Virtual memory ,Allocaton of frame & Trashing
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This document summarizes key concepts of virtual memory including: - Virtual memory separates logical from physical memory allowing for larger virtual address spaces. - It uses demand paging to bring pages from disk to memory as needed. - Frame allocation schemes like equal and proportional allocation are used to assign frames to processes. - Thrashing occurs if a process does not have enough frames, causing high page fault rates and low CPU utilization. - The principle of locality explains why virtual memory works efficiently due to processes accessing only a small working set of pages over time.
Virtual memory ,Allocaton of frame & Trashing
- 1. WWW.EASYDESIGN.COM DESIGN BY IMDAD HUSSAIN Operatin Sysrem COMSATS Sahiwal
- 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