size of xK is given a partition of size 2i*K, where 2i  x and i is the smallest such integer Any two free buddies have to merge

Buddy system tree 512M 256M 128M 64M 32M : Processed : Allocated : Available Figure shows certain state of a small 512 Mega Bytes (MB) main memory. A black circle represents a partition that has been broken and no longer exists. Dark circles represent a partition that is occupied by a program. An unfilled circle represents a free (available) partition Advantage: easy to find a partition for a program Disadvantage: Internal fragmentation increases

Summary There are variety of memory management The simplest, single contiguous memory management The most complex, multilevel page table page-based virtual memory with segmentation and cache memory Older managements are good for special purpose computer such as embedded control systems and intelligent machinery electronics This lecture introduced static and dynamic partition memory managements Base register is a central concept to dynamic partition MM

Find out What the degree of multiprogramming is in your computer Practical systems which use single partition MM If we can have a buddy system in which buddies are not the same size In buddy system, why can’t any two adjacent partitions the size of 2i*K, join to form a partition the size of 2i+1*K In what situations worst-fit will perform better than other partition selection algorithms

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