1. Chapter 4 Memory Management 4.1 Basic memory management 4.2 Swapping
4.3 Virtual memory
4.4 Page replacement algorithms
4.5 Modeling page replacement algorithms
4.6 Design issues for paging systems
4.7 Implementation issues
4.8 Segmentation

2. Memory Management Ideally programmers want memory that is
large
fast
non volatile
Memory hierarchy
small amount of fast, expensive memory – cache
some medium-speed, medium price main memory
gigabytes of slow, cheap disk storage
Memory manager handles the memory hierarchy

3. Basic Memory Management Monoprogramming without Swapping or Paging
Three simple ways of organizing memory
- an operating system with one user process

4. Multiprogramming with Fixed Partitions
Fixed memory partitions
separate input queues for each partition
single input queue

5. Modeling Multiprogramming
Degree of multiprogramming
CPU utilization as a function of number of processes in memory

6. Uso da CPU Fazendo 80% I/O:
chance de 1 processo usar CPU: = 0.2
chance de 2 processos usarem a CPU: *0.8 = 0.36
chance de N processos usarem a CPU:
** N

7. Analysis of Multiprogramming System Performance
Arrival and work requirements of 4 jobs
CPU utilization for 1 – 4 jobs with 80% I/O wait
Sequence of events as jobs arrive and finish
note numbers show amout of CPU time jobs get in each interval

8. Tempo de uso de CPU 1 processo, tempo 0 -10
- rodou 10minutos * 0.2 = 2 minutos
2 processos, tempo 10-15
Cada um rodou
5 minutos * 0.18 = 0.90 minutos

9. Relocation and Protection
Cannot be sure where program will be loaded in memory
address locations of variables, code routines cannot be absolute
must keep a program out of other processes’ partitions
Use base and limit values
address locations added to base value to map to physical addr
address locations larger than limit value is an error

10. Minix Relocacao e protecao via registradores base e limite.
Precisa de MMU.
Como fica o fork, exec e exit no minix?

11. Swapping (1) Memory allocation changes as
processes come into memory
leave memory
Shaded regions are unused memory

12. Swapping (2) Allocating space for growing data segment
Allocating space for growing stack & data segment

13. minix
Minix nao usava swapping, nem virtual memory.

14. Memory Management with Bit Maps
Part of memory with 5 processes, 3 holes
tick marks show allocation units
shaded regions are free
Corresponding bit map
Same information as a list

15. Memory Management with Linked Lists
Four neighbor combinations for the terminating process X

16. De onde pega memoria livre?
first fit
next fit
best fit
worst fit
Como isso funciona no minix ao fazer fork, exec, exit?

17. Virtual Memory Paging (1)
The position and function of the MMU

18. Paging (2)
The relation between virtual addresses and physical memory addres- ses given by page table

19. Page Tables (1)
Internal operation of MMU with 16 4 KB pages

20. memoria virtual ldr r0, =0x1000 ldr r0,[r0]
em processo 1, eh diferente de
em processo 2.

21. tabela de paginas Onde se encontra?
dentro da MMU
fora da MMU
quais os problemas para dentro e fora da MMU?
resposta: ANOTEM.

22. tabela de paginas Temos: resposta:
uma tabela de paginas para o sistema?
uma tabela de paginas por processo?
resposta:

23. entradas na tabela de paginas
endereco de 32 bits.
Vamos separar em paginas de 4Kbytes; ou
20 bits - numero de paginas
12 bits - paginas de 4Kbytes.
Helloword precisa de tabela de paginas com 1M entradas.

24. Page Tables (2) 32 bit address with 2 page table fields
Second-level page tables
Top-level
page table
32 bit address with 2 page table fields
Two-level page tables

25. Page Tables (3) Typical page table entry
Por que nao temos o numero do processo?

26. TLBs – Translation Lookaside Buffers
A TLB to speed up paging

27. TLB memoria associativa dentro da MMU ausencia do pid na TLB, por que?
miss, hit
no miss, qual entrada sai? Quem decide? hw? sw?

