Understanding Virtual Memory Management in Operating Systems

1. Rensselaer Polytechnic Institute CSCI-4210 – Operating Systems David Goldschmidt, Ph.D. Operating Systems{week 13}

2. Noncontiguous allocation (i) • A noncontiguous memory allocation scheme avoids the external fragmentation problem • Slice up physical memory intofixed-sized blocks called frames • Sizes typically range from 216 and up! • Slice up logical memory intofixed-sized blocks called pages • Allocate pages into frames • Note that frame size equals page size

3. process Pi == in use main memory == free Noncontiguous allocation (ii) • When a process of size n pages is ready to run, operating system finds n free frames • The OS keepstrack of pagesvia a page table

4. Paging via a page table • Page tables map logical memoryaddresses to physical memoryaddresses

5. page offset page number p d Address translation • Covers a logical addressspace of size 2m withpage size 2n (m – n) (n)

6. Address translation

7. Translation look-aside buffer • Use page tablecaching at thehardware levelto speed addresstranslation • Hardware-leveltranslation look-aside buffer(TLB)

8. Virtual memory (i) • Only part of a process needs to be loaded into memory for it to start its execution • Virtual memory further separates logical memory and physical memory • Logical (or virtual) address space can be larger than physical address space • Allows physical address space to be sharedby several processes • Enables quicker process creation

9. Virtual memory (ii) • Unused pages are stored on disk

10. Shared libraries • Multiple processes can share common libraries or data by mapping virtual pages to shared physicalpages • More efficientuse of physicalmemory space

11. Demand paging • When a page of memory is requested, if it’s not in physical memory,load page from disk • i.e. on demand • Less I/O required • Less physical memory • Faster user responsetimes (usually) • More user processes

12. Virtual memory policies • Virtual memory policies include: • The fetch policy governs when a pageshould be loaded into memory • The placement policy specifies where apage is loaded into physical memory • The replacement policy determines whichexisting page in memory should be replaced

13. Page-to-process allocation • Page allocation: • In a static allocation scheme,the number of frames per process is fixed • In a dynamic allocation scheme,the number of frames per process varies • In an equal allocation scheme, all processeshave an equal number of frames • In a proportional allocation scheme, processesare allocated frames in proportion to size, priority, etc.

14. Virtual memory page table • Associate a valid/invalid bit witheach page table entry • Initially set all entries to i • During address translation,if valid/invalid bit is v, pageis already in memory • Otherwise, if bit is i,a page fault occurs valid/invalid bit frame # v v v v i …. i i page table

15. Page faults (i) • Page faults are trapped by the OS: • When an invalid reference occurs in the page table • When a page is not yet in the page table • Page fault recovery: • Get free frame from physical memory • Swap desired page into free frame • Reset page table entry • Set validation bit to v • Restart instruction that caused the page fault

16. Page faults (ii) • Page faultsare costly!

17. Page faults (iii)

18. Demand paging performance (i) • The page fault rate p is in the range [0.0, 1.0]: • If p is 0.0, no page faults occur • If p is 1.0, every page request is a page fault • Typically p is very low.... • The effective memory-access time is • (1 – p) xphysical-memory-access +px ( page-fault-overhead +swap-page-out + swap-page-in +restart-overhead )

19. Demand paging performance (ii) • Given: • Memory access time is 200 nanoseconds • Average page-fault service time is 8 milliseconds • The effective memory access time is • (1 – p) x 200ns + px 8ms = • 200ns – 200ns p + px 8,000,000ns = • 200ns + 7,999,800 p

20. Thrashing (i) • If a process does not have enough pages, the page-fault rate is high, leading to thrashing • Process is busy swapping pagesin and out of memory • Low CPU utilization • Operating system might thinkit needs to increase the degreeof multiprogramming! • More processes added, further degrading performance

21. Thrashing (ii) • Remember the Principle of Locality

22. Principle of locality (i) • Future memory references in a given process will likely be local to previous memory references • This phenomenon is calledthe principle of locality • A process executes ina series of phases, spendinga finite amount of time performingmemory references in each phase

23. Principle of locality (ii) • Example graph of page faults versus total number of allocated frames

24. Principle of locality (iii) • Operating system should allocate enough frames for the current locality of a process: • What happens when too fewframes are allocated? • What happens whentoo many frames are allocated?

25. Principle of locality (iv) • Example of asingle process:

26. Principle of locality (v) • Dynamic page fault frequency scheme

27. Page replacement • How do weidentify the victim?

28. Page replacement algorithms • Page replacement algorithms include: • First-in-first-out (FIFO) • Optimal (OPT) • Least recently used (LRU) • Least frequently used (LFU) • Page fault frequency scheme (introduced earlier) • Working set apply these algorithms to apage reference stream

29. First-in-first-out (FIFO) algorithm • The above is a 3-frame memory • How many page faults occur if we usea 4-frame memory instead?

30. Belady’s anomaly (i) • Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 • How many page faults occur with a 3-frame memory? • How many page faults occur with a 4-frame memory?

31. Belady’s anomaly (ii) • Belady’s Anomaly: • More frames may lead to more page faults!

32. Optimal (OPT) algorithm • Replace pages that will not be used forthe longest amount of time FIFO: OPT:

33. Least recently used (LRU) algorithm • Replace pages that have not been used forthe longest amount of time

34. Least frequently used (LFU) algorithm • Similar to LRU, but replaceleast frequently used pages • Requires usage counts • Initial page replacements areswapped out quickly becausetheir usage counts are 1

35. Working set algorithm • For each process, maintain a working set of pages referenced by the process during the most recent w page references • How do we choose w?

36. Total demand of frames • Let d be the sum of the sizes of the working sets of all active processes • Let F be the total number of frames • If d < F, then the operating system can allow additional processes to enter the system • If d > F, then the operating system must suspend one or more active processes • Otherwise thrashing will occur!

37. Windows XP example (i) • Windows XP uses demand paging with clustering (which loads pages surrounding the page fault) • Processes are assigned a working set minimum and a working set maximum • The working set minimum is the minimum number of pages a process is guaranteed to have in physical memory • Likewise, a process may be assigned pages up to its working set maximum

38. Windows XP example (ii) • When the amount of free memory in the system falls below a threshold, automatic working set trimming is performed • Increases the amountof free memory • Removes pages fromprocesses that have pagesin excess of their workingset minimum