SMD149 - Operating Systems - VM Management

Transcription

1 SMD149 - Operating Systems - VM Management Roland Parviainen November 17, / 35

2 Outline Overview Virtual memory management Fetch, placement and replacement strategies Placement strategies Paging, segmentation Demand and anticipatory paging strategies 2 / 35

3 Locality Overview Most strategies relies on locality Temporal and spatial locality Empirical property - altough reasonable Loops, functions, procedures: temporal Array traversals, sequential code execution, variables: spatial 3 / 35

4 Demand paging Overview At execution of process: load page with first instruction Load pages when explicitly referenced Only needed pages are loaded One page at a time Can create long wait times 4 / 35

5 Space time product Overview Execution time x space in main memory Measures time process is spent waiting and amount of space that can t be used 5 / 35

6 Overview 6 / 35

7 Anticipatory paging Overview Anticipatory paging (prefetching, prepaging) Predict what pages are needed Criteria: Prepaged allocation - main memory Number of pages at once Which pages Often combined with demand paging Load contiguous pages as needed Contiguous on disk? More efficient than demand paging? 7 / 35

8 OPT, RAND, FIFO, LRU, etc... Out of page frames: what page to replace? Generally tries to reduce page faults If page is modified (dirty), it need to be written to disk Flushing OPT/MIN Optimal Minimizes page faults In general, not possible Useful to compare against 8 / 35

9 RAND Random Page Replacement Easy, low overhead Fair Might replace wrong pages 9 / 35

10 FIFO First-In-First-Out Page Replacement Replace page that been in the system the longest Might be wrong page Uses age (not locality) Often perform worse than RAND VAX/VMS 10 / 35

11 FIFO Anomaly More page frames - fewer page faults? Not with some patterns with FIFO FIFO anomaly, Belady s anomaly Not common in real systems 11 / 35

12 FIFO Anomaly 12 / 35

13 LRU Least Recently Used Uses a process recent past behavior Replace the page that has been in memory the longest without being referenced List structure At page reference, move page to front Replace pages from end LRU: never more than N-times more page faults than OPT Near-optimal performance in theory Expensive to implement in practice N page frames, looping over N + 1 pages? Page fault on every access Switch to other strategy OS/390 uses global LRU approximation and falls back to random replacement when LRU performance degenerates. 13 / 35

14 LFU Least-Frequently-Used Least frequently used or least intensively referenced Counter updated each page reference Does not consider age 14 / 35

15 NUR Not-Used-Recently (Not-Recently-Used) Approximating LRU Two hardware bits: referenced/accessed and modified bit Initially: referenced = 0 Page types: Category 0: not referenced, not modified Category 1: not referenced, modified Category 2: referenced, not modified Category 3: referenced, modified All pages referenced eventually? 15 / 35

16 Second chance Modified FIFO Better, little cost Also uses the referenced bit When choosing a page (head of list) If referenced, move to back of list (turn of bit) If not, replace Modified pages must be flushed Less expensive than LRU Clock page replacement Circular list Replaces first not referenced page 16 / 35

17 Far Page Replacement Programs reference functions and data in predictable patterns Creates an access graph of pages Initially all pages are unreferenced Replace page furthest away from any referenced page Complex 17 / 35

18 18 / 35

19 Working set model Locality: only a subset of pages are necessary Working set theory of program behavior: what pages? Working set window size: w Working set: W(t, w) (from time t - w to t) Choice of w is important! Changes during execution 19 / 35

20 20 / 35

21 21 / 35

22 22 / 35

23 23 / 35

24 24 / 35

25 PFF Page fault frequency page replacement Ideal: between thrashing and no page faults PFF: change resident page set based on page fault frequency Lower overhead than working set model When page fault: compare time from last fault to an upper and a lower threshold less than lower: add page to resident page set more than upper: release unreferenced pages Corresponds to choice of w 25 / 35

26 Page release Pages that are not needed should be removed from working set Can take a long time Voluntary page release? 26 / 35

27 Page sizes Small or large page sizes? Large: Large range of memory in TLB Less time in I/O transfers Smaller page tables Small: Locality over small address space - less memory waste Less internal fragmentation Multiple page sizes 27 / 35

28 28 / 35

29 Global vs. Local Page Replacement : all processes or individual processes Global vs. Local Page Replacement Linux: global Windows XP: local 29 / 35

30 Linux page replacement Variation of second chance/clock algorithm approximate glru The memory manager uses two linked lists Active list Most-recently used pages are near head of active list Inactive list Least-recently used pages near tail of inactive list Only pages in the inactive list are replaced New page: inactive list, reference bit = 1 30 / 35

31 31 / 35

32 Windows page replacement Working set Pages a process currently has in main memory Balance set manager Responsible for trimming working sets Localized Least Recently Used Similar to LRU Localized by process 32 / 35

33 Localized LRU Trim all pages above working set maximum Place trimmed pages into Standby Page List Modified Page List Modified No-Write Page List Return to Valid Page List if process access page Otherwise place on Free Page List Zero bits of free pages Move pages to Zeroed Page List Allocate zeroed page to process that requests new page 33 / 35

34 34 / 35

35 Summary Next time: Disk Performance Optimization Hard drives Disk scheduling RAID 35 / 35