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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 10.1

Silberschatz, Galvin, and Gagne  1999

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 10.2

Silberschatz, Galvin, and Gagne  1999

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 10.3

Silberschatz, Galvin, and Gagne  1999

Consistency Problem -- Whenever data is replicated --

Dirty

A 1. read A 2. modified A -- 최신정보 Applied Operating System Concepts 10.4

Invalid

A 1. read A 2. modified • Strong / Weak consistency • Write through / Write back • Pull / Push • Overhead vs QoS Silberschatz, Galvin, and Gagne  1999

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 10.5

Silberschatz, Galvin, and Gagne  1999

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: 1.

Invalid reference? eg bad address, protection violation …? Just not in memory? Then continue….

3. Get an empty page frame. ( 없으면 뺏어 올 것 - replace)  abort process.

4. Read the page into frame from disk.

이 disk I/O 가 끝나기 까지 이 프로세스는 Disk read 가 끝나면 CPU 를 preempt 당함 (wait) page tables entry 기록 , validation bit = “valid”.

다시 CPU ready queue 에 process 를 insert – dispatch later 이 프로세스가 아까 중단됬던 CPU 에 다시 run – 위의 instruction 을 다시 시작 page fault trap 이 이제야 끝난 것임 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 – Paging system 이 성능이 좋게 되는 근거임 reference 하는 program behavior Applied Operating System Concepts 10.6

Silberschatz, Galvin, and Gagne  1999

실제

HW design

의 어려움 When does Page Fault occur?

on instruction fetch

on operand fetch

– restart 후 다시해야  (instruction fetch, decode, operand-fetch) 3.

최악의 경우  한 기계어가 “여러 location 을 변화”시키는 도중에 발생 – 예 : “ block copy” instruction: copy count from_address to_address source destination – 중간에서 page fault 가 나면 ?

 Undo 시켜야 함 (block copy 자체를 )  그러려면 관련된 값들을 관리하는 HW 가 추가로 필요 Applied Operating System Concepts 10.7

Silberschatz, Galvin, and Gagne  1999

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 10.8

Silberschatz, Galvin, and Gagne  1999

Performance of Demand Paging

• Page Fault Rate 0 

 1.0

– if

= 0 no page faults – if

= 1, every reference is a fault • Effective Access Time (EAT) EAT = (1 –

) x memory access +

( [OS & HW page fault overhead] + [swap page out if needed ] + [swap page in] + [OS & HW restart overhead] ) Applied Operating System Concepts 10.9

Silberschatz, Galvin, and Gagne  1999

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 10.10

Silberschatz, Galvin, and Gagne  1999

First-In-First-Out (FIFO) Algorithm

• • Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

frames (3 pages can be in memory at a time per process) 1 2 1 2 4 1 5 3 ( 1, 2, 3, 4, 1, 2, 5 , 1, 2, 3, 4 , 5) 9 page faults 3 3 2 4 More page faults?

•

frames 1 1 5 4 2 2 1 5 3 3 2 4 4 3 • FIFO Replacement – Belady’s Anomaly – more frames  less page faults Applied Operating System Concepts 10.11

10 page faults ( 1, 2, 3, 4, 1, 2 , 5, 1, 2, 3, 4, 5 ) Silberschatz, Galvin, and Gagne  1999

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 1 2 3 6 page faults 4 • • How do you know this?

Used for measuring how well your algorithm performs. ( 비교기준용 ) Applied Operating System Concepts 10.12

Silberschatz, Galvin, and Gagne  1999

Least Recently Used (LRU) Algorithm

• Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 1 2 3 4 • 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 Applied Operating System Concepts 10.13

Silberschatz, Galvin, and Gagne  1999

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 into page 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 10.14

Silberschatz, Galvin, and Gagne  1999

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 예 OS : 00000000 10101010 – – 가 이 8 비트를 shift (to high order) 한번도 안사용됨 홀수 주기마다 사용됨 Applied Operating System Concepts 10.15

Silberschatz, Galvin, and Gagne  1999

(

계 속

)

• 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 – – 한바퀴 되돌아와서도 ( 그림 (second chance) 0 10.13 중간 비트들 참조 이면 그때에는 replace 당함 ) – – 자주 사용되는 페이지라면 최악의 경우 모든 bit 이 1 second chance 이면 FIFO 가 됨 가 올때 1 • Enhanced Second chance 빈번사용 ? I/O 유발 ?

