csci2413 lecture 5 operating systems memory management 1 phones off (please)

CSCI2413 Lecture 5

Operating Systems

Memory Management 1

phones off (please)

© De Montfort University, 2004 CSCI2413 - L5 2

Lecture Outline

• Background

• Memory Types

• Memory Management..– Overlays– Swapping

• Allocation– First fit, Best fit, Worst fit


Background

• The process scheduling determines– when a process will run

• The memory manager determines– where a process will run


The problem !

• No matter how much memory we install, some programmer will always want to use more!!

• How do we get a quart into a pint pot ??

Hence, MEMORY Management …


Memory Types– CPU registers

– cache• on board, or very close to, the microprocessor

– Primary or main memory – Read And Write Memory (RAM) or Read Only Memory (ROM)

– secondary• hard disks or other offline storage devices


Memory Speeds

• The clock for a 500 MHz processor ticks at a rate of 500 million times per second– each tick takes 2 nanoseconds

• cache memory, built into the microprocessor, – access time

• typically a few nanoseconds (around 10 nsec.)

• Primary memory • around 50 - 100 nanoseconds

• Hard disk, about 10 milliseconds• or ten million nanoseconds!


Memory Management Requirements

• Relocation– Programmer does not know where the program will

be placed in memory when it is executed

– While the program is executing, it may be swapped to disk and returned to main memory at a different location (relocated)

– Memory references in the code must be translated to actual physical memory addresses


A Simple Model• In the simplest memory management model

– one process is in memory at a time• that process is allowed to use as much memory as available

userprogram 1

userprogram 2

operatingsystem 0x0000

0x1000

0xFFFF


Overlays

• Split the program into small blocks

• Keep in main only those blocks that are needed at any given time.

• Needed when process is larger than amount of memory allocated to it.


Overlays …• it is possible to increase the amount of memory

available through the use of overlays

main: initialisationinputprocessingoutput

initialisation



input


processing


output

operatingsystem 0x0000

0x1000

0xFFFF


Swapping

• Moving processes to and fro between main memory and hard disk is called swapping

• Roll out, roll in – swapping variant used for priority-based scheduling algorithms;

• lower-priority process is swapped out so higher-priority process can be loaded and executed.


Schematic View of Swapping

Main Memory Backing store

Operating Systems

Process1

User Space

Process2

Swap out

Swap in


Fixed Partitioning

• Main memory is divided into equal- (or unequal-) sized partitions and these partitions are assigned to processes.

• Any program, no matter how small, occupies an entire partition. This is called internal fragmentation. Main memory use is inefficient.

• The operating system keeps a track of which partitions are being used and which are free– processes are allocated to partitions when free


Dynamic Partitioning

• Partitions are of variable length and number

• Process is allocated exactly as much memory as required

• Eventually get holes in the memory. This is called external fragmentation

• Must use compaction to shift processes so they are contiguous and all free memory is in one block


Memory Allocation Methods

1. Single Process Systems

User Process

Operating System

Unused Space



2. Fixed Partition Memory

Operating System

Process A

180K

Process B

200K

Process C

300K

200K

300K

400K

0K

1000K



3. Variable Partition Memory

Operating System

Process A

180K

Process B

200K

Process C

300K


Contiguous Allocation

• Main memory usually into two partitions:– Resident operating system, usually held in low

memory with interrupt vector.– User processes then held in high memory.

• contiguous storage – allocates memory only in a single block

• the memory is in a single, continuous block, with no ‘holes’ or ‘gaps’


Contiguous Allocation …

• Hole – blocks of memory; holes of various size scattered throughout memory.– When a process arrives, it is allocated memory from a

hole large enough to accommodate it.– Operating system maintains information about:

a) allocated partitions b) free partitions (hole)

process 2

process 5

process 9

OS

process 2

process 5

OS

process 2

process 5

OS

process 2

process 5

process 9

OS

process 9

process 10


Non-contiguous …

• Allocate memory in multiple blocks, or segments, which may be placed anywhere in memory– the blocks are not necessarily next to each other– this is called non-contiguous storage allocation


Placement Algorithms

• Operating system must decide which free block to allocate to a process

– First-fit: scan the list segment and choose the block that is closest to the request.

– Best-fit: search the entire list and allocate the smallest hole that is big enough;

– Worst-fit: allocate the largest hole; must also search entire list. Produces the largest leftover hole.


Example:

A variable partition memory has the following hole sizes in memory order:

200K, 600K, 400K, 800K, 350K, 70K

A new process of size 300K enters the system. Determine where it will go according to the best-fit, first-fit and worst-fit algorithms and update the hole size status of the memory after the process has been added.


Placement Policies

Another Example

Hole sizes are:

475K 425K 350K

2 processes of sizes 370K and 100K

Best–fit ? First-fit ?


Dynamic relocation using a relocation register


Summary

• A number of processes can coexist in memory using a variety of allocation techniques and placement policies

• Main problem is the creation of unusable holes as processes terminate making it sometimes difficult to accommodate new processes

• Compaction can be used to shuffle processes and create larger more usable space but the large overheads involved make it not always feasible to carry out in real time.

csci2413 lecture 5 operating systems memory management 1 phones off (please)

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