1 Virtual Memory in the Real World Implementing exact LRU Approximating LRU Hardware Support Clock Algorithm Thrashing Cause Working Set

2 Implementing Exact LRU On each reference, time stamp page When we need to evict: select oldest page = least-recently used A, B, C, B, C, C, D

3 Implementing Exact LRU On each reference, time stamp page When we need to evict: select oldest page = least-recently used A1A1 A, B, C, B, C, C, D

4 Implementing Exact LRU On each reference, time stamp page When we need to evict: select oldest page = least-recently used A1A1 B2B2 A, B, C, B, C, C, D

5 Implementing Exact LRU On each reference, time stamp page When we need to evict: select oldest page = least-recently used A1A1 B2B2 C3C3 A, B, C, B, C, C, D

6 Implementing Exact LRU On each reference, time stamp page When we need to evict: select oldest page = least-recently used A1A1 B4B4 C3C3 A, B, C, B, C, C, D

7 Implementing Exact LRU On each reference, time stamp page When we need to evict: select oldest page = least-recently used A1A1 B4B4 C5C5 A, B, C, B, C, C, D

8 Implementing Exact LRU On each reference, time stamp page When we need to evict: select oldest page = least-recently used A1A1 B4B4 C6C6 A, B, C, B, C, C, D

9 Implementing Exact LRU On each reference, time stamp page When we need to evict: select oldest page = least-recently used A1A1 B4B4 C6C6 A, B, C, B, C, C, D D7D7 LRU page How should we implement this?

10 Implementing Exact LRU: Data Structures Hash table: optimize the common case (memory hit) Location of a page in memory: apply hash function to a page number update: O(1), eviction: O(n) (n: # of pages to process) Expensive: on every reference, compute hash of page address; update time stamp doubly-linked list Move items to front when referenced LRU items at end of list Still too expensive: Linear lookup time to find a page 4-6 pointer updates per reference

11 Virtual Memory in the Real World Implementing exact LRU Approximating LRU Reference-bit algorithm Clock Algorithm Thrashing Cause Working Set

12 Reference-bit algorithm Hardware support: reference bit Maintain reference bit for every page On each access, set reference bit to 1 Periodically resets reference bits Evict page with reference bit = 0

13 Reference-bit algorithm: example Maintain reference bit for every page On each access, set reference bit to 1 Periodically resets reference bits Evict page with reference bit = 0 A1A1 B1B1 C1C1 A, B, C, B, C, C, D

14 Reference-bit algorithm: example Maintain reference bit for every page On each access, set reference bit to 1 Periodically resets reference bits Evict page with reference bit = 0 A0A0 B0B0 C0C0 A, B, C, B, C, C, D reset reference bits

15 Reference-bit algorithm: example Maintain reference bit for every page On each access, set reference bit to 1 Periodically resets reference bits Evict page with reference bit = 0 A0A0 B1B1 C0C0 A, B, C, B, C, C, D

16 Reference-bit algorithm: example Maintain reference bit for every page On each access, set reference bit to 1 Periodically resets reference bits Evict page with reference bit = 0 A0A0 B1B1 C1C1 A, B, C, B, C, C, D

17 Reference-bit algorithm: example Maintain reference bit for every page On each access, set reference bit to 1 Periodically resets reference bits Evict page with reference bit = 0 A0A0 B1B1 C1C1 A, B, C, B, C, C, D

18 Reference-bit algorithm: example Maintain reference bit for every page On each access, set reference bit to 1 Periodically resets reference bits Evict page with reference bit = 0 A0A0 B1B1 C1C1 A, B, C, B, C, C, D D1D1

19 Virtual Memory in the Real World Implementing exact LRU Approximating LRU Reference-bit algorithm Clock Algorithm Thrashing Cause Working Set

20 The Clock Algorithm Set reference bit to 1 for an access Consider frames in circle Pointer: 0: initially Only advance pointer when page fault happens On page fault, OS: Repeat Checks reference bit of the current pointer If reference bit = 0, replace page, set bit to 1; advance pointer to next frame; break; If reference bit = 1, set bit to 0, advance pointer to next frame

21 The Clock Algorithm: example A, B, C, D, B, C, E, F, C, G

22 The Clock Algorithm: Summary Variant of FIFO & LRU LRU: ? FIFO: ? No need to reset reference bit periodically

23 Enhancing Clock Idea: favor eviction of unmodified pages We don’t write back unmodified pages Extend hardware to keep another bit: modified bit Total order of tuples: (ref bit, mod bit) (0,0), (0,1), (1,0), (1,1) Evict page from lowest nonempty class

24 Page Replacement in Enhanced Clock OS may scan multiple times: Page (0,0) – replace that page Page (0,1) – replace and write out page Page (1,0) – replace that page Page (1,1) – replace and write out page Fast, but still coarse approximation of LRU

25 Virtual Memory in the Real World Implementing exact LRU Approximating LRU Reference-bit algorithm Clock Thrashing Cause Working Set

26 Thrashing A process is busy swapping pages in and out; no useful work is done low CPU utilization OS adds processes → even more page swapping & lower CPU utilization

27 Cause of Thrashing Locality A set of pages that are actively used together Process migrates from one locality to another Total locality of processes > total memory size Process cannot keep in memory all pages that it is currently using

28 Working Set Strategy Working set = pages referred in last  references (approximate of locality) OS monitors working set of each process; allocate enough frames to process Another process can be started if there are enough extra frames Suspend process(es) if the sum of working-set sizes exceeds RAM

29 Working Set Problems Algorithm relies on key parameter,  How do we set  ? Is there one correct  ? Different processes have different timescales over which they refer pages Not acceptable (or necessarily possible) to suspend processes altogether Not really used Very rough variant used in Windows