VIRTUAL MEMORY AND PAGE REPLACEMENT ALGORITHM

Presented to

Sir Tahir Presented By Ch Muhammad Awais

2695/FBAS/BSCS4/F13(A)

M Mansoor Ul Haq 2736/FBAS/BSCS4/F13(A)

Nouman Dilshad 2709/FBAS/BSCS4/F13(A)

Faheem Akhtar 2710/FBAS/BSCS4/F13(A)

Muddasir Shabbir 2739/FBAS/BSCS4/F13(A)

VIRTUAL MEMORY

Now the Question is

What Is Virtual Memory ??

Virtual memory

“separation of user logical memory from physical memory.”

Only part of the program needs to be in memory for execution Logical address space can therefore be much larger than physical

address space Allows address spaces to be shared by several processes Allows for more efficient process creation

Virtual Memory which is larger than Physical Memory

Virtual Address Space

Advantages Of Virtual Memory:Multitasking

Allocating Memory Is Easy and Cheap

More Efficient Swapping

Process may even be larger than all of the physical memory.

This concept is very helpful in implementing multiprogramming environment.

Disadvantage's Of Virtual Memory:

Longer Memory Access Time As HDD Is Slower.

Memory Requirement.

Applications run slower if the system is using virtual memory.

It takes more time to switch between applications.

Less hard drive space for your use.

PAGE REPLACEMENT

Prevent over-allocation of memory by modifying page-fault service routine to include page replacement

Use modify bit to reduce overhead of page transfers – only modified pages are written to disk

Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory

Page Replacement

Need For Page Replacement

1. Find the location of the desired page on disk

2. Find a free frame: - If there is a free frame, use it - If there is no free frame, use a page replacement algorithm to select a victim frame

3. Bring the desired page into the (newly) free frame; update the page and frame tables

4. Restart the process

Basic Page Replacement

Page Replacement

Why and When We Use Page Replacement Algorithm ?

Want lowest page-fault rate

Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string

In all our examples, the reference string is

1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

ALGORITHMS

FIFO(FIRST IN FIRST OUT) ALGORITHMTreats page frames allocated to a process as a circular buffer:When the buffer is full, the oldest page is replaced. Hence first-in, first-out:A frequently used page is often the oldest, so it will be repeatedly paged out by FIFO.

Simple to implement: requires only a pointer that circles through the page frames of the process.

Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 3 frames (3 pages can be in memory at a time per process)

4 frames

First In First Out Algorithms

1

2

3

4

1

2

5

3

4

1

2

3

9 page faults

1

2

3

1

2

3

5

1

2

4

5 10 page faults

44 3

• Replace page that will not be used for longest period of time• 4 frames example

1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

How do you know this? Used for measuring how well your algorithm performs

Optimal Algorithm

1

2

3

4

6 page faults

4 5

• Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

• Counter implementation• Every page entry has a counter; every time page is referenced through this

entry, copy the clock into the counter• When a page needs to be changed, look at the counters to determine which are

to change

5

2

4

3

1

2

3

4

1

2

5

4

1

2

5

3

1

2

4

3

Least Recent Used (LRU) Algorithm

Use of a stack to Record The Most Recent Page Reference

LRU Approximation Algorithms • Reference bit

o With each page associate a bit, initially = 0o When page is referenced bit set to 1o Replace the one which is 0 (if one exists)

• Second chanceo Need reference bito Clock replacemento If page to be replaced (in clock order) has reference bit = 1 then:

set reference bit 0 leave page in memory replace next page (in clock order), subject to same rules

LRU(LEAST RECENT USED) ALGORITHMAdvantages :- LRU policy is often used as a page replacement algorithm It is quite good algorithm. It is easy to choose that has already page faulted and not in use for long period.

Disadvantages:- Problem is how to implement LRU replacement Require substantial hardware assistance

Problem is determine the frame order by the time of last use.

Counting Algorithms

Keep a counter of the number of references that have been made to each page

LFU Algorithm: replaces page with smallest count

MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used

ANY QUESTION

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