28. Inverted Page Tables
Comparison of a traditional page table with an inverted page table

29. tab pags invertida uma por sistema ou uma por processo? resp: ANOTE
entrada
PID + virtual page number

30. Page Replacement Algorithms
Page fault forces choice
which page must be removed
make room for incoming page
Modified page must first be saved
unmodified just overwritten
Better not to choose an often used page
will probably need to be brought back in soon

31. FIFO Page Replacement Algorithm
Maintain a linked list of all pages
in order they came into memory
Page at beginning of list replaced
Disadvantage
page in memory the longest may be often used

32. Modeling Page Replacement Algorithms Belady's Anomaly
FIFO with 3 page frames
FIFO with 4 page frames
P's show which page references show page faults

33. Optimal Page Replacement Algorithm
Replace page needed at the farthest point in future
Optimal but unrealizable
Estimate by …
logging page use on previous runs of process
although this is impractical

34. Modeling Page Replacement Algorithms Belady's Anomaly
FIFO with 3 page frames
FIFO with 4 page frames
P's show which page references show page faults

35. Not Recently Used Page Replacement Algorithm
Each page has Reference bit, Modified bit
bits are set when page is referenced, modified
Pages are classified
not referenced, not modified
not referenced, modified
referenced, not modified
referenced, modified
NRU removes page at random
from lowest numbered non empty class

36. FIFO Page Replacement Algorithm
Maintain a linked list of all pages
in order they came into memory
Page at beginning of list replaced
Disadvantage
page in memory the longest may be often used

37. Second Chance Page Replacement Algorithm
Operation of a second chance
pages sorted in FIFO order
Page list if fault occurs at time 20, A has R bit set (numbers above pages are loading times)

38. The Clock Page Replacement Algorithm

39. Least Recently Used (LRU)
Assume pages used recently will used again soon
throw out page that has been unused for longest time
Must keep a linked list of pages
most recently used at front, least at rear
update this list every memory reference !!
Alternatively keep counter in each page table entry
choose page with lowest value counter
periodically zero the counter

40. Simulating LRU in Software (1)
LRU using a matrix – pages referenced in order 0,1,2,3,2,1,0,3,2,3

41. Simulating LRU in Software (2)
The aging algorithm simulates LRU in software
Note 6 pages for 5 clock ticks, (a) – (e)

42. working set
paginas referenciadas: 1, 2, 2, 1, 2, 4, 5 nos tempos: 1,2,3,4,5,6,7,8,
w(k,t) = paginas referenciadas entre t-k e t
exemplo:
w(2,3) = {2}; size(w(2,3)) = 1;
w(3,3) = {1,2}; size(w(3,3)) = 2;

43. The Working Set Page Replacement Algorithm (1)
The working set is the set of pages used by the k most recent memory references
w(k,t) is the size of the working set at time, t

44. The Working Set Page Replacement Algorithm (2)
The working set algorithm

45. The WSClock Page Replacement Algorithm
WSClock algorithm; 2202->2204

46. Review of Page Replacement Algorithms

47. Design Issues for Paging Systems Local versus Global Allocation Policies (1)
Original configuration
Local page replacement
Global page replacement

48. Local versus Global Allocation Policies (2)
Page fault rate as a function of the number of page frames assigned

49. Load Control Despite good designs, system may still thrash
When PFF algorithm indicates
some processes need more memory
but no processes need less
Solution : Reduce number of processes competing for memory
swap one or more to disk, divide up pages they held
reconsider degree of multiprogramming

50. Page Size (1) Small page size Advantages Disadvantages
less internal fragmentation
better fit for various data structures, code sections
less unused program in memory
Disadvantages
programs need many pages, larger page tables

51. Exercicio 4.12
Below is the listing of a short assembly language program for a computer with 512- byte pages. The program is located at address 1020, and its stack pointer is at (the stack grows toward 0). Give the page reference string generated by this program. Each instruction occupies 4 bytes (1 word), and both instruction and data references must appear in the reference string.

