Operating System Concepts and

Techniques Lecture 10

Memory Management-3

M. Naghibzadeh

ReferenceM. Naghibzadeh, Operating System Concepts and Techniques, First ed., iUniverse Inc., 2011.

To order: www.iUniverse.com, www.barnesandnoble.com, or www.amazon.com

Virtual MemoryGoals

Reduce memory waste Eliminate program size restriction Increase number of live programs

Move inactive parts of data, program, and results out of main memory

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Page-Based Virtual MMExtended Page memory management to support

virtual memoryWith virtual memory, disk becomes a virtual main

memoryDuring execution of a program, disk virtual area is

the permanent place for the programMain memory is a temporary place for the program

To load a program, any page can take any page frame

Executable files from slow devices such as CD has to be moved to virtual area to improve

performance

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Page-Based Virtual MM...Page table knows what page is where

No need to bring all pages to start running itCan start running even if no page is brought to

main memory Prefetch is possible, if you like and there is space

Page table is extended to show which page is in main memory and which page is not

This is done by adding a column called present/absent

In theory, a very small main memory is adequate to run a very large program

When the system wants to execute an instruction the instruction and its operands have to be in main

memory

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Page-Based Virtual MM...Otherwise, a page fault occurs

the process is suspended until the page is brought to main memory by memory manager

If the processor spends most of its time suspending and resuming processes due to frequent page faults,

then a thrashing condition occursA reference to a logical address which has already

been loaded into main memory is called page successIn short time intervals, the program usually refers to a

small number of pages; this phenomenon is called locality of reference

Page Frame Table (PFT) knows what page frames are free

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Ready to goPut the first executable address of the program in PC and we are ready to go,

even if no page is brought to main memory

Every logical address has to be translated to a physical address

Next we talk about actual address translation

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Address translation Using PT

Address translation using page table within MMU

Page table is too big to fit in MMU; use cache memory

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Page number Offset

Page frame number Offset

Logical address

Physical address aaddress

Offset is directly copied from logical address to physical address

Page map table of the running process with

present/absent field in each entry

Present?

Yes

No

Interrupt

Address translation Using PFT

Have to use a content addressable memory called Translation Lookaside

Buffer (TLB)The content addressable memory has to

house page frame tableWe cannot have such a huge associative

memoryWill use Inverted Page Table (IPT)

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Address translation Using IPT

Rows of IPT are created as needed

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Virtual page# Page frame# Protection Free/Allocated Other fields

Virtual page# Offset

Page frame number Offset

Logical address

Physical address aaddress

Offset is directly copied from logical address to physical address

TLB

Present?

Yes

No

Interrupt

Concepts before page removal

If a running program need a page, OS must bring it to main memory

If there is no free page frame, a page must be removed

We have to respect: Working set; every process is given a credit of MM

Per process or overall; do we remove pages from the same process or from any process

Page locking; some pages are locked which means they cannot be removed

What to do with removed pages? If necessary copy them back to disk

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First-In-First-Out Page removal

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FIFO behavior with three page frames, H=3, M=9, h=3/12, and m=9/12.

Simple to implement: use a single linked list with two pointers

Page trace 1 2 3 4 1 2 5 1 2 3 4 5

Page frame 0 1 2 3 4 1 2 5 5 5 3 4 4

Page frame 1 1 2 3 4 1 2 2 2 5 3 3

Page frame 2 1 2 3 4 1 1 1 2 5 5

Hit? - - - - - - - + + - - +

FIFO (Belady’s) AnomalySometimes by increasing memory

frames page faults increase

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Page trace 1 2 3 4 1 2 5 1 2 3 4 5

Page frame 0 1 2 3 4 4 4 5 1 2 3 4 5

Page frame 1 1 2 3 3 3 4 5 1 2 3 4

Page frame 2 1 2 2 2 3 4 5 1 2 3

Page frame 3 1 1 1 2 3 4 5 1 2

Hit? - - - - + + - - - - - -

FIFO behaviour with four page frames, H=2, M=10, h=2/12, and m=10/12

FIFO does not have stack property

Clock policy/Second chance policy

Use reference bit

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Page trace 4 0 2 3 0 4 3 2 4

Page frame 0 41 01 21 40 00 20 31 31 00 41 41 30 21 21

Page frame 1 41 01 21 40 00 20 20 31 00 00 41 30 30

Page frame 2 41 01 21 40 00 01 20 31 31 00 41 41

Hit? - - - - + - + - +

Second chance behavior with three page frame, H=3, M=6, h=3/9, and m=6/9

Least Recently Used

LRU with a main memory of three frames, H=2, M=10, h=2/12, and m=10/12

• Stack algorithm, but not practical

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Page trace 1 2 3 4 1 2 5 1 2 3 4 5

Page frame 0 1 2 3 4 1 2 5 1 2 3 4 5

Page frame 1 1 2 3 4 1 2 5 1 2 3 4

Page frame 2 1 2 3 4 1 2 5 1 2 3

Hit? - - - - - - - + + - - +

Some other page removals

Not Recently Used, uses modified bit and reference bit

Least Frequently Used, uses a reference counter

Most Frequently Used, Uses a reference counter

Not Frequently used, uses a reference bit and a reference bit

update interval

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AgingAging shifts every R bit to its corresponding

shift counter at the end of intervalIt forgets far history

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Page trace 1 2 3 2 5 3 4 2 4 5

Page frame 0 10000 10

000 10000 10

000 10000 50

000 50000 50

000 40000 40

000 41000 40

100 40100

Page frame 1 20000 20

000 20000 21

000 21000 21

000 20100 20

100 21100 21

100 20110 20

110

Page frame 2 30000 30

000 30000 30

000 31000 30

100 30100 30

100 30100 30

010 50000

Hit? - - - + - + - + + -

Aging with a main memory of three frames, H=4, M=6, h=4/10, and m=6/10

SummaryVirtual memory management is the de facto

MM policy of contemporary computersIt has many good properties such as being able

to run extremely large programs and running numerous simultaneous programs

With virtual memory management, comes a variety of page removal methods with different

complexities and efficienciesIt has disadvantages too; increased execution

time and the need for special hardware module called MMU are two main ones

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Find outThe smallest size main memory which can run

any program though inefficientlyOptimal page size

The main property which makes aging to become a good page replacement algorithmThe total space needed for all page tables of say 1000 programs of size 4giga bytes each

The format of a logical address in a four level page table environment

The effective main memory access time with TLB and a non-virtual page table

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Any questions?