II’nd PART OPERATING SISTEMS

LECTURE 10

MEMORY MANAGEMENT

Ştefan Stăncescu

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MEMORY MANEGEMENT

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Main Memory – cf. von Neumann arh. (nonremanent – fast ) :

• Cell row where info are deposed, modified, kept

• Code & data space where programs act as processes

Memory management :• Free/allocated memory space management

• Allocate/free memory spaces to processes in run

• Swap => MP – disk transfer

• Transfers between hierarchical levels of memories

• Virtual memory management – page fault mechanisms

MEMORY MANEGEMENT

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No MM

MSDOS – Only one user program in memory – cooperativ SO – one program after each, in batch processing

Al resources available – no security

Multiprocessing only with collaborative threads in time sharing UC -

- embedded systems

User programs

OS

OS

User programs

DD(I/O)User

programs

OS

MEMORY MANEGEMENT

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Fixed memory partitions managementTime shared UC for separate processes –allocation at boot

• Each process runs only in previously fixed space

• In each memory fixed partition

• Batch processing management in each partition

• Process queue management at each fixed partition

• Process memory space allocation at fixed partition accordingly with partition resources (space)

Empty partitions with satisfied queues and a lot of partitions with processes in wait for memory space

=> only one queue for all

but inappropriate space allocation

Partitia 1

Partitia 2

Partitia 3

Partitia 4

Partitia 5

MEMORY MANEGEMENT

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Swap memory – variable “partitions”Disk (external memory) resident programs

• when a process run is needed by process scheduler

program is loaded and run from disc in MM

with instant space required by process

• when a process run is ended,

all data needed are saved on disk and

memory space in MM is released

• Disc main memory = swapSpace allocation/released accountedFree space compaction and management

Partitia 1

Partitia 2

Partitia 3

Partitia 4

Partitia n

MEMORY MANEGEMENT

Variable memory partitions – swap

=> Free space management

Process memory space allocation optimized

PROCES B

PROCES A

PROCES C

PROCES B

PROCES A

PROCES D

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PROCES B

PROCES A

PROCES D

Prtitia 3PROCES E

PROCES A

PROCES F

PROCES D

PROCES E

PROCES A

MEMORY MANEGEMENT

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Rule 50%

N processes N/2 holes1 process => up 50% probability hole/process

down 50% probability hole/process Total 2 processes for 1 hole

MEMORY MANEGEMENT

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Unused memory rule(N/2) * kS = M-NS

M=NS(1+k/2)

f=% unused space in memory

f=(N/2) * (kS /M) // no. holes * hole dim/ all memory dim.

f=NkS/2M=NkS/2(NS*(1+k/2)) // with M calculated before f=k/(2+k) // f is dependent only of k

Ex: k=1/2 => f=(1/2)/(2+1/2)=1/5=20%

k=1/4 => f=11%

S = medium process dimkS = medium hole dimM = all memory dimN = no. processesn/2= no. holes (50% rule)

MEMORY MANEGEMENT

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Process allocation in holes with holes list1. First fit => first hole in list sufficient for process => fast

2. Next fit => first fit in rest list => little worse as first fit

3. Best fit => most near dim in list => worsest

4. Worst fit => anti best fit, small process in greater hole

All alg are slow, by hole list analysis time improvements => sorting, separate lists, etc.

5. Quick fit => separate hole lists of fixed dims

=> Problem=> merging little holes – working with big holes

MEMORY MANEGEMENT

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Modele in gestiunea de spatii de memorie

Harta de biti -Bitmap1 1 1 1 1 0 0 0

0 1 1 1 1 1 1 1

1 1 0 0 1 1 1 1

1 1 1 1 1 0 0 0

1 1 1 0 0 0 0 0

0 0 0 0 0 0 0 0

5 4 9 2 9 3 3

MEMORY MANEGEMENT

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Modele in gestiunea de spatii de memorie

Liste inlantuite:

5 4 9 2 9 3 3

1 5 0 4 1 9

0 2

0 1001 3

0 31 9

MEMORY MANEGEMENT

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Modele in gestiunea de spatii de memorie

Modificari la liste inlantuite

A X B A B

X B

A

B

A X

X

MEMORY MANEGEMENT

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Modele in gestiunea de spatii de memorie Buddy sistem32k 64k 128 256 512

A A 128 256 512

A A B 128 256 512

A A B C C 256 512

B C C 256 512

B D C C 256 512

D C C 256 512

C C 256 512

256 512

MEMORY MANEGEMENT

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Page Replacement Algorithms at Page FaultsOptimal Page Replacement Algorithm

Not Recently Used – NRU

First In First Out – FIFOSecond chance

ClockLeast Recently Used - LRUNot Frequently Used - NFU

Not Frequently Used – NFU w/AgingWorking set - WS

WS w/Clock

MEMORY MANEGEMENT

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Page Replacement Algorithms at Page FaultsOptimal Page Replacement Algorithm

Ideal one, not realizable

Optimality:

First the program runs with fixed data

Note the page demanding list

Decide best replacements by keeping minimum PF

(the most delayed use page will be replaced)

At each PF => calculate the most delayed use page

replace that one with the demanded page

MEMORY MANEGEMENT

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Some HW adjacent each physical page

w info needs in decision about page replacement.