52. Exercicio 4.12 minix3 Load word 6144 into register 0
Push register 0 onto the stack
Call a procedure at 5120, stacking the return address
Subtract the immediate constant 16 from the stack pointer
Compare the actual parameter to the immediate constant 4
Jump if equal to 5152

53. internal fragmentation
Page Size (2)
Overhead due to page table and internal fragmentation
Where
s = average process size in bytes
p = page size in bytes
e = page entry in bytes
s/p = no. de entradas na tab. de paginas
s/p. e = espaco ocupado pela tab. de pags.
s = 1MB, e = 8B -> p = 4KB.
page table space
internal fragmentation
Optimized when

54. Separate Instruction and Data Spaces
One address space por processo.
Separate I and D spaces por processo.

55. Two processes 1, 2 sharing same program sharing its page table
Shared Pages
Two processes 1, 2 sharing same program sharing its page table

56. Sharing data copy on write
Processo 1 fork -> processo 2
dados sao identicos para ambos os processos e sao compartilhados como read only.
Soh deixa de compartilhar quando alguem modifica -> viola o read only, excessao duplica dado.

57. shared libraries windows - DLL, Dynamic Link Libraries.
programa linkado com um stub e nao com a rotina na biblioteca de fato.
Problema de relocacao: 2 processos compartilhando DLL colocam mapeiam DLL em posicoes diferentes de memoria virtual; (fisica eh mesma).
solucao: usar instrucoes de jump relativo

58. Cleaning Policy Need for a background process, paging daemon
periodically inspects state of memory
When too few frames are free
selects pages to evict using a replacement algorithm
It can use same circular list (clock)
as regular page replacement algorithm

59. Implementation Issues Operating System Involvement with Paging
Four times when OS involved with paging
Process creation
determine program size
create page table
Process execution
MMU reset for new process
TLB flushed
Page fault time
determine virtual address causing fault
swap target page out, needed page in
Process termination time
release page table, pages

60. Page Fault Handling (1) Hardware traps to kernel
General registers saved
OS determines which virtual page needed
OS checks validity of address, seeks page frame
If selected frame is dirty, write it to disk

61. Page Fault Handling (2) OS brings schedules new page in from disk
Page tables updated
Faulting instruction backed up to when it began
Faulting process scheduled
Registers restored
Program continues

62. An instruction causing a page fault
Instruction Backup
An instruction causing a page fault

63. Locking Pages in Memory
Virtual memory and I/O occasionally interact
Proc issues call for read from device into buffer
while waiting for I/O, another processes starts up
has a page fault
buffer for the first proc may be chosen to be paged out
Need to specify some pages locked
exempted from being target pages

64. Backing Store (a) Paging to static swap area
(b) Backing up pages dynamically

65. Separation of Policy and Mechanism
Page fault handling with an external pager
onde coloca o page replacement algorithm?

66. Segmentation (1) One-dimensional address space with growing tables
One table may bump into another

67. Allows each table to grow or shrink, independently
Segmentation (2)
Allows each table to grow or shrink, independently

68. Comparison of paging and segmentation

69. Implementation of Pure Segmentation
(a)-(d) Development of checkerboarding
(e) Removal of the checkerboarding by compaction

70. Segmentation with Paging: MULTICS (1)
Descriptor segment points to page tables
Segment descriptor – numbers are field lengths

71. Segmentation with Paging: MULTICS (2)
A 34-bit MULTICS virtual address

72. Segmentation with Paging: MULTICS (3)
Conversion of a 2-part MULTICS address into a main memory address

73. Segmentation with Paging: MULTICS (4)
Simplified version of the MULTICS TLB
Existence of 2 page sizes makes actual TLB more complicated

74. Segmentation with Paging: Pentium (1)
A Pentium selector

75. Segmentation with Paging: Pentium (2)
Pentium code segment descriptor
Data segments differ slightly

76. Segmentation with Paging: Pentium (3)
Conversion of a (selector, offset) pair to a linear address

77. Segmentation with Paging: Pentium (4)
Mapping of a linear address onto a physical address

78. Segmentation with Paging: Pentium (5)
Level
Protection on the Pentium