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Module 10: Virtual Memory

Module 10: Virtual Memory. Background Demand Paging Performance of Demand Paging Page Replacement Page-Replacement Algorithms Allocation of Frames Thrashing Other Considerations Demand Segmenation. Background. Virtual memory – separation of user logical memory

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Module 10: Virtual Memory

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  1. Module 10: Virtual Memory • Background • Demand Paging • Performance of Demand Paging • Page Replacement • Page-Replacement Algorithms • Allocation of Frames • Thrashing • Other Considerations • Demand Segmenation Applied Operating System Concepts

  2. Background • Virtual memory – separation of user logical memory from physical memory. • Only part of the program needs to be in memory for execution. • Logical address space can therefore be much larger than physical address space. • Need to allow pages to be swapped in and out. • Virtual memory can be implemented via: • Demand paging • Demand segmentation Applied Operating System Concepts

  3. Demand Paging • Bring a page into memory only when it is needed. • Less I/O needed • Less memory needed • Faster response • More users • Copy a page back to disk only when it is needed. • Dirty (modified) bit • Less I/O needed • Lazy write • Page is needed  reference to it • invalid reference  abort • not-in-memory  bring to memory Applied Operating System Concepts

  4. Consistency Problem-- Whenever data is replicated -- Invalid Dirty 1. read A A 1. read A A 2. modified 2. modified • Strong / Weak consistency • Write through / Write back • Pull / Push • Overhead vs QoS --- 최신정보 A Applied Operating System Concepts

  5. Valid-Invalid Bit • “Invalid” means both: • not-in-memory: this page has never been loaded from disk before • not legal: it is from disk, but-disk copy (original) has been updated eg: KAL reservation system: one global disk (본사) – N computers / 영업점 • Initially all entries are set to invalid • During address translation, if invalid bit is set  “page fault”. Applied Operating System Concepts

  6. Page Fault • Access to invalid page causes HW (MMU) trap – page fault trap • Trap handler is within OS address space  page fault handler is invoked • OS handles the page fault as follows: • Invalid reference? eg bad address, protection violation …?  abort process. • Just not in memory? Then continue…. • Get an empty page frame. (없으면 뺏어 올 것 - replace) • Read the page into frame from disk. • 이 disk I/O 가 끝나기 까지 이 프로세스는 CPU를 preempt 당함 (wait) • Disk read 가 끝나면 page tables entry 기록, validation bit = “valid”. • 다시 CPU ready queue에 process 를 insert – dispatch later • 이 프로세스가 CPU 에 다시 run – 위의 page fault trap이 이제야 끝난 것임 • 아까 중단됬던 instruction을 다시 시작 • Pure paging • Never swap-in until referenced – start program with no page in memory • Locality of reference • 실제 대부분의 프로그램에서 관찰되는 현상으로써 • 일정 시간에는 극히 일부 page 만 집중적으로 reference 하는 program behavior • Paging system 이 성능이 좋게 되는 근거임 Applied Operating System Concepts

  7. 실제 HW design의 어려움 When does Page Fault occur? • on instruction fetch • on operand fetch • restart 후 다시해야  (instruction fetch, decode, operand-fetch) • 최악의 경우 한 기계어가 “여러 location을 변화”시키는 도중에 발생 • 예: “block copy” instruction: copy count from_address to_address <op-code> <byte수> source destination • 중간에서 page fault 가 나면? • Undo 시켜야 함 (block copy 자체를) • 그러려면 관련된 값들을 관리하는 HW가 추가로 필요 Applied Operating System Concepts

  8. What happens if there is no free frame? • Page replacement – 빼앗아올 (replace) page 를 결정해야 • Will not be used soon? • Swap it out if necessary (if modified). • Algorithm for choosing target page for replacement • Goal – minimum number of page faults. • Same page may be brought into memory several times during run. Applied Operating System Concepts

  9. Performance of Demand Paging • Page Fault Rate 0  p  1.0 • if p = 0 no page faults • if p = 1, every reference is a fault • Effective Access Time (EAT) EAT = (1 – p) x memory access + p ( [OS & HW page fault overhead] + [swap page out if needed ] + [swap page in] + [OS & HW restart overhead] ) Applied Operating System Concepts

  10. Page-Replacement Algorithms • Want lowest page-fault rate. • Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string. • In all our examples, the reference string is 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5. Applied Operating System Concepts