Reference-Modify HW cell memories:

R M

0 0 No referred page, w/o read/write - modify

1 0 Recent read page, w/o write-modify

0 1 Old read page, w/old write-modify

1 1 Recent read page, w/old or recent write-modify

R erased at clock IT (“forget” old unnecessary reads)

MEMORY MANEGEMENT

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NRU (not recently used)

w/RM help in page ranking

R M

0 0 Class 3 Max priority

1 0 Class 2

0 1 Class 1

1 1 Class 0 min priority

NRU => At PF eject at random any page

from the less priority nonempty class

Simple, no optimal, reasonably efficient, no big mistakes

MEMORY MANEGEMENT

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FIFO (First In First Out)

FIFO queue w/ index requested pagesAlg:Fill the queue and reuse any pages in queue is requestedAfter queue full – PF

eject page in front (earliest in)

Simple, efficient, butmay eject page in current use,

should be kept for next intensive work,although the page has spent much time in

memory

MEMORY MANEGEMENT

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FIFO Second Chance

FIFO queue w/ index requested pages and RMAlg.: AT PF

if (R=0) then

replace //old page, unreffered

else

R=0 //pretend to not be read, although R=1

move from front to end of FIFO queue

//consider as a new loaded page

go to next position

(if same page found, replace, without remorse)

MEMORY MANEGEMENT

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FIFO Clock

Hand points to the oldest page

On a page fault, follow the hand to inspect pages

Apply clock second chance:

if (R=1),

R=0 and advance the hand

if (R=0)

replace page

(only if all pages are R=1, second chance get first page)

Long round

MEMORY MANEGEMENT

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Least Recently Used LRU

NR couter/register attached to each page to count accesses

Alg:

at each clock IT

all NR are increments with R (0/1)

//each NR keeps reference page rank

at PF, page with smallest rank is ejected

=> old heavy runs are kept also! – wrong!

Idea: aging = replace bold text w/

shift all NR 1 bit right

add R(0/1) at left (msb position bit in NR)

GESTIUNE DE MEMORIE

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Least Recently Used LRU

0 1 2 3 2

1 0 3 23

0 1 1 1

0 0 0 0

0 0 0 0

0 0 0 0

0 0 1 1

1 0 1 1

0 0 0 0

0 0 0 0

0 0 0 1

1 0 0 1

1 1 0 1

0 0 0 0

0 0 0 0

1 0 0 0

1 1 0 0

1 1 1 0

0 0 0 0

1 0 0 0

1 1 0 1

1 1 0 0

0 0 0 0

1 0 1 1

1 0 0 1

1 0 0 0

0 1 1 1

0 0 1 1

0 0 0 1

0 0 0 0

0 1 1 0

0 0 1 0

0 0 0 0

1 1 1 0

0 1 0 0

0 0 0 0

1 1 0 1

1 1 0 0

0 1 0 0

0 0 0 0

1 1 0 0

1 1 1 0

MEMORY MANEGEMENT

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Not Frequently Used - NFU

NR couter/register attached to each page

And a overall process time counter

Alg:

At each reference overall increment time counter

and transfer content to referred page NR

// NR keeps page data access time stamp

at PF

eject the page with oldest data stamp (lowest content)

GESTIUNE DE MEMORIE

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Not Frequently Used - NFU

0 1 2

3 4 5

Pagina

CK

0 1 2 3 4 5

0 1 0 1 0 1 1

1 1 1 0 0 1 0

2 1 1 0 1 0 1

3 1 0 0 0 1 0

4 0 1 1 0 0 0

1 0 0 0 0 0 0

1 1 0 0 0 0 0

1 1 1 0 0 0 0

1 1 1 1 0 0 0

0 1 1 1 1 0 0

0 0 0 0 0 0 0

1 0 0 0 0 0 0

1 1 0 0 0 0 0

0 1 1 0 0 0 0

1 0 1 1 0 0 0

1 0 0 0 0 0 0

0 1 0 0 0 0 0

0 0 1 0 0 0 0

0 0 0 1 0 0 0

1 0 0 0 1 0 0

0 0 0 0 0 0 0

0 0 0 0 0 0 0

1 0 0 0 0 0 0

0 1 0 0 0 0 0

0 0 1 0 0 0 0

1 0 0 0 0 0 0

1 1 0 0 0 0 0

0 1 1 0 0 0 0

1 0 1 1 0 0 0

0 1 0 1 1 0 0

1 0 0 0 0 0 0

0 1 0 0 0 0 0

1 0 1 0 0 0 0

0 1 0 1 0 0 0

0 0 1 0 1 0 0

GESTIUNE DE MEMORIE

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Least Recently Used - LRU /Not Frequently Used – NFU

LRU update at each access (instruction)

NFU update at each IT (many instructions)

a page may count a lot of references between IT’s

same rank for a few or a lot of references

pages w/first references between IT are at

same rank as pages w/last references in interval