Modeling Page Replacement Algorithms

7/28/2019 Modeling Page Replacement Algorithms

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I.S.L Sarwani,

Asst prof, IT,

ANITS

Modeling Page Replacement

Algorithms


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Modeling Page Replacement Algorithms

Belady's Anomaly

FIFO with 3 page frames FIFO with 4 page frames

P's show which page references show page faults


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Stack Algorithms

State of memory array, M, after each item in reference stringis processed


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Contd..

Paging system can be categorized by 3 items:

1. The reference string of executing process

2. the page replacement algorithm

3. the number of page frames available in memory i.e.

mM- internal array keeps track of state of memory

Top part- all the page entries currently in memory

Bottom part- pages referenced once, swapped out and

are not in memory


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The Distance String

A page reference will be denoted by the distance

from top of stack where reference page was

located.

Pages that have not yet been reference distance infinity.

Distance string depends on reference string and

paging algorithm.


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Predicting page fault rates

Computation of page fault rate from distance string the Cvector

the Fvector


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Contd..C number of occurrences of i or infinity in distance

string

F number of page faults


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Design issues for paging systems


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Local versus Global Allocation Policies (1)

Original configuration Local page replacement

Global page replacement


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Local when a process gets page fault , the

page replacement algorithm try to find the LRU

page considering pages that are currently

allocated to only that process.

Global if the page with lowest age value is

removed without regard to whose page it is.


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Contd..

Local allocates fixed memory to process.

If working set grows thrashing will result. So

wasting memory.

Global dynamical allocation among runnable

processes.Here OD continually decides how many page

frames to assign to each process.

Page Fault Frequency (PFF) algorithm: Tells when

to increase or decrease process page allocation but

not which page to remove


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Load Control

Despite good designs, system may still thrash

When PFF algorithm indicates some processes need more memory

but no processes need less

Solution :

Reduce number of processes competing for memory

swap one or more to disk, divide up pages they held reconsider degree of multiprogramming


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Page Size (1)

Small page size Advantages

less internal fragmentation

better fit for various data structures, code sections

less unused program in memory

Disadvantages

programs need many pages, larger page tables


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Separate Instruction and Data Spaces

One address space Separate I and D spaces


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Shared Pages

Two processes sharing same program sharing its

page table


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Contd..

If I space and D space are supported 2 or

more processes use the same page table for

their I space but different D space.

Processes share same code problem occurs

Copy-on-write: Those pages that are never

written need not be copied.


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Cleaning Policy

Need for a background process, paging daemon periodically inspects state of memory

When too few frames are free

selects pages to evict using a replacement algorithm

It can use same circular list (clock)

front hand controlled by paging daemon Back hand page replacement


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Segmentation


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Segmentation (1)

One-dimensional address space with growing tables

One table may bump into another


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Segmentation (2)

Allows each table to grow or shrink, independently


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Segmentation (3)

Comparison of paging and segmentation


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Implementation of Pure Segmentation

(a)-(d) Development of checkerboarding

(e) Removal of the checkerboarding by compaction

Documents

Modeling Page Replacement Algorithms