basic memory management

8/8/2019 Basic Memory Management

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Memory Management


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Readings

4.1 and 4.2 of the text book


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Introduction

Our machines today have 10,000 times more memory than the IBM 7094

leadingedgemachineofthe1960 s

Cost of memory had dropped dramatically

Bill Gates (former chair of Microsoft) once said 640K should be enough

Our programs tend to expand to fill the memory available

Technology does not allow for each program to have infinitely large and fast

memory

Operating systems must manage memory


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Whatifaprogramislargerthanphysical

memory?

Registers

On-chip Cache

Main MemoryMagnetic (Hard) Disk

Magnetic Tape

Memory Hierarchy


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Memory Management

We will review different approaches:

r Memorymanagementintheabsenceofmultiprogramming

r Memory management using fixed partitions

r Memory management using dynamic partitions

r Virtual memory


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Basic Memory Management

Three different ways to organize an operating system withone

user process

(Palm computers, iPhones) (MS-DOS)

BIOS


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Basic Memory Management

One user program at a time

When a command is typed, the OS copies the requested program from

disk to memory and executes it

Newprogramstobeloadedoverridetheprogramcurrentlyinmemory


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Multiprogramming with Fixed Partitions

Having multiple processes running at once means that when one

process is blocked waiting for I/O to finish another can use the CPU

Need to divide up memory among programs

Simpleapproach:Dividememoryupintonpartitions

r Partitions do not necessarily have to be of the same size


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Multiple Queues

r Jobisputintotheinputqueueforthesmallestpartitionlargeenoughto

hold it

Single Queuer Job that is closest to the front of the queue that fits in it could be loaded

into the empty partition

r Alternatively search the entire input queue whenever a partition becomes

free and pick the largest job that fits


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(a) Separate input queues for each partition

(b) Single input queue


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Questions

Single Queue:

r Whatistheproblemwithtakingthefirstjobthatfitsintothepartition?

r What is the problem with taking the largest job that fits into the

partition?r What do you thinkinternal fragmentationis?

Multiple Queues:

r What is the disadvantage?


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Was used by OS/360 on large IBM mainframes for many years

Incoming jobs were queued until a suitable partition was available

Today no modern OS uses fixed partitions


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Swapping

Dynamic partitions

r Allocate as much memory as needed by each process

One approach to managing memory is to useswapping.

Swappingconsists of:r Bringing in each process in its entirely

r Running it for a while

r Putting it back on the disk.

The number of partitions, location and size vary dynamically asprocesses come and go


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Graphical Depiction of Swapping


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Swapping - Example

Memory allocation changes as

r processes come into memory

r leave memory

Shadedregionsareunusedmemory


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Questions

What would happen if you swapped a process memory space that is not

idle?

WhatwouldhappenifyouswappedoutaprocesswaitingforanI/O

operationthatputsdataintobuffers?


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How Much Memory to Allocate

How about allocating a fixed size of memory when the processes is

created or swapped in?

r Simple, but a process s data segment may grow through dynamic

allocation of memory How about dynamic allocation of memory?

r Let s say there is a unused contiguous memory (hole) adjacent to the

process s memory

r That s great it can be used.

r What if there isn t?


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How Much Memory to Allocate

How about dynamic allocation of memory?

r Let ssaythatthememoryadjacenttoaprocess sallocatedmemoryis

being used by another process?

r

Do you move the process that needs more memory?r Do we swap out other processes to make room to create a large enough

hole?

r Why not allocate a little additional memory to allow for growth?

Note: It can still run out


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How Much Memory to Allocate?

(a) Allocating space for growing data segment

(b) Allocating space for growing stack & data segment


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Question

Should unused memory allocated to a process be swapped to disk?


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Keeping Track of Holes

When a process terminates:

r Compactmemory?

Move all processes above the hole down in memory.

Can be very slow: 256MB of memory, copy 4 bytes in 40ns compacting memory in 2.7 sec

Almost never used

Result: OS needs to keep track ofholes.

Problem to avoid: memoryfragmentation.

Two approaches to keeping track of holes:r Bit maps

r Linked lists


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Memory Management with Bitmaps

Memory is divided up into allocation units

r Variesfromafewwordstoseveralkilobytes

A bit in a bitmap is associated with each allocation unit


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Swapping Data Structure: Bit Maps

Partofmemorywith5processes,3holes

r tickmarksshowallocationunits

r shaded regions are free

Corresponding bit map


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Properties of Bit-Map Swapping

Allocation unit is k bits

bitmapusesM/kbits,whereMistheamountofmemory;Couldbequite

large.

Searchingbit-mapforaholeisslow


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Memory Management using Linked Lists

Maintain a linked list of allocated and free memory segments

A segment is a process or a hole between two processes

Each entry in the list specifies

r A hole (H) or process (P)

r The address at which it starts,

r The length

r Pointer to the next entry


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Swapping Data Structure: Linked Lists

Keep a list of blocks (process=P, hole=H)

As time goes on, memory becomes more and more fragmented; Memory

utilizationdeclines

This is referred to asexternal fragmentation.


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What Happens When a Process Terminates?

What happens when a process terminates (or is swapped out)?

There are four possibilities as seen on the next slide


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Merge neighboring holes to create a bigger hole


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Searching for the memory allocated is easy since most operating

systems have a pointer to the memory allocated for the process


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Hole Selection Strategy

We have a list of holes of sizes

10, 20, 10, 50, 5

A new process is created that needs size 4.

Which hole to use?

Lots of algorithms


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First/Best/Next/Worst/Quick Fit

First fit: For a process of sizes, find first hole larger thans

Best fit: For a process of sizes, use smallest hole that

has size(hole) >= s.

Worst fit: find the biggest hole that fits.

Quick Fit: maintain separate lists for common block sizes.


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Problems

Relocation: The mechanism for fixing memory references in memoryr Cannot be sure where program will be loaded in memory: address locations of variables, code routines

cannot be absolute

Protection: One process should not be able to access another processes memory partition

Solutions:r Relocationduringloading

r Program Status Word (protection)

r Base and limit (protection and relocation)


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Relocation

Alogical address(orvirtual address) is a reference to a memory

location independent of the current assignment of data

Arelative addressis a logical address

r Addressisexpressedasalocationrelativetosomeknownpoint

Aphysical addressis an actual location in main memory


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Relocation using Base and Limit Registers

Thebase registeris loaded with the starting address in main memory of the

program

Thebounds registerthat indicates the ending location of the program

These values must be set when the program is loaded into memory or when the

processimageisswappedin

Mapping logical address to physical address:

r Value in the base register is added to the relative address to produce a physical address

r Resulting address is compared with the value in the bounds register


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Where Found?

Older versions of Unix

Microsoft Windows 3.1 operating systems


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Summary

Using fixed-size and variable-size partitions are inefficient in the use

of memory

Fixed-size results in internal fragmentation

Variablesizeresultsinexternalfragmentation

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