Operating System Concepts and Techniques Lecture 11

Operating System Concepts and

Techniques Lecture 11

Memory Management-4

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

Optimal page replacement policyAlthough not practical, there is an optimal algorithm

Removes the page which will be referred to in the farthest future

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

Page frame 1 2 2 23 22 21 2 23 2 2 2i 1

Page frame 2 3 34 33 32 3 3i 5 5 5i 5

Page frame 3 46 17 53 4 42 4 4 4i 4

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

Optimal with a main memory of three frames, H=4, M=7, h=4/11, and m=7/11

Multilevel Page-TableSo far page table was exempted from

page removalWith large page tables memory

efficiency reducesWe have to have a way of knowing

which part (page) of page table is in main memory and which is not

Two level page table serves this purposeWill discuss details in Windows case

study

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Cache memory management

Cache is a read/write Random access memory similar to main memory but faster

It sits between CPU and main memoryIt is a temporary place for some main

memory locations

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CPU (registers)

Cache memory

Main memory

Secondary storage

Can be write through or non-write through

Cache memory management…

Most main memory management policies can be adopted to cache

However, there are special policies tooInformation transfer to cache is in block unit, a block

is much smaller than a pageLet’s assume we have 1Mega bytes of cache, block

size of 128 bytesMain memory can be divided into blocks of size 128

bytesAn address in main memory can be shown as

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Row number Column number Offset 12 bits 13 bits 7 bits

Cache managementWith this policy, only one block of each column of main memory is

in cache

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0 . . . 213 -1 10

1

2

.

.

.

212-1

2

Are inputs equal?

Yes, address in cache

No, address not in cache

Register array

Physical address

Main memory

Cache memory

The structure to check whether a physical address is in cache or not

Effective access time Suppose cache success rate is 0.9

Therefore, in only 10% of the times an actual access to main memory will take place.

If cache access time is say 20 nanoseconds and main memory access time is 50 nanoseconds, on

the average, it will take 0.9*20 + 0.1*(x + 50) nanoseconds to access what we need

x is the time that takes to fine out what we need to access is in the cache or not.

For x=5 nanosecons effective main memory access time will become 0.9*20 + 0.1*(5 + 50) =

23.5 nanoseconds; much better than 50 nanoseconds

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Windows case studyTwo-level page tablePage size of 4K bytesAn address looks like

It uses dynamic working set, with default value at the beginning

It uses per process page removalPage removal algorithm is FIFO

It prefetches pages of main memory

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31 30 … 22 21 20 … 12 11 10 … 2 1 0 Partial Page table Page number Offset

Address translation in Windows

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Partial page table number Page number Offset

Page frame number Offset

Logical address

Physical address

The partial page table corresponding to left most 10 bits of the logical address

Present?

Yes

No

Interrupt

.

.

.

.

.

.

YesNo

Interrupt

Present?

.

.

.

.

.

.

Directory

Unix case studyOne-level page table

Page size of usually 1K bytesAn address looks like

It uses dynamic working set, with default value at the beginning

It uses global page removalPage removal algorithm is a modified

version of clock

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31 30 … 22 21 20 … 12 11 10 … 2 1 0 Page number Offset

Unix page removal algorithm

UNIX likes to have a certain minimum number of free page frames all the times, minfree (or

low water level)If it becomes lower, page daemon agent will try

to free page frames until the number of page frames reaches lotsfree (or high water level)

Think of filling up a water tank when the water level is low

It uses a modified version of the clock algorithm to decide which pages to remove

more than one page is removed every time page daemon agent is activated

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Unix page removal algorithm..

If many frames are scanned by page daemon, but the number of free frames have not

reached lotsfree the process swapper complements the page removal process

Process swapper will swap out some processesThese processes will be swapped back when

the situation improvesWith this view, take one more look at Unix

state transition diagram

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SummaryCache memory is part of all computers, MM would be incomplete without talking about

cache and the way it is managedIt improves effective memory access time

depending on cache hit ratioTwo case studies were discussed

UNIX uses special concepts such as process swapping, lotsfree and minfree

Windows, on the other hand, uses two-level page table memory management which

allows virtual memory for page tables

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Find outA page replacement policy which always gives

the highest page faultsThe total size of directory and partial page

tables for a 4giga bytes program in WindowsEffective memory access in Windows MM with

two-level page table and cache memory for address traslation

In UNIX, what are the rationales for stopping the page daemon agent when “enough” frames are

scanned but not enough pages are freedThe criteria for selecting a process to swap out

in Unix and vice versa

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Any questions?