Chapter 7 Memory Management Basic requirements of Memory

Chapter 7 Memory Management • Basic requirements of Memory Management • Memory Partitioning • Basic blocks of memory management – Paging – Segmentation 1

The need for memory management • Memory is cheap today, and getting cheaper – But applications are demanding more and more memory, there is never enough! • Memory Management involves swapping blocks of data from secondary storage. • Memory I/O is slow compared to a CPU – The OS must cleverly time the swapping to maximise the CPU’s efficiency 2

Memory Management A program must be loaded into main memory to be executed. Memory needs to be allocated to ensure a reasonable supply of ready processes to consume available processor time – Otherwise, for much of the time all of the processes will be waiting for I/O and the processor will be idle. 3

Memory Management Requirements • Relocation • Protection • Sharing • Logical organisation • Physical organisation 4

Requirements: Relocation • The programmer does not know where the program will be placed in memory when it is executed, – it may be swapped to disk and return to main memory at a different location (relocated) • But, OS knows because it is managing memory and is responsible for bringing this process into main memory 5

Addressing The processor and OS must be able to translate the memory references found in the code of the program into actual physical memory addresses (to be discussed) 6

Requirements: Protection • Processes should not be able to reference memory locations in another process without permission • Impossible to check absolute addresses at compile time • Must be checked at run time 7

Requirements: Sharing • Allow several processes to access the same portion of memory – Better to allow each process executing the same program access to the same copy of the program rather than have their own separate copy – Processes that are cooperating on some task may need to share access to the same data structure 8

Requirements: Logical Organization • Memory is organized linearly (usually) • In contrast, programs are organized into modules – Modules can be written and compiled independently – Different degrees of protection can be given to different modules (read-only, execute-only) – Modules can be shared among processes • Segmentation helps here 9

Requirements: Physical Organization • Cannot leave the programmer with the responsibility to manage memory – Memory available for a program plus its data may be insufficient – Programmer does not know how much space will be available • The task of moving information between the two levels of memory should be a system responsibility 10

Partitioning • An early method of managing memory – Pre-virtual memory – Not used much now • But, it will clarify the later discussion of virtual memory if we look first at partitioning – Virtual Memory has evolved from the partitioning methods 11

Types of Partitioning • • • Fixed Partitioning Dynamic Partitioning Simple Paging Simple Segmentation Virtual Memory Paging Virtual Memory Segmentation 12

Fixed Partitioning • Equal-size partitions (see fig 7. 3 a) – Any process whose size is less than or equal to the partition size can be loaded into an available partition • OS can swap a process out of a partition – If none are in a ready or running state 13

Fixed Partitioning Problems • A program may be too big to fit into a partition • Main memory use is inefficient. – Any program, no matter how small, occupies an entire partition. – This is results in internal fragmentation. 14

Solution – Unequal Size Partitions • Lessen both problems – But doesn’t solve completely – Larger programs can be accommodated – Smaller programs can be placed in smaller partitions, reducing internal fragmentation 15

Placement Algorithm • Equal-size – Placement is trivial (no options) • Unequal-size – Can assign each process to the smallest partition within which it will fit – Queue for each partition – Processes are assigned in such a way as to minimize wasted memory within a partition 16

Fixed Partitioning It is possible that a partition is unused even though some smaller process could have been assigned to it Select the smallest available partition that will hold the process 17

Remaining Problems with Fixed Partitions • The number of active processes is limited by the system – i. e. , limited by the pre-determined number of partitions • Partition sizes are preset at system generation time, small jobs will not use space efficiently 18

Dynamic Partitioning • Dynamic partitioning can overcome some of the difficulties with fixed partitioning • Partitions are of variable length and number • Process is allocated exactly as much memory as required 19

Dynamic Partitioning Example OS (8 M) P 2 P 1 (14 M) (20 M) Empty (6 M) Empty P 4(8 M) P 2 (56 M) (14 M) Empty (6 M) P 3 (18 M) • External Fragmentation – Memory external to all processes is fragmented • Can resolve using compaction – OS moves processes so that they are contiguous – Time consuming and wastes CPU time Empty (4 M) Refer to Figure 7. 4 20

Dynamic Partitioning • Operating system must decide which free block to allocate to a process • Best-fit algorithm – Chooses the block that is closest in size to the request – Worst performer overall • Since smallest block is found for a process, the fragment left behind is too small to satisfy other requests • Memory compaction must be done more often 21

