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Working set model, Page fault frequency

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Recap

In the last class, you have learnt

Concepts of thrashing

Allocation of frames

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Objective

On completion of this period, you would be able

to know

Working set model

Page fault frequency

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The pages used by a process within a window of time are called its working setThe working-set model is based on the assumption of locality

…2 6 1 5 7 7 7 7 5 1 6 2 3 4 1 2 3 4 4 4 3 4 3 4 4 4 1 3 2 3 4

Changes continuously - hard to maintain an accurate

numberHow can the system use this number to give optimum

memory to the process?

WS(t1) = {1,2,5,6,7} WS(t2) = {3,4}

Working Set Model

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Working Set Model

defines the working set window

If a page is in active use, it will be in the working

set

If it is not in the use, it will be drooped from the

working set

Working – set is the approximation of the

program’s locality

The accuracy of the working set depends on the

selection of the

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Working Set Model

Important property of the working set is the size

Compute working set size for each process in the system WSSi

D=WSSi, where D is the total demand for frames

If D>m where m is the total no. of available frames thrashing will occur

The operating system monitors the working-set of each process and allocates them enough frames

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Working Set Model

Working-set strategy prevents thrashing

Degree of multiprogramming will be high

Disadvantage

Keeping track of working set

Working set is a moving window

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The Working Set Model

The working set is the set of pages used by the k most recent memory references

w(k,t) is the size of the working set at time, t

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Steps in handling a Page Fault

The process has touched a page not currently in memory

Check an internal table for the target process to determine if the reference was valid (do this in hardware)

If it was valid, but page isn't resident, then try to get it from secondary storage

Find a free frame; a page of physical memory not currently in use (May need to free up a page)

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Steps in handling a Page Fault

Schedule a disk operation to read the desired

page into the newly allocated frame

When memory is filled, modify the page table to

show the page is now resident

Restart the instruction that failed

Repeat these steps

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Page Fault Frequency 

This is a good indicator of thrashing

If the process is faulting heavily, allocate it more frames

If faulting very little, take away some frames

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Summary

In this class, you have learnt

Working set model

Page fault frequency

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Frequently Asked Questions

1. Explain the working-set model ?

2. Explain the page fault frequency ?

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Quiz

1. If a page is in active use, it will be in the a) FIFOb) LRU c) Working set

d) None

2. Important property of the working set is the

a) FIFO b) LRU c) Size

d) None

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Quiz

3. Working – set is the approximation of the program’s

a) Locality

b) LRU

c) FIFO

d) None

4. D=WSSi, where D is the total demand for

a) LRU

b) Frames

c) FIFO

d) None

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