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Memory Management Memory Management

COSC513 – Spring 2004COSC513 – Spring 2004

Student Name: Nan QiaoStudent Name: Nan Qiao

Student ID#: 104454Student ID#: 104454

Professor: Dr. Morteza AnvariProfessor: Dr. Morteza Anvari

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Goals of Memory ManagementGoals of Memory Management

Relocation Protection Sharing Logical Organization Physical Organization

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RelocationRelocation

Location of a program is not known until the program is executed Where the program is loaded depends on what

other programs are in the system A program may be swapped out and then back into

a different location Need to be able to execute the program

regardless of where it is loaded

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ProtectionProtection

User processes cannot be allowed to read or write outside their boundaries

Protection must be done at runtime Programs can be loaded anywhere in memory Programs can dynamically allocate memory

Check for valid addresses during address translation Verify that address is within its bound

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SharingSharing

Several processes may need access to the same data Shared operating system library (i.e. DLL) Shared data buffer

More efficient to allow shared access than to copy the data

Contrary to protection schemes Cannot just allow all processes access to all

memory

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Logical StructureLogical Structure

A program is typically constructed based on different types of modules i.e. code, data, stack, heap Access to each type may be different (I.e.

read/write/shared/execute) Modules are written and compiled

separately References are resolved at runtime Typically implemented with segmentation

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Physical StructurePhysical Structure

Need the ability to swap between main and secondary storage Secondary storage is large and non-volatile Main memory is fast

Solution: Divide memory into units that can be moved between disk and memory

Movement is coordinated by user or the OS OS knows current system resources OS can perform movement more efficiently

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Memory PartitioningMemory Partitioning

Fixed partitioning Processes loaded into regions of fixed size

Dynamic partitioning Processes loaded into variable sized regions

Paging Fixed sized regions of small size

Segmentation Large dynamic regions

Virtual memory Uses pages and/or segments

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Fixed PartitioningFixed Partitioning

Memory is divided into a number of fixed sized partitions Equal sized: all pages are the same Unequal sized: small and large pages

The entire process is loaded into contiguous memory

Advantage: Simple to implement Disadvantage: Can be wasteful

Page sizes are determined by the a-priori

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Equal Sized PartitionsEqual Sized Partitions

Load processes into first available region Memory utilization can be low Large processes may not fit

8M

8M

8M

8M

8M

8M Process

10M Process

Too largeCannot be

loaded

4M Process

2M Process

Operating System

InternalFragmentation

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Unequal Sized PartitionsUnequal Sized Partitions

Load process into: Smallest free region that fits Region closest to size (may require waiting)

Advantages Accommodates large and small processes May have better utilization than a fixed scheme

Disadvantages Memory utilization low if processes are small Limits the number of active processes

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Unequaled Sized Partitions (example)Unequaled Sized Partitions (example)

Only processes larger than their partitions waste memory Less internal fragmentation

2M

4M

8M

16M

Operating System

10M Process

8M Process

4M Process

2M Process

InternalFragmentation

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Dynamic PartitioningDynamic Partitioning

Partitions are of variable size Allocate only the amount of memory needed

Number of partitions only limited by the amount of free memory

Load entire process into contiguous memory Advantages:

No internal fragmentation Disadvantages:

Fragmentation still possible (external) More complex (requires placement scheme)

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Dynamic Partitioning (example)Dynamic Partitioning (example)

Load processes contiguously in memory Holes are created when processes release

memory Operating System

32M

8M Process

24M

3M Process

21M

1M Process

20M

11M Process

9M

10M Process

Does notfit

ExternalFragmentation

7M Process

2M

32M

2M

1M

8M

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Placement AlgorithmPlacement Algorithm

The OS must decide which hole to fill when allocating memory to a process Best fit: Find the smallest hole that satisfies the

allocation Worst fit: Use the biggest free region First fit: Use the first region in memory that satisfies

the allocation Next fit: like first fit, but search starting from the last

allocation

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Internal FragmentationInternal Fragmentation

i.e. wasted memory within a partition Occurs for fixed sized partitions Occurs when a process does not use the

entire partition Prevent by dynamically partitioning

However this causes external fragmentation

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External FragmentationExternal Fragmentation

i.e. wasted memory outside of the partition Occurs in variable sized partitions Occurs when holes are not large enough to

hold a new process Solution:

