VIRTUAL MEMORY MANAGEMENT

1. VIRTUAL MEMORY MANAGEMENT

2. DEMAND PAGING • Pages loaded on demand is known as demand paging • Never bring a page into memory until it is required • VM handler loads only one page of a program to start with. • Three concepts are important in understanding operation of demand paging • Page fault • Page in and page out • Page replacement

3. Page fault • When a program tries to access locations that are not in memory, the hardware traps to operating system (page fault). • The operating system reads desired page into memory and restarts the process as though the page had always been in memory

4. Steps handling page fault • Check the page table to determine whether the reference is valid or invalid memory access • If the reference was invalid, terminate the process. If it was valid, but have not yet brought in that page, page in the latter. • Find a free frame(from the free frame list) • schedule a disk operation to read the desired page into the newly allocated frame • When the disk read is complete, modify the internal table kept with the process and the page table to indicate that the page is now in memory. • Restart the instruction that was interrupted by the illegal address trap. The process can now access the page as though it had always been in memory

5. Page In & Page Out • While initiating execution of a program, an area is allocated on the paging device for its legal address space and its code and data are copied into the space. This space is known as swap space of a program. • When a page fault occurs during reference to a page say Pi, the VM handler finds a free page frame in memory and loads Pi in it. This is known as Page In operation for Pi. • If no free frame exist in memory some page Pk existing in memory is written out into the swap space to free its page frame. This is a Page Out operation for Pk. • Page I/O. • The term Page Traffic is used to describe movements of pages in and out of memory.

6. Page I/O Swap space of the program Pk Page being Written out Page out operation Pi Page being loadedin Page in operation • A program which encounters a page fault becomes blocked till • The required page is loaded in memory. Hence the execution • Performance of the program suffers

7. Transfer of a Paged Memory to Contiguous Disk Space

8. Valid-Invalid Bit • With each page table entry a valid–invalid bit is associated(v in-memory,i  not-in-memory) • Initially valid–invalid bit is set to i on all entries • Example of a page table snapshot:During address translation, if valid–invalid bit in page table entry is I  page fault Frame # valid-invalid bit v v v v i …. i i page table

9. Page Table When Some Pages Are Not in Main Memory

10. Page Fault • If there is a reference to a page, first reference to that page will trap to operating system: page fault • Operating system looks at another table to decide: • Invalid reference  abort • Just not in memory • Get empty frame • Swap page into frame • Reset tables • Set validation bit = v • Restart the instruction that caused the page fault

11. Steps in Handling a Page Fault

12. Page Replacement • Prevent over-allocation of memory by modifying page-fault service routine to include page replacement • Use modify (dirty) bitto 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

13. Need For Page Replacement

14. Basic Page Replacement • 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 • Bring the desired page into the (newly) free frame; update thepage and frame tables • Restart the process

15. Page Replacement

16. Page Replacement Algorithms • 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 0100, 0432, 0101,0612,0102,0103,0104,0101,0611, 0102,0103,0104,0101,0610,0103,0104,0101,0609 100 bytes per page is reduced to following reference string 1,4,1,6,1,6,1,6,1,6

17. Belady’s anomaly • It is found that under FIFO page replacement, certain page reference pattern cause more page faults when the number of page frames allocated to a process is increased • By Belady, Shedler and Nelson • This phenomenon is known as FIFO anomaly or Belady’s anomaly • Try doing the following eg: • Reference string 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 Page fault for 4 frames and 3 frames

18. Graph of Page Faults Versus The Number of Frames

19. First-In-First-Out (FIFO) Algorithm • 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 • Belady’s Anomaly: more frames  more page faults 1 1 4 5 2 2 1 3 9 page faults 3 3 2 4 1 1 5 4 2 2 1 10 page faults 5 3 3 2 4 4 3

20. FIFO Page Replacement

21. Optimal Algorithm • The principle of optimality states that to obtain optimum performance, replace the page that will not be used for a 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 1 4 2 6 page faults 3 4 5

22. Optimal Page Replacement

23. Least Recently Used (LRU) Algorithm • 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 1 1 1 5 1 2 2 2 2 2 5 4 3 4 5 3 3 4 3 4

24. LRU Page Replacement

25. LRU Algorithm- using stack • Stack implementation – keep a stack of page numbers in a double link form: • Page referenced: • move it to the top • requires 6 pointers to be changed • No search for replacement

26. Use Of A Stack to Record The Most Recent Page References