Operating System Concepts and Techniques Lecture 8

1. 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

2. 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

3. A Brief list of MM policies

4. 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

5. 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

6. 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

7. 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 fragmentationoccurs 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

8. 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=

9. Partition allocation • First-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

10. 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

11. 512M 256M 128M 64M 32M : Processed : Allocated : Available Buddy system tree 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

12. 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

13. 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

14. Any questions?