  11. First-In-First-Out (FIFO) Algorithm • Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 • 3 frames (3 pages can be in memory at a time per process) (1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4,5) • 4 frames (1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5) • FIFO Replacement – Belady’s Anomaly • more frames  less page faults 1 1 4 5 2 2 1 3 9 page faults 3 3 2 4 More page faults? 1 1 5 4 2 2 1 10 page faults 5 3 3 2 4 4 3 Applied Operating System Concepts

  12. Optimal Algorithm • Replace page that will not be used for longest period of time. • 4 frames example 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 • How do you know this? • Used for measuring how well your algorithm performs. (비교기준용) 1 2 6 page faults 3 4 Applied Operating System Concepts

  13. Least Recently Used (LRU) Algorithm • Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 • 문제는 이것을 어떻게 implement 하느냐이다. • 있는 그대로 구현하려면, 매 memory reference 마다 • (페이지 마다 시간을 기록해야하고) extra memory (page table) traffic • (모든 페이지중 최소치 시간을 찾아내야 한다) • 즉 space, time 이 너무 많이 들어감 이렇게는 불가능함 • 여러 종류의 유사 implementation 방법들도 나옴 1 2 3 4 Applied Operating System Concepts

  14. LRU Algorithm (구현방법들) • Counter implementation • Every page entry has a counter; • CPU counter is incremented at every memory reference (logical clock) • CPU counter = 총 memory reference 수 • When page A is accessed, copy the CPU clock intopage A’s counter. • At replacement, search page table for minimum counter • Extra memory access (counter write time) – every memory access 마다 • Search (time) overhead – replacement 시 • Counter (space) overhead, …. • Stack implementation – keep a stack of page numbers in a double link form: • Page A referenced: • move page A to the top • requires 6 pointers to be changed • No search for replacement 참조됨 Applied Operating System Concepts

  15. LRU Approximation Algorithms • Reference bit • Initially -= 0 • 1 means page has been referenced • Replace the one which is 0 (if one exists). • We do not know the order, however. • Additional reference bit • 8 bits for reference bit. • 주기적으로 OS 가 이 8 비트를 shift (to high order) • 예: 00000000 – 한번도 안사용됨 • 10101010 – 홀수 주기마다 사용됨 . Applied Operating System Concepts

  16. (계속) • Second chance (clock) algorithm • Ref bit = 0 : never referenced • = 1 : yes, referenced • Circular queue of pages (이 프로세스에 속한) • Advance pointer until it finds reference bit 0 (never referenced) • 포인터 이동하는 중에 reference bit 1 은 모두 0 으로 바꿈 • (그림 10.13 중간 비트들 참조) • 한바퀴 되돌아와서도 (second chance) 0이면 그때에는 replace 당함 • 자주 사용되는 페이지라면 second chance 가 올때 1 • 최악의 경우 모든 bit 이 1 이면 FIFO 가 됨 • Enhanced Second chance 빈번사용? I/O 유발? • Not-Referenced, not-modified - 첫번째로 replace • Referenced, modified - 가장 나중 replace Applied Operating System Concepts

  17. Counting Algorithms • LFU Algorithm: • replaces page with smallest count. • MFU Algorithm: • based on the argument that the page with the smallest count was probably just brought in and has yet to be used. Applied Operating System Concepts

  18. Allocation of Frames • LRU 복습 • Page fault 발생시 • choose victim? (they assumed fixed allocation, not variable allocation) • Why not consider allocating one more page frame at page fault? • Allocation Problem – 매번 fault 시 마다 allocation size 다시 결정 • 참고: Each process needs minimum number of pages. • HW 측면: IBM 370 – 6 pages to handle SS MOVE instruction: • instruction is 6 bytes, might span 2 pages. • 2 pages to handle from. • 2 pages to handle to. • SW 측면: • Loop 내의 page 는 한꺼번에 allocate 되는 것이 유리함 • 그렇지 않으면 매 loop 마다 page fault – CPU/disk load 심한 불균형? • Two major allocation schemes. • fixed allocation • priority allocation Applied Operating System Concepts

  19. Fixed Allocation • Equal allocation – e.g., (100 frames, 5 processes) 20 pages each. • Proportional allocation – Allocate according to the size of process. Applied Operating System Concepts

  20. Priority Allocation • Use a proportional allocation scheme using priorities rather than size. • High priority process – give more memory – finish early Global vs. Local Replacement • Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another. • Local replacement – each process selects from only its own set of allocated frames. Applied Operating System Concepts

