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Memory Management

Memory Management. How is the OS chose the location in the main memory to place an incoming process?. Process E 8 KB (computation heavy). Process D 8 KB (I/O heavy). Process H 2 KB. Process C 1 KB. Process G 2 KB. Process F 2 KB. Memory (18 KB). Process A 1 KB. Process B 1 KB.

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Memory Management

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  1. Memory Management How is the OS chose the location in the main memory to place an incoming process? Process E 8 KB (computation heavy) Process D 8 KB (I/O heavy) Process H 2 KB Process C 1 KB Process G 2 KB Process F 2 KB Memory (18 KB) Process A 1 KB Process B 1 KB

  2. Adjacent Placement • First-fit: Choose the first block that is large enough to hold the new process. • Best-fit: Chose the block that leaves the smallest amount of space unused when the new process is placed. • Worst-fit: Chose the block that leaves the largest amount of space unused when the new process is placed. 6 KB 6 KB 6 KB 6 KB 12 KB 14 KB 14 KB 14 KB 14 KB 19 KB 19 KB 19 KB 19 KB 11 KB 11 KB 11 KB 11 KB 13 KB 13 KB 13 KB 13 KB

  3. P1 P2 P3 P4 P5 P6 P7 Arrival time 0 12 18 25 29 3 7 CPU time 5 3 9 10 16 2 8 Memory req. 3 5 8 12 2 6 9 Example Memory Size: 35 units Operating system: 10 units

  4. Garbage Collection Over time as processes are placed and removed from the main memory, there would be a number of used and used areas in the main memory distributed in a random fashion Unused areas by themselves may not be sufficiently big enough to place an incoming process All used areas can be collected in one part of the main memory leaving one big chunk of unused area to place incoming processes This process is referred to as garbage collection operation 6 KB 48 KB 48 KB 76 KB 14 KB 19 KB 76 KB 64 KB 19 KB 10 KB 64 KB 51 KB 11 KB 10 KB 13 KB

  5. Garbage Collection Garbage collection operation requires CPU to read the instructions and data of each process and write them to another place in the main memory During this time CPU can not execute user programs Hence garbage collection is not effective process It is not a preferred operation either but has to be executed from time to time

  6. Paging Divide the main memory into fixed size blocks (e.g.,8 KB) Divide each process into same sized blocks as well Main memory Sayfa 1 In this model, it is not required that all the pages of a process be in main memory If the instruction to execute is not in the current page in the main memory a page fault is generated and the page is brought in to the main memory 8 KB Sayfa 2 16 KB Sayfa 3 24 KB Sayfa 4 32 KB Sayfa 5 40 KB Sayfa 6 48 KB Sayfa 7 Process 52 KB

  7. Paging Small or Large Block Size When a process is divided into fized size blocks (pages), the number of data in the last page can be smaller than the page size (i.e. A 51 KB process is divided into 8 KB blocks) and this unused area can NOT be used by another process (wasted) As the block size increases the amount of wasted space in the main memory increases As the page size decreases the number of page faults is expected to increase. Furthermore small page size would mean large number of entries to manage corresponding data structures (page tables, etc.)

  8. D-4 D-1 E-1 E-4 D-7 D-5 D-2 E-2 E-5 E-7 E-8 D-3 E-3 D-8 D-6 E-6 Process C 1024 Proses E Process A 1024 Proses D F-1 Process B 1024 F-2 Proses F G-1 Proses G G-2 H-1 Proses H H-2

  9. First Last LLT LLT for A LLT for B LLT for C LLT for D A B C D E F G H LLT for E LLT for F LLT for G LLT for H Process/Page Table

  10. Paging Algorithms • The objective is to minimize the page faults • Work fast and use minimal resources to execute • Optimum Algorithm • Replace the page that will not be used for the longest timeBUT we do not know this • Used to determine how well a proposed algoritm work 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 1 1 4 Four pages can be Hold in the main Memory simultanously 2 2 2 3 3 3 4 5 5

  11. First Come First Served 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 3 pages in the main memory 1 2 3 4 1 1 1 2 5 5 2 3 4 1 2 2 2 5 3 3 3 4 1 2 5 5 5 3 4 4 The number of page faults increases as the number of pages in the memory increases Four pages in the main memory 2 3 4 1 2 1 5 1 1 2 3 4 5 2 3 2 2 1 3 3 3 4 5 1 3 4 2 4 4 4 5 1 2 3 4 5

  12. Least Recently Used (LRU) • Her sayfa için bir sayaç kullanılır. Bir sayfaya referans geldiğinde sistem saati bu alana yazılır • Ana bellekten sayacı en küçük olan sayfayı çıkar 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 4 pages in the main memory 4 4 1 2 5 1 4 2 3 1 2 5 2 3 2 1 3 4 3 2 5 1 3 4 1 2 4 2 4 1 5 1 2 3 4 5

  13. 1, 2, 5, 4, 1, 3, 5, 4, 2, 1, 3, 5 1 1 5 1 5 5 3 3 3 3 2 5 4 5 4 4 5 5 5 5 5 4 3 4 3 3 2 1 1 1 1 1 2 5 4 3 5 4 2 1 2 3 5 4 1 5 4 2 1 3 5 5 4 1 3 4 2 1 3 5

  14. Vald/Invalid Bit • Use a valid/invalid bit for every page in the page table(1: in memory; 0: not in memory) • Inıtiazie all valid/invalid bits to 0 • If the address of the instruction is in a page and the valid/invalid bit of that page is 0, generate an interrupt • When a page is written to the main memory, set the corresponding valid/invalid bit to 1 Valid/Invalid bit Page no 1 Page no 0 Page no 0 Page no 1 0 Page no Page Table

  15. Page Table/Page Fault 1 0 1 2 3 4 5 6 7 4 0 1 2 3 4 5 6 7 A B C DEFG H • A reference to an instruction in a new page causes an interrupt • OS looks at the table and makes a decision. If it is an illegal reference, ends thew process. Otherwise brings in the new page to the memory • Find an unused page in the main memory • Read the page from disk and write it to main memory Set the valid bit in the page table to 1 0 4 A 6 1 0 6 C A B 0 9 1 C D E . . . 0 0 F G 9 F Virtual Memory Page Table H Main memory

  16. Page Fault Process 1 0 1 2 3 4 5 6 7 4 0 3. Page is İn disk A B 1. Referans 6 1 2. Trap Operating system Load page H 0 C D E 0 6. Restartinstruction 9 1 F G 0 0 Page Table H 4 A 5. Update Page table 6 C 4. Bring the page To the main memory . . . 9 F Main memory

  17. When there is no empty page in the main memory? • Page replacement- find some page in memory but not really in use and swap it out • Note: Page replacement completes seperation between logical and physical memory.. Large virtual memory can be provided on a smaller physical memory • Find the location of the desired page on disk • 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 • Read the desired page into the (newly) free frame, update the page and frame tables • Restart the process

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