CS4500/5500 Operating Systems Page Replacement Algorithms and Segmentation

Transcription

1 Operating Systems Page Replacement Algrithms and Segmentatin Yanyan Zhuang Department f Cmputer Science UC. Clrad Springs Ref. MOSE, OS@Austin, Clumbia, Rchester

2 Recap f last Class Virtual memry Memry verlad What if the demanded memry page is nt in memry? } Page fault What if sme ther pages need t be kicked ut? } Page replacement algrithms A single virtual address per prcess Multiple virtual addresses à segmentatin Ref. MOSE, OS@Austin, Clumbia, Rchester

3 Page Replacement Algrithms Like cache miss, a page fault frces chice f Which page must be remved Make rm fr incming pages Mdified page must first be saved Mdified/Dirty bit Unmdified just verwritten Better nt t chse an ften used page Will prbably need t be brught back sn } Tempral lcality Metric Lw page-fault rate Ref. MOSE, OS@Austin, Clumbia, Rchester

4 Optimal Page Replacement Algrithm Ref. MOSE, Clumbia, Rchester Replace page needed at the farthest pint in future Optimal but unrealizable OS has t knw when each f the pages will be referenced next Gd as a benchmark fr cmparisn } Take tw runs: first run gets trace, and secnd run uses trace fr the replacement } Still, it is nly ptimal with respect t that specific prgram Reference string: page faults

5 Nt Recently Used (NRU) Each page has R bit (referenced) and M bit (mdified) Bits are set when page is referenced and mdified OS clears R bits peridically (by clck interrupts) Pages are classified 0. nt referenced, nt mdified. nt referenced, mdified. referenced, nt mdified. referenced, mdified NRU remves a page at randm Frm the lwest numbered nn-empty class } Better remve a mdified page that hasn t been referenced in at least ne clck tick (typically ~0ms) than a clean page that is still in use Perfrmance nt ptimal, but adequate Class ccurs when a class page has its R bit cleared by a clck interrupt 5 Ref. MOSE, OS@Austin, Clumbia, Rchester

6 FIFO Page Replacement Algrithm Maintain a linked list f all pages in the rder they came int memry Page at beginning f list replaced (the ldest ne): n page fault, the page at head remved, new page added t tail Disadvantage page in memry the lngest (ldest) may be ften used Reference string: page faults Ref. MOSE, OS@Austin, Clumbia, Rchester

7 Secnd Chance Page Replacement Algrithm OS clears R bits peridically (by clck interrupts) R bit is cleared, page is put nt end f list, lad time is updated Secnd chance (FIFO-extended): lks fr an ldest and nt referenced page in the previus clck interval; if all referenced, FIFO (a) Pages srted in FIFO rder (numbers abve pages are lading times); (b) Page list if a page fault ccurs at time 0, and A has R bit set (c) What if A has R bit cleared? Unnecessarily inefficient because it is cnstantly mving pages arund n its list 7 Ref. MOSE, OS@Austin, Clumbia, Rchester

8 The Clck Page Replacement Algrithm Hand pints t the ldest page hand The clck page replacement algrithm differs Secnd Chance nly in the implementatin N need t mve pages arund n a list Instead, rganize a circular list as a clck, with a hand pints t the ldest page 8 Ref. MOSE, OS@Austin, Clumbia, Rchester

9 Least Recently Used (LRU) Assume pages used recently will be used again sn Thrw ut the page that has been least used recently Must keep a linked list f pages Mst recently used at frnt, least at rear Update this list every memry reference!!! } Finding, remving, and mving it t the frnt Special hardware: Equipped with a 6-bit cunter C Keep a cunter field in each page table entry } At each memry reference, C is updated fr the page referenced Chse page with lwest value cunter Expensive, but mre simulatin alternatives Reference string: page faults Ref. MOSE, OS@Austin, Clumbia, Rchester

10 Nt Frequently Used (NFU) NFU (Nt Frequently Used): uses a sftware cunter per page t track hw ften each page has been referenced, and chse the least t kick ut OS adds R bit (0 r ) t the cunter at each clck interrupt Prblem: never frgets anything A small mdificatin t NFU makes it able t simulate LRU quite well 0 Ref. MOSE, OS@Austin, Clumbia, Rchester

11 Aging - Simulating LRU/NFU in Sftware Aging: the cunters are each shifted right bit befre the R bit is added in; the R bit is then added t the leftmst The page whse cunter is the lwest is remved when a page fault ccurs The aging algrithm simulates LRU in sftware, 6 pages fr 5 clck ticks, (a) (e) Ref. MOSE, OS@Austin, Clumbia, Rchester

