Lecture Operating system principles - Chapter 7: Memory management

After studying this chapter, you should be able to: Discuss the principal requirements for memory management, understand the reason for memory partitioning and explain the various techniques that are used, understand and explain the concept of paging,...

Chapter 7 Memory Management

• Basic requirements of Memory

Management

• Memory Partitioning

• Paging

• Segmentation

Memory Management

The principal operation of memory

management is to bring processes into

main memory for execution by the

processor.

A program must be loaded into main memory to be executed.

Memory Management

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

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 CPU

– The OS must cleverly time the swapping to

maximise the CPU’s efficiency

• But, OS knows because it is managing

memory and is responsible for bringing

this process into main memory

Relocation

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

program in main memory is unpredictable.

• Must be checked at run time by the

processor.

• 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

Logical Organization

• Memory is organized linearly (usually)

• In contrast, programs are organized into

– Modules can be shared among processes

• Segmentation helps here

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 or where the space will be

• The task of moving information between

different levels of memory should be a

system responsibility

• 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

• Virtual Memory Paging

• Virtual Memory Segmentation

Fixed Partitioning

• Partition memory into regions with

fixed boundaries

• Equal-size partitions

– Any process whose size is less than

or equal to the partition size can be

loaded into an available partition

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

(wasted space internal to a partition)

Fixed Partitioning

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

Fixed Partitioning

Placement Algorithm

• Equal-size

– Placement is trivial: a process can be loaded

into any available partition

• 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

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

Fixed Partitioning

Placement Algorithm

• 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

Fixed Partitioning

Remaining Problems

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

• Can resolve using

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

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 such that small free partitions must be searched over on each subsequent

pass

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

Dynamic Partitioning

Allocation

Buddy System

• A reasonable compromise to overcome the

disadvantages of both the fixed and dynamic

partitioning schemes

• Entire space available is treated as a single

block of 2U

• 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

Buddy System

Example

Buddy System

Tree Representation

A binary tree representation of the buddy allocation immediately after the Release B request

The leaf nodes represent the current

partitioning the memory

• The actual (absolute) memory locations

are determined when program is loaded

into memory

• A process may occupy different partitions

which means different absolute memory

locations during execution

– Swapping

– Compaction

• Logical

– Reference to a memory location independent of the

current assignment of data to memory

– A relative address is expressed as a location relative

to some known point (typically the program origin)

to produce an absolute address

2 Compare the resulting address

to the value in the bounds register

– Ending location of the process

• These values are set when the process is loaded or when the process is swapped in

• 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 within bounds, instruction

execution may proceed

– Otherwise, an interrupt indicating error is

generated to OS

Relocation

Registers Used during Execution

• Partition memory into small equal

fixed-size 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

Paging

Processes and Frames

A.0 A.1 A.2 A.3 B.0 B.1 B.2 C.0 C.1 C.2 C.3

D.0 D.1 D.2

D.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

• 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

Paging

Page Table

page number

frame number

Paging

Logical Addresses

• Using a page size that

is a power of 2, a

logical address (page

no., offset) is identical

to its relative address

number, k 3 The physical address is

Consider an address of n+m bits, where the leftmost n bits

are the page no and the

rightmost m bits are the offset

• 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)

• 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.

Segmentation

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=

2 12 =4096

2.Use the segment no as

an index into the process

segment table to find the

starting physical address

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

Consider an address of n + m bits, where the leftmost n bits are the segment no and the rightmost m bits are the offset