This document discusses the concepts of virtual memory, including its implementation through demand paging. It describes how memory frames are allocated to processes, including equal and proportional allocation schemes. The document also discusses the problem of thrashing that can occur if a process does not have enough frames allocated, resulting in high page fault rates and decreased system performance. Solutions proposed include using local replacement algorithms and providing enough frames to meet each process's working set size.

1. WWW.EASYDESIGN.COM
DESIGN BY
IMDAD HUSSAIN
Operatin Sysrem COMSATS Sahiwal
Virtual Memory
Alloction of Frames
Trashing

2. 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

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

4. 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

5. 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.

6. VITUAL MEMORY CONCEPT
The conceptual
separation of user
logical memory from
physical memory.
Thus we can have
large virtual memory
on a small physical
memory

7. TRANSFER OF A PAGED MEMORY TO CONTIGUOUS DISK SPACE

8. IMPLEMENTATION OF VIRTUAL MEMORY
• DEMAND PAGING
WHEN A PAGE IS TOUCHED, BRING IT FROM SECONDARY TO MAIN MEMORY
• DEMAND SEGMENTATION

9. 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.

10. 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

11. 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

12. ALLOCATION SCHEMES
•FIXED ALLOCATION
•PRIORITY ALLOCATION

13. 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 SIZEWILL BE: V = ΣV(P).
• ALLOCATE (V(P) /V) * N FRAMES TO PROCESS P.

14. 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

15. THRASHING
IF A PROCESS DOES NOT HAVE ‘‘ENOUGH’’ PAGES, THE PAGE-FAULT RATE IS VERY HIGH
• LOW CPU UTILIZATION
• OPERATING SYSTEM THINKS THAT IT NEEDSTO INCREASE THE DEGREE OF MULTIPROGRAMMING
• ANOTHER PROCESS ADDED TO THE SYSTEM.
•THRASHING= A PROCESS IS BUSY SWAPPING PAGES IN AND OUT.

16. EXAMPLE OF THRASHING
CPU
Utilization
Degree of
Multiprogramming
Trashing

17. SADVANTAGES
• INCREASES THE DEGREE OF MULTIPROGRAMMING
•
• SYSTEM THROUGHPUT DECREASES
• PAGE FAULT RATE INCREASES
• EFFECTIVE ACCESS TIME INCREASES

18. SOLUTION
1. 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

19. 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

20. 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