12 The Wrking Set and Pre-Paging Demand paging vs. pre-paging Wrking set: the set f pages that a prcess is currently using Thrashing: a prgram causing page faults every few instructins Observatin: wrking set des nt change quickly due t lcality Pre-paging wrking set fr prcesses in multiprgramming Example: 0,,, 5,, 5, Upn page fault: find a page that s nt in the wrking set and evict it The wrking set is the set f pages used by the k mst recent memry references w(k,t) is the size f the wrking set at time, t Ref. MOSE, OS@Austin, Clumbia, Rchester

13 The Wrking Set Page Replacement Algrithm instead f defining wrking set as pages used in previus k memry references, define it as pages used during past τ ms τ >> a clck tick age = (current virtual time) (time f last use) Upn page fault: The wrking set algrithm Ref. MOSE, OS@Austin, Clumbia, Rchester

14 The WSClck Page Replacement Algrithm Upn page fault: basic wrking set algrithm: the entire page table has t be scanned at each page fault If R bit is, R bit is set t 0, hand advanced t next page If R bit is 0, and age> τ, replace with new page Operatin f the WSClck algrithm Ref. MOSE, OS@Austin, Clumbia, Rchester

15 Review f Page Replacement Algrithms Optimal evicts the page that will be referenced furthest in the future there is n way t determine which page this is, s in practice this cannt be used. It is useful as a benchmark NRU divides pages int classes depending n the state f R/M bits. A randm page frm lwest-numbered class is chsen. Easy t implement, but very crude 5 Ref. MOSE, OS@Austin, Clumbia, Rchester

16 Review f Page Replacement Algrithms FIFO keeps track f the rder f pages by keeping them in a linked list. Remving the ldest page is trivial, but that page might still be in use. nd chance is a mdificatin t FIFO: checks if a page is in use befre remving. If it is, the page is spared. Greatly imprves perfrmance. Clck: a diff implementatin f nd chance, less time t execute 6 Ref. MOSE, OS@Austin, Clumbia, Rchester

17 Review f Page Replacement Algrithms LRU cannt be implemented withut special hardware. NFU is a crude attempt t apprx LRU: nt very gd. Aging is a better apprx t LRU, can be implemented efficiently Wrking set gives reasnable perfrmance, but expensive t implement. WSClck is a variant: gives gd perfrmance, als efficient t implement. 7 Ref. MOSE, OS@Austin, Clumbia, Rchester

18 Design Issues fr Paging Systems Lcal page replacement vs. glbal page replacement Hw memry shuld be allcated amng the cmpeting prcesses? Glbal algrithms wrk better (a) Original cnfiguratin. (b) Lcal page replacement. (c) Glbal page replacement. 8 Ref. MOSE, OS@Austin, Clumbia, Rchester

19 Design Issues fr Paging Systems () Lcal page replacement: static allcatin What if the wrking set f sme prcess grws? What if the wrking set f sme prcess shrinks? The cnsideratin: thrashing and memry utilizatin Glbal page replacement: dynamic allcatin Hw many page frames assigned t each prcess Lcal algrithms allcate every prcess a fixed - Keep mnitring the wrking set size fractin f memry. Glbal algrithms dynamically allcate page frames amng runnable prcesses - Allcating an equal share f available page frames - Allcating a prprtinal share f available page frames - Or hybrid allcatin, using PFF (page fault frequency) Lcal: wrking set grws, thrashing ccurs; wrking set shrinks, waste memry 9 Ref. MOSE, OS@Austin, Clumbia, Rchester

20 Page Fault Frequency (PFF) PFF: cntrl the size f allcatin set f a prcess When and hw much t increase r decrease a prcess page frame allcatin Replacement: what page frames t be replaced start each prcess up with sme # f pages prprtinal t prcess size, but allcatin updated dynamically as prcesses run page fault rate unacceptably high: faulting prcess is given mre page frames t reduce fault rate page fault rate lw: prcess has t much mem, page frames may be taken away frm it Page fault rate as a functin f the number f page frames assigned 0 Ref. MOSE, OS@Austin, Clumbia, Rchester

21 Lad Cntrl Despite gd designs, system may still have thrashing Whenever cmbined wrking sets f all prcesses exceed the capacity f memry When PFF algrithm indicates Sme prcesses need mre memry But n prcesses need less There is n way t give mre memry t prcesses withut hurting ther prcesses Slutin Swap ne r mre t disk, divide up pages amng thers Even with paging, swapping still needed: t reduce number f prcesses cmpeting fr memry Ref. MOSE, OS@Austin, Clumbia, Rchester

22 Page Size () Small page size Advantages Less unused prgram in memry (due t internal fragmentatin) Better fit fr varius data structures, cde sectins Disadvantages Prgrams need many pages, larger page tables Lng access time f page frames } Transfers t and frm the disk are generally a page at a time Als may have mre paging peratins due t page faults Ref. MOSE, OS@Austin, Clumbia, Rchester

