CSE 3320 Operating Systems Page Replacement Algorithms and Segmentation Jia Rao

Transcription

1 CSE 0 Operating Systems Page Replacement Algrithms and Segmentatin Jia Ra Department f Cmputer Science and Engineering

2 Recap f last Class Virtual memry Memry verlad What if the demanded memry page is nt in memry? } Paging What if sme ther pages need t be kicked ut? } Page replacement algrithms A single virtual address per prcess Multiple virtual addresses à segmentatin

3 Page Replacement Algrithms Like cache miss, a page fault frces chice 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 in sn Tempral lcality Metrics Lw page-fault rate

4 Optimal Page Replacement Algrithm 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, the first run gets the trace, and the secnd run uses the 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. nt referenced, nt mdified. nt referenced, mdified. referenced, nt mdified 4. referenced, mdified NRU remves a page at randm Frm the lwest numbered nn-empty class

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) Disadvantage page in memry the lngest (ldest) may be ften used page faults Reference string:

7 Secnd Chance Page Replacement Algrithm OS clears R bits peridically (by clck interrupts) 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 (b) Page list if a page fault ccurs at time 0, and A has R bit set (numbers abve pages are lading times); (c) what if A has R bit cleared?

8 The Clck Page Replacement Algrithm 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

9 Least Recently Used (LRU) Assume pages used recently will be used again sn thrw ut 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 64-bit cunter keep a cunter field in each page table entry chse page with lwest value cunter peridically zer the cunter (NRU) And mre simulatin alternatives Reference string: page faults

10 LRU Supprt in Hardware Fr a RAM with n page frames, maintain a matrix f n x n bits; set all bits f rw k t, and then all bits f clumn k t 0. At any instant, the rw whse binary value is lwest is the least recently used. Page reference rder: 0 0

11 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 0r ) t the cunter at each clck interrupt Prblem: never frgets anything

12 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 The aging algrithm simulates LRU in sftware, 6 pages fr 5 clck ticks, (a) (e)

13 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 a few instructins Observatin: wrking set des nt change quickly due t lcality Pre-paging wrking set fr prcesses in multiprgramming Example: 0,,, 5,, 5, 4 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

14 The Wrking Set Page Replacement Algrithm The wrking set algrithm

15 The WSClck Page Replacement Algrithm Operatin f the WSClck algrithm

16 Review f Page Replacement Algrithms

17 Design Issues fr Paging Systems Lcal page replacement vs. glbal page replacement Hw is memry allcated amng the cmpeting prcesses? Glbal algrithms wrk better (a) Original cnfiguratin. (b) Lcal page replacement. (c) Glbal page replacement.

18 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 - Keep mnitring the wrking set size - Allcating an equal share f available page frames - Allcating a prprtinal share f available page frames - Or hybrid allcatin, using PFF (page fault frequency)

19 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 Page fault rate as a functin f the number f page frames assigned

20 Lad Cntrl Despite gd designs, system may still have thrashing When 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 Slutin : swap ne r mre t disk, divide up pages they held recnsider degree f multiprgramming } CPU-bund and I/O-bund mixing Reduce number f prcesses cmpeting fr memry

21 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 (cmpared t transfer time) Als maybe mre paging actins due t page faults

22 Page Size () Tradeff: verhead due t page table and internal fragmentatin Where s = average prcess size in bytes p = page size in bytes e = page entry size in bytes verhead se = + p page table space p internal fragmentatin s=m, e=8b à p=4kb Optimized/minimized when f (p)=0 p = se

23 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 addr. spaces can be paged, each has its wn page table One address space Separate I and D spaces

24 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

25 Shared Pages () What t d when a page replacement ccurs t a prcess while ther prcesses are sharing pages with it? Minr page faults. Hw sharing data pages, is cmpared t sharing cde pages? UNIX frk() and cpy-n-write Generating a new page table pint t the same set f pages, but nt duplicating pages until A vilatin f read-nly causes a trap

26 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

27 Implementatin Issues Fur times when OS invlved with paging. Prcess creatin - determine prgram size - create page table. Prcess executin - MMU reset fr new prcess - TLB flushed (as invalidating the cache). Page fault time - determine virtual address causing fault - swap target page ut, needed page in 4. Prcess terminatin time - release page table, pages

28 Page Fault Handling. Hardware traps t kernel. General registers saved. OS determines which virtual page needed 4. 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

29 Backing Stre Disk Management Hw t allcate page space n the disk in supprt f VM? Static swap area (pages cpied): adding the ffset f the page in the virtual address space t the start f the swap area Dynamic swapping (page nt cpied, a table-per-prcess needed) (a) Paging t static swap area. (b) Backing up pages dynamically

30 Segmentatin () Why t 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

31 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 Allws each table t grw r shrink, independently

32 Cmparisn f Segmentatin and Paging

33 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

34 Implementatin f Pure Segmentatin An essential difference f paging and segmentatin Segments have different sizes while pages are fixed size! (a)-(d) Develpment f checkerbarding (external fragmentatin) in physical memry, if segments are small; (e) Remval f the checkerbarding by cmpactin

35 Segmentatin with Paging: MULTICS What if the memry is nt large enugh fr a single segment? MULTICS (Hneywell 6000) Paged segment with wrd (4B) addressing Multi-dim VM up t ^8 segments, each up t 64K (-bit) wrds 4-bit virtual address (seg #, page #, page ffset) Physical memry 6M wrds (4-bit physical address) Page size: 04 wrds; r 64 wrds (64-wrd alignment) 8-bit segment number fr page table address A 4-bit MULTICS virtual address

36 MULTICS Virtual Memry One descriptr segment and 6-bit segment descriptrs What if the page table f a segment is nt in the memry? Descriptr segment pints t page tables. Segment descriptr.

37 MULTICS Virtual Address à Physical Address What if the descriptr segment is paged (ften it is)? Hw t speed up the searching &cnversin? Cnversin f a -part MULTICS address int a main memry address

38 MULTICS TLB MULTICS TLB Simplified versin f the MULTICS TLB (LRU replacement), which has 6 mst recently referenced pages.

39 Case Study: Linux Page Replacement Algrithm Page Frame Reclaiming Algrithm (PFRA) Unreclaimable Swappable SRC/include/linux/mm_types.h: page Syncable discardable

40 Case Study: Segmentatin in Linux and X86_ Linux uses segmentatin in a limited way Paging is simple and effective Prtability t ther architecture, e.g., RISC

41 Segmentatin in X86_ Hardware

42 Summary Page replacement algrithms Cmparisn? Design issues Implementatin issues Segmentatin Why? Linux Self-reading Instructin Backup Lcking Pages in Memry Separatin f Plicy and Mechanism Segmentatin with Paging: Pentium