– Not-Referenced, not-modified – Referenced, modified 첫번째로 가장 나중 replace replace Applied Operating System Concepts 10.16

Silberschatz, Galvin, and Gagne  1999

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 10.17

Silberschatz, Galvin, and Gagne  1999

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

– SW 측면 :   Loop 내의 page 는 한꺼번에 그렇지 않으면 매 loop 마다 allocate 되는 것이 유리함 page fault – CPU/disk load 심한 불균형 ?

• Two major allocation schemes.

– fixed allocation – priority allocation Applied Operating System Concepts 10.18

Silberschatz, Galvin, and Gagne  1999

Fixed Allocation

• • Equal allocation – e.g., (100 frames, 5 processes) 20 pages each.

Proportional allocation – Allocate according to the size of process.

s S m i

 size of process

p i

 

s i

 total number of frames

a i

 allocation for

p i



s i S



m m

 64

s i

 10

2  127

1  10 137  64  5

2  127 137  64  59 Applied Operating System Concepts 10.19

Silberschatz, Galvin, and Gagne  1999

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 10.20

Silberschatz, Galvin, and Gagne  1999

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() { for (I=1, 100000) { A = B + X }} main() X B Applied Operating System Concepts 10.21

Silberschatz, Galvin, and Gagne  1999

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 10.22

Silberschatz, Galvin, and Gagne  1999

Working-Set Model

•   working-set window  a fixed number of page references Example: 10,000 instruction •

WS(t i

) = { pages referenced in [

t i,

, – if 

t i

 ] } too small will not encompass entire locality.

– if  – if  too large will encompass several localities.

=   will encompass entire program.

123123123248024802480248033666666336666666336666633666

• • • • •

WSS i

= Working set size for process P

i D

= 

WSS i

 total demand frames if

 Thrashing (m is total number of available frames) Policy if

> 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 Applied Operating System Concepts 즉 (WSS 전체 ) or (suspended) 중 택일 10.23

Silberschatz, Galvin, and Gagne  1999

Working-Set Model

• •

WS(t i

) = { pages referenced in [

t i,

t i

 ] } 만일 페이지 P 가

t i

에

WS(t i

) 에 속하였으면 keep in memory 안 속하였으면 out of memory • • • • 이 원칙에 따라 replace, allocate 를 결정 따라서 working set model 은 allocate/replace 를 같이 결정함 시시로 allocation size 가 달라질 수 있음

WS(t i

) 가 모두 보장되어야만 run, 아니면 suspend. • “ 시간 ” 개념 – Process (virtual) time – 이 프로세스가 – Real time (wall-clock time) run 할 동안만 운행 Applied Operating System Concepts 10.24

Silberschatz, Galvin, and Gagne  1999

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,

.ref

“1” clk “0” Timer interrupts: every 5K time units.

– Extra 2 bits for each page.

copy

reset – 5K ref 마다 timer interrupt : (2 bit 를 LEFT shift) (Right bit  ZERO) , – Reference 되면 가장 right-most bit 를 – (If one of the bits = 1)  1 로 set 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?

Applied Operating System Concepts 10.25

Silberschatz, Galvin, and Gagne  1999

Page-Fault Frequency Scheme

• Establish “acceptable” page-fault rate.

– If actual rate too low, process loses frame. – If actual rate too high, process gains frame.

(or dec  ) (or inc  ) Applied Operating System Concepts 10.26

Silberschatz, Galvin, and Gagne  1999

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 10.27

Silberschatz, Galvin, and Gagne  1999

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 이 커질수록 – 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 10.28

Silberschatz, Galvin, and Gagne  1999

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 P

old

requests disk I/O to page A, wait for I/O – CPU is given to P

new,

– P

new

generates page fault, replaces page A – disk for P

old

is ready, DMA begins transfer to page A for P

old

– 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 10.29

Silberschatz, Galvin, and Gagne  1999