23 Page Size () Tradeff: verhead due t page table and internal fragmentatin Let s = average prcess size in bytes p = page size in bytes e = page table entry size in bytes verhead se = + p page table space p internal fragmentatin s=m, e=8b à p=kb Optimized/minimized when f (p)=0 p = se Ref. MOSE, OS@Austin, Clumbia, Rchester

24 Separate Instructin and Data Spaces What if the single virtual address space is nt enugh fr bth prgram and data? Dubles the available virtual address space, and ease page sharing f multiple prcesses Bth address spaces can be paged, each has its wn page table One address space Separate I and D spaces Ref. MOSE, OS@Austin, Clumbia, Rchester

25 Shared Pages Hw t allw multiple prcesses t share the pages when running the same prgram at the same time? One prcess has its wn page table(s) Tw prcesses sharing same prgram sharing its I-page table 5 Ref. MOSE, OS@Austin, Clumbia, Rchester

26 Shared Pages () What t d when a page replacement ccurs t a prcess while ther prcesses are sharing pages with it? Minr page faults Prcesses A and B share pages Scheduler remves A frm memry à B will generate a large # f page faults t bring them back in Hw sharing data pages is cmpared t sharing cde pages? UNIX frk() and cpy-n-write A new page table per prcess, pinting t same set f pages Data pages are mapped as read-nly Nt duplicating pages until A vilatin f read-nly causes a trap A cpy is made f the ffending page Each cpy is read/write 6 Ref. MOSE, OS@Austin, Clumbia, Rchester

27 Cleaning Plicy Need fr a backgrund prcess, paging daemn Peridically inspects state f memry T ensure plenty f free page frames When t few frames are free Selects pages t evict using a replacement algrithm Write back plicy Write dirty pages back when the rati f dirty pages exceeds a threshld /prc/sys/vm/dirty_rati 7 Ref. MOSE, OS@Austin, Clumbia, Rchester

28 Implementatin Issues Fur times when OS invlved with paging: what has t be dne?. Prcess creatin - Create page table. Prcess executin - MMU reset fr new prcess - TLB flushed (as invalidating the cache). Page fault time - Determine virtual address causing fault - Evict target page, mve needed page in (restart faulting instructin). Prcess terminatin time - Release page table, pages 8 Ref. MOSE, OS@Austin, Clumbia, Rchester

29 Page Fault Handling. Hardware traps t kernel. General registers saved. OS determines which virtual page needed. OS checks validity f address, seeks page frame 5. If selected frame is dirty, write it t disk 6. OS brings the new page in frm disk 7. Page tables updated 8. Faulting instructin backed up t when it began 9. Faulting prcess scheduled 0. Registers restred. Prgram cntinues 9 Ref. MOSE, OS@Austin, Clumbia, Rchester

30 Backing Stre Disk Management Hw t allcate page space n the disk in supprt f virtual memry? Static swap area (pages cpied): adding the ffset f the page in the virtual address space t the start f the swap area; but prcess can increase size Dynamic swapping (page nt cpied, a table per-prcess needed) (a) Paging t static swap area. 0 (b) Backing up pages dynamically Ref. MOSE, OS@Austin, Clumbia, Rchester

31 Segmentatin () Why have tw r mre separate virtual address spaces? Hw t free a prgrammer frm the issues f expanding and cntracting tables? One-dimensinal address space with grwing tables fr cmpiling, ne table may bump/interfere int anther Ref. MOSE, OS@Austin, Clumbia, Rchester

32 Segmentatin () Segments: many independent virtual address spaces A lgical entity, knwn and used by the prgrammer Tw-dimensinal memry: the prgram must supply a tw-part address, a segment number and an address within an segment A segment des nt cntain a mixture f types: Different segments can have different prtectin Allws each table t grw r shrink, independently Ref. MOSE, OS@Austin, Clumbia, Rchester

33 Cmparisn f Segmentatin and Paging Ref. MOSE, OS@Austin, Clumbia, Rchester

34 Paged v.s. Segmented Virtual Memry Paged virtual memry Memry divided int fixed sized pages Segmented virtual memry Memry divided int variable length segments paging segmentatin Ref. MOSE, Clumbia, UCSD

35 Implementatin f Pure Segmentatin An essential difference f paging and segmentatin Segments have different sizes while pages are fixed size! If the segments are large: incnvenient r impssible t keep them in memry in their entirety Paging segments: nly thse pages f a segment that are actually needed have t be arund (a)-(d) Develpment f checkerbarding (external fragmentatin) in physical memry, if segments are small; (e) Remval f the checkerbarding by cmpactin 5 Ref. MOSE, OS@Austin, Clumbia, Rchester

36 Summary Page replacement algrithms Cmparisn? Design issues Implementatin issues Segmentatin Why? Linux 6 Ref. MOSE, OS@Austin, Clumbia, Rchester