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Chapter 7Memory Management

• Basic requirements of Memory

Management

• Memory Partitioning

• Basic blocks of memory management

– Paging

– Segmentation

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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

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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.

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

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Memory Management Requirements

• Relocation

• Protection

• Sharing

• Logical organisation

• Physical organisation

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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

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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)

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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

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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

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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

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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

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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

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Types of Partitioning

• Fixed Partitioning

• Dynamic Partitioning

• Simple Paging

• Simple Segmentation

• Virtual Memory Paging

• Virtual Memory Segmentation

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Fixed Partitioning

• Equal-size partitions (see fig 7.3a)

– 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

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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.

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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

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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

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Fixed PartitioningIt 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

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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

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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

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Dynamic Partitioning Example

• 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

OS (8M)

P1 (20M)

P2(14M)

P3(18M)

Empty (56M)

Empty (4M)

P4(8M)

Empty (6M)

P2(14M)

Empty (6M)

Refer to Figure 7.4

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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

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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

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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

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Allocation

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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 2U

• If a request of size s where 2U-1 < s 2U

– 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

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Example of Buddy System

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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

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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

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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

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Hardware Support for Relocation

0. Initialize base and bounds registers when a process is assigned to the Running state

1. Add the value in the base register to the relative address to produce an absolute address

2. Compare the resulting address to the value in the bounds register

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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

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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

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Paging

• 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

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Processes and Frames

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

C.0C.1C.2C.3

D.0D.1D.2

D.3D.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

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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

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Page Table

page number

frame number

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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

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Paging Logical to Physical Address Translation

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

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

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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)

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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.

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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

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SegmentationLogical to Physical Address Translation

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

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