WWW.EASYDESIGN.COM

DESIGN BY

IMDAD HUSSAIN

Operatin Sysrem COMSATS Sahiwal

GROUP MEMBERS

IMDAD HUSSAIN SP13-BCS-B-91

KASHIF BILAL SP13-BCS-B-90

MOHSIN SHABIR SP13-BCS-B-88

SUBMITTED TO

Sir.Farhan-u-Allah Khan

Operating System COMSATS Sahiwal

IN THE NAME OF ALLAH WHO IS MOST BEFICIAL AND MOST MERCIFUL

OUTLINES• VIRTUAL MEMORY

• VITUAL MEMORY CONCEPT

• IMPLEMENTATION OF VIRTUAL MEMORY

• ALLOCATION OF FRAMES

• GLOBAL VERSES LOCAL ALLOCATION

• HOW MAYNY FRAME ASSIGN TO EACH PROCESS

• TRASHING

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.

VITUAL MEMORY CONCEPT

The conceptual separation of user logical memory fromphysical memory. Thus we can have large virtual memory on a small physical memory

TRANSFER OF A PAGED MEMORY TO CONTIGUOUS DISK SPACE

IMPLEMENTATION OF VIRTUAL MEMORY

• DEMAND PAGING

WHEN A PAGE IS TOUCHED, BRING IT FROM SECONDARY TO MAIN MEMORY

• DEMAND SEGMENTATION

ALLOCATION OF FRAMES EACH PROCESS NEEDS MINIMUM NUMBER OF PAGES

• EXAMPLE: IBM 370 - 6 PAGES TO HANDLE ,SS MOVE INSTRUCTION:

• A) INSTRUCTION IS 6 BYTES, MIGHT SPAN 2 PAGES.

• B) 2 PAGES TO HANDLE FROM.

• C) 2 PAGES TO HANDLE TO.

FRAME ALLOCATION

•GLOBAL REPLACEMENT: A PROCESS CAN REPLACE ANY PAGE IN THE

MEMORY.

• LOCAL REPLACEMENT: EACH PROCESS CAN REPLACE ONLY FROM ITS

OWN RESERVED SET OF

ALLOCATED PAGE FRAMES

EXAMPLE PROPOTIONAL ALLOCATION

CONSIDER A SYSTEM HAVING 64 FRAMES AND THERE ARE 4 PROCESSES WITH THE FOLLOWING VIRTUAL

MEMORY SIZES: V(1) = 16, V(2) = 128, V(3) = 64 AND V(4) = 48.

EQUAL ALLOCATION: ASSUME THAT THERE ARE N FRAMES, AND P PROCESSES, THEN N/P FRAMES ARE

PROPORTIONAL ALLOCATION: V = 16 + 128 + 64 + 48 = 256. IT ALLOCATES:

(16 / 256) * 64 = 4 FRAMES TO PROCESS 1,

(128 / 256) * 64 = 32 FRAMES TO PROCESS 2,

(64 / 256) * 64 = 16 FRAMES TO PROCESS 3,

(48 / 256) * 64 = 12 FRAMES PROCESS 4

ALLOCATION SCHEMES

• FIXED ALLOCATION

• PRIORITY ALLOCATION

HOW FRAME ASSIGN TO EACH PROCESS• EQUAL ALLOCATION: IF THERE ARE N FRAMES AND P PROCESSES, N/P

FRAMES ARE ALLOCATED TO

• EACH PROCESS.

• PROPORTIONAL ALLOCATION: LET THE VIRTUAL MEMORY SIZE FOR

PROCESS P BE V(P). LET THERE

• ARE M PROCESSES AND N FRAMES. THEN THE TOTAL VIRTUAL MEMORY SIZE WILL BE: V = ΣV(P).

• ALLOCATE (V(P) /V) * N FRAMES TO PROCESS P.

FIXED ALLOCATION EQUAL ALLOCATION

• IF 100 FRAMES AND 5 PROCESSES, GIVE EACH PROCESS 20 FRAMES.

PROPORTIONAL ALLOCATION

• ALLOCATE ACCORDING TO THE SIZE OF PROCESS

SI = SIZE OF PROCESS PI

S = S SI

M = TOTAL NUMBER OF FRAMES

AI = ALLOCATION FOR PI =SI/S *M

THRASHINGIF A PROCESS DOES NOT HAVE ‘‘ENOUGH’’ PAGES, THE PAGE-FAULT RATE IS VERY HIGH

• LOW CPU UTILIZATION

• OPERATING SYSTEM THINKS THAT IT NEEDS TO INCREASE THE DEGREE OF MULTIPROGRAMMING

• ANOTHER PROCESS ADDED TO THE SYSTEM.

•THRASHING = A PROCESS IS BUSY SWAPPING PAGES IN AND OUT.

EXAMPLE OF THRASHING

CPUUtilization

Degree ofMultiprogrammin

g

Trashing

SADVANTAGES

• INCREASES THE DEGREE OF MULTIPROGRAMMING

•

• SYSTEM THROUGHPUT DECREASES

• PAGE FAULT RATE INCREASES

• EFFECTIVE ACCESS TIME INCREASES

SOLUTION1.WE CAN REDUCE THE EFFECT OF THRASHING BY USING THE LOCAL

REPLACEMENT ALGORITHM

2.TO PREVENT THRASHING PROVIDE AS MANY FRAMES AS IT NEEDS

3.TO KNOW THE NUMBER OF FRAMES USE WORKING SET STRATEGY

WHY DOES PAGING WORK?• LOCALITY MODEL

PROCESS MIGRATES FROM ONE LOCALITY TO ANOTHER

LOCALITIES MAY OVERLAP

• WHY DOES THRASHING OCCUR?

SIZE OF LOCALITY > TOTAL MEMORY SIZE

PRINCIPLE OF LOCALITY

1.ONLY A FEW PIECES OF A PROCESS WILL BE NEEDED OVER A SHORT PERIOD OF TIME

2.POSSIBLE TO MAKE INTELLIGENT GUESSES ABOUT WHICH PIECES WILL BE NEEDED IN THE FUTURE

3.THIS SUGGESTS THAT VIRTUAL MEMORY MAY WORK EFFICIENTLY