  21. Thrashing • If a process does not have “enough” pages, • the page-fault rate is very high. This leads to: • low CPU utilization. • operating system thinks that it needs to increase the degree of multiprogramming. • another process added to the system (higher MPD). • Thrashing  a process is busy swapping pages in and out. CPU is idle most of the time – low throughput A main() main() { for (I=1, 100000) { A = B + X }} X B Applied Operating System Concepts

  22. Thrashing Diagram • Why does paging work?Locality model • Process migrates from one locality to another. • Localities may overlap. • Why does thrashing occur?( size of locality) > (total allocated memory size) Applied Operating System Concepts

  23. Working-Set Model •   working-set window  a fixed number of page references Example: 10,000 instruction • WS(ti ) = { pages referenced in [ ti, , ti -  ] } • if  too small will not encompass entire locality. • if  too large will encompass several localities. • if  =   will encompass entire program. 123123123248024802480248033666666336666666336666633666 • WSSi = Working set size for process Pi • D =  WSSi  total demand frames • if D > m  Thrashing (m is total number of available frames) • Policy if D > m, then suspend one of the processes. • Working set model - If a process is not allocated memory greater than its WSS rather choose to be suspended  MPD 를 결정 함 즉 (WSS 전체) or (suspended) 중 택일 Applied Operating System Concepts

  24. Working-Set Model • WS(ti ) = { pages referenced in [ ti, , ti -  ] } • 만일 페이지 P 가 ti에 WS(ti )에 속하였으면 keep in memory 안 속하였으면 out of memory • 이 원칙에 따라 replace, allocate 를 결정 • 따라서 working set model은 allocate/replace 를 같이 결정함 • 시시로 allocation size 가 달라질 수 있음 • WS(ti ) 가 모두 보장되어야만 run, 아니면 suspend. • “시간” 개념 • Process (virtual) time – 이 프로세스가 run 할 동안만 운행 • Real time (wall-clock time) Applied Operating System Concepts

  25. Keeping Track of the Working Set • 구현: 매 ref 마다 각 page 들의 최근 reference time 을  와 비교?  too expensive (space for ref-time field, time for 비교) • Approximate with interval timer + a reference bit • Example:  = 10K, • Timer interrupts: every 5K time units. • Extra 2 bits for each page. • 5K ref 마다 timer interrupt : (2 bit를 LEFT shift) (Right bit  ZERO) , • Reference 되면 가장 right-most bit 를 1로 set • (If one of the bits = 1)  page belongs to the working set ( = 10K) . • Why is this not completely accurate? • Improvement = 10 bits and interrupt every 1000 time units. • Q: How do you decide window size? “1” 1.ref “0” clk 3. reset 2. copy Applied Operating System Concepts

  26. Page-Fault Frequency Scheme • Establish “acceptable” page-fault rate. • If actual rate too low, process loses frame. (or dec ) • If actual rate too high, process gains frame. (or inc ) Applied Operating System Concepts

  27. Page Management Policy Issues • Fetch policy • Prefetch • On-demand • Replacement policy • Recency based • Frequency based • … • Allocation policy • Equal vs unequal • Fixed vs dynamic Applied Operating System Concepts

  28. Other Considerations • Prepaging (prefetching, lookahead, ….) • 10.7.2. Page size selection • Internal fragmentation vs table fragmentation (table size) • Disk transfer efficiency – seek/rotation time vs. transfer time • I/O 횟수 • Improved Locality • Smaller page size isolate only needed info within page • Trend • Larger page size – speed gap 이커질수록 • DRAM: 7%/yr, Disk: 1800 5400 rpm/50yr • (CPU speed, memory capacity) – improves faster than disk speed. • Page fault (relative) penalty is becoming more costly these days. Applied Operating System Concepts

  29. Other Consideration (Cont.) • 10.7.4. Program structure • Locality of reference: reference 를 국지에 국한 • Data structure, program structure 가 영향을 준다 • 프로그램 언어 • C ----- pointers ------- random access (non-local variable) • Java --no pointers -- locality • Compiler, loader 도 영향을 준다 • program restructuring – 자주 쓰는 프로그램 • I/O interlock • 시나리오 Pold requests disk I/O to page A, wait for I/O • CPU is given to Pnew, • Pnew generates page fault, replaces page A • disk for Pold is ready, DMA begins transfer to page A for Pold . • Solution 1. No I/O with user space page – only to kernel buffer • then to user space (yes, extra copy) • 2. Lock the page reserved for disk I/O. Applied Operating System Concepts

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