Compaction

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CompactionCompaction

Eliminate holes by moving processes Copy operation is expensive

Operating System

32M

8M Process

24M

3M Process

21M

1M Process

20M

11M Process

9M7M Process

1M

32M

2M

1M

8M

Operating System

3M Process

11M Process

7M Process

11M

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Disadvantages of PartitioningDisadvantages of Partitioning

Fragmentation is unavoidable Must load entire process into memory

Limits the number of active processes Solution: use paging and/or segmentation Why study partitioning

Historical reasons Paging and segmentation are based on it May still be used for dynamic (heap) allocation

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PagingPaging

Memory is divided into small page frames

Pages of process are loaded into frames (need not be contiguous)

Less external fragmentation Small pages limit internal

fragmentation Page table translates page

addresses to frames

Main MemoryFrame

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

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Paging (Example)Paging (Example)

Load pages into empty frames

Sufficient frames must exist for entire process

No compaction needed (pages can be loaded into non-contiguous frames)

Main MemoryFrame

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

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Process A (1)

Process A (2)

Process A (3)

Process A (4)

Process B (1)

Process B (2)

Process B (3)

Process B (4)

Process B (5)

Process B (6)

Process C (1)

Process C (2)

Process C (3)

Process C (4)

Process D (1)

Process D (2)

Process D (3)

Process D (4)

Process D (5)

Process D (6)

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Page TablesPage Tables

One page table for each process

Page table translates logical addresses to physical addresses

Main MemoryFrame

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

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Process B (1)

Process B (2)

Process B (3)

Process B (4)

Process B (5)

Process B (6)

Process C (1)

Process C (2)

Process C (3)

Process C (4)

Process D (1)

Process D (2)

Process D (3)

Process D (4)

Process D (5)

Process D (6)

Page Frame0 41 52 63 74 85 9

Process B

Page Frame0 101 112 123 13

Process C

Page Frame0 01 12 23 34 145 15

Process D

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Logical to Physical TranslationLogical to Physical Translation

Translation done in hardware Upper N bits translated to frame number Lower M bits contain offset into page

Pag

e 0

Pag

e 1

Pag

e 2

...

0 0 0 0 1 0 1 1 0 0

0 0 0 0 1 0 1 1 0 0

Page OffsetPage Number

Logical Address

0 0 1 0 1 0 1 1 0 0

0 0 1 0

0 0 1 1

Physical Address

Page Table

Frame Number

Frame Number

Page Offset

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Advantages of Logical AddressingAdvantages of Logical Addressing

Logical addressing provides a means for process relocation

Each process uses the same logical addresses Separate page tables map logical addresses of

each process to different physical addresses Process segments start from a fixed location Makes compiling/loading easier

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SegmentationSegmentation

Process memory is divided into segments One segment for each type of data

(data,code,stack,heap…) Each segment can be loaded anywhere

Segments are of variable size Address translation is required

Address contains segment number and offset Must ensure that process does not address beyond

the segment limit

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Comparing Paging and SegmentationComparing Paging and Segmentation

Size of regions Small and fixed size for paging Large and variable size for segmentation

Address translation Simple bit shift for paging More complicated addition for segmentation

Advantages Paging: very little fragmentation Segmentation: logical structure

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Combining Paging and SegmentationCombining Paging and Segmentation

Combine to gain advantages of both

A process’s segments consist of a number of fixed sized pages

Requires two address translations First logical segment to logical page address Next logical page to physical page address

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How Paging/Segmentation Meet How Paging/Segmentation Meet RequirementsRequirements Relocation

Logical to physical translation allow segments or pages to be loaded anywhere

Protection Address translation gives hardware an opportunity

to check for valid addresses Sharing

Shared regions can be divided into segments Two logical segments can be mapped to one

physical shared segment

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How Paging/Segmentation Meet How Paging/Segmentation Meet RequirementsRequirements Logical Organization

Program is divided into segments Physical Organization

Pages are convenient units to swap in and out of memory