Dynamic Partitioning • First-fit algorithm – Scans memory from the beginning and chooses the first available block that is large enough – Simplest and fastest – May have many process loaded in the front end of memory that must be searched over when trying to find a free block 22

Dynamic Partitioning • Next-fit – Scans memory from the location of the last placement – More often allocate a free block at the end of memory where the largest block is found • The largest block of free memory is quickly broken up into smaller blocks • Compaction is required more frequently 23

Allocation 24

Buddy System • A reasonable compromise to overcome the disadvantages of both the fixed and variable partitioning schemes • Entire space available is treated as a single block of 2 U • If a request of size s where 2 U-1 < s 2 U – entire block is allocated • Otherwise block is split into two equal buddies – Process continues until the smallest block greater than or equal to s is generated 25

Example of Buddy System 26

Tree Representation of Buddy System A binary tree representation of the buddy allocation immediately after the Release B request The leaf nodes represent the current partitioning the memory 27

Relocation • The actual (absolute) memory locations are determined when program are loaded into memory • A process may occupy different partitions which means different absolute memory locations during execution – Swapping – Compaction 28

Addresses • Logical – Reference to a memory location independent of the current assignment of data to memory • Relative – Address expressed as a location relative to some known point (typically the program origin) • Physical or Absolute – The absolute address or actual location in main memory A translation must be made to a physical address before memory access can be achieved 29

Hardware Support for Relocation 1. Add the value in the base register to the relative address to produce an absolute address 0. Initialize base and bounds registers when a process is assigned to the Running state 2. Compare the resulting address to the value in the bounds register 30

Registers Used during Execution • Base register – Starting address for the process • Bounds register – Ending location of the process • These values are set when the process is loaded or when the process is swapped in 31

Registers Used during Execution • The value of the base register is added to a relative address to produce an absolute address • The resulting address is compared with the value in the bounds register • If the address is not within bounds, an interrupt is generated to the operating system 32

Paging • Partition memory into small equal fixedsize chunks (frames) and divide each process into the same size chunks (pages) • Pages of process could be assigned to available frames of memory • No external fragmentation but little internal fragmentation consisting of only a fraction of the last page of a process 33

Processes and Frames A. 0 A. 1 A. 2 A. 3 D. 0 B. 0 D. 1 B. 1 D. 2 B. 2 C. 0 C. 1 C. 2 C. 3 D. 4 OS finds free frames and loads the pages of Process A, B & C Process B is suspended and is swapped out of main memory All of the processes in main memory are blocked, and OS brings in Process D 34

Paging • Operating system maintains a page table for each process which contains the frame location for each page in the process • Given a logical address (page number, offset), the processor uses the page table to produce a physical address 35

Page Table page number frame number 36

Logical Addresses • Using a page size that is a power of 2, a logical address (page no. , offset) is identical to its relative address • Example • • • 16 -bit address 210 =1024 -byte page 10 -bit offset 6 -bit page number A maximum of 26 =64 pages 37

Paging Logical to Physical Address Translation Consider an address of n+m bits, where the leftmost n bits are the page no. and the rightmost m bits are the offset 1. Extract the page no. 2. Use the page no. as an index into the process page table to find the frame number, k 3. The physical address is constructed by appending the offset to k 38

Segmentation • A program can be subdivided into segments – Segments may vary in length – There is a maximum segment length • Segmentation is similar to dynamic partitioning – But, a program may occupy more than one partition • No internal fragmentation but suffers from external fragmentation (as does dynamic partitioning) 39

Segmentation • A logical address consists of two parts – a segment number and – an offset • There is a segment table for each process and a list of free blocks of main memory. • Each segment table entry would have to give – the starting address in main memory of the corresponding segment. – the length of the segment, to assure that invalid addresses are not used. 40

Logical Addresses • There is no simple relationship between a logical address (segment no. , offset) and the physical address • Example • • 16 -bit address 12 -bit offset 4 -bit segment number maximum segment size= 212=4096 41

Segmentation Logical to Physical Address Translation Consider an address of n + m bits, where the leftmost n bits are the segment no. and the rightmost m bits are the offset 1. Extract the segment no. 2. Use the segment no. as an index into the process segment table to find the starting physical address of the segment 3. If the offset the length of the segment, the address is invalid 4. The physical address is the sum of the starting physical address of the segment plus the offset 42