CS 423 Operating System Design: Virtual Memory Mgmt Professor - PowerPoint PPT Presentation

CS 423 
 Operating System Design: Virtual Memory Mgmt Professor Adam Bates Spring 2018 CS 423: Operating Systems Design

Goals for Today • Learning Objective: • Navigate the history of memory systems in OS • Announcements, etc: • MP1 Due Tonight! • MP2 Out later This Week Reminder : Please put away devices at the start of class CS 423: Operating Systems Design 2

Storage Hierarchy CPU Registers Size Performance 32-64 bits Cache 4-128 words Memory 512-16k words Secondary Storage CS 423: Operating Systems Design 3

Problem Statement We have limited amounts of fast resources, and large amounts of slower resources… How to create the illusion of an abundant fast resource? CS 423: Operating Systems Design 4

History: Mem Overlays Secondary Storage 0K Overlay 1 Main Program 5k Overlay Manager Overlay 2 7k Overlay Area Overlay 3 12k Used when process memory requirement exceeded the physical memory space 5 CS 423: Operating Systems Design

History: Mem Overlays Secondary Storage 0K Overlay 1 Main Program 5k Overlay Manager Overlay 2 7k Overlay 1 Overlay 3 12k Used when process memory requirement exceeded the physical memory space 6 CS 423: Operating Systems Design

History: Mem Overlays Secondary Storage 0K Overlay 1 Main Program 5k Overlay Manager Overlay 2 7k Overlay Area Overlay 3 12k Used when process memory requirement exceeded the physical memory space 7 CS 423: Operating Systems Design

History: Mem Overlays Secondary Storage 0K Overlay 1 Main Program 5k Overlay Manager Overlay 2 7k Overlay 2 Overlay 3 12k Used when process memory requirement exceeded the physical memory space 8 CS 423: Operating Systems Design

History: Mem Overlays Secondary Storage 0K Overlay 1 Main Program 5k Overlay Manager Overlay 2 7k Overlay Area Overlay 3 12k Used when process memory requirement exceeded the physical memory space 9 CS 423: Operating Systems Design

History: Mem Overlays Secondary Storage 0K Overlay 1 Main Program 5k Overlay Manager Overlay 2 7k Overlay 3 Overlay 3 12k Used when process memory requirement exceeded the physical memory space 10 CS 423: Operating Systems Design

History: Mem Overlays Secondary Storage 0K Overlay 1 Main Program 5k Overlay Manager Overlay 2 7k Overlay Area Overlay 3 12k Used when process memory requirement exceeded the physical memory space 11 CS 423: Operating Systems Design

History: Mem Overlays Secondary Storage 0K Overlay 1 Main Program 5k Overlay Manager Overlay 2 7k Overlay 1 Overlay 3 12k Used when process memory requirement exceeded the physical memory space 12 CS 423: Operating Systems Design

History: Fixed Partitions 0k • Approach: Multiprogramming 4k with fixed memory partitions • Divides memory into n fixed partitions (possible unequal) 16k • Problem? 64k Free Space 128k CS 423: Operating Systems Design 13

History: Fixed Partitions 0k • Approach: Multiprogramming 4k with fixed memory partitions Program 1 • Divides memory into n fixed partitions (possible unequal) • Problems? 16k Program 2 64k Free Space Program 3 128k CS 423: Operating Systems Design 14

History: Fixed Partitions 0k • Approach: Multiprogramming 4k with fixed memory partitions Program 1 • Divides memory into n fixed partitions (possible unequal) • Problems? 16k • Internal Fragmentation! Also, Program 2 • Level of Multiprogramming 64k Free Space Program 3 128k CS 423: Operating Systems Design 15

History: Fixed Partition Allocation Placement Algorithms for Fixed Partitions ■ Trivial for equal size partitions. For unequal… ■ Multiple Queues: ■ Assign (i.e., enqueue) each incoming job to the smallest partition within which it fits ■ Decreases fragmentation ■ when the queue for a large partition is empty but the queue for a small partition is full. Small jobs have to wait to get into memory even though plenty of memory is free. ■ Single Queue: ■ Assign each process to the smallest available partition within which it fits ■ Increases amount of multiprogramming on the expense of fragmentation CS 423: Operating Systems Design 16

History: Relocation ■ Correct starting address when a program should start in the memory ■ Different jobs will run at different addresses When a program is linked, the linker must know at what address the ■ program will begin in memory. ■ Logical addresses Logical address space , range (0 to max) ■ Physical addresses, Physical address space range (R+0 to R+max) for ■ base value R. User program never sees the real physical addresses ■ ■ Relocation register Mapping requires hardware with the base register ■ CS 423: Operating Systems Design 17

History: Relocation Register Base Register BA Physical Logical CPU Address Address + Instruction Memory MA MA+BA Address Relocation => “Variable Partition Allocation” CS 423: Operating Systems Design 18

History: Variable Partition Allocation 1 Monitor Job 1 Job 2 Job 3 Job 4 Free 2 Monitor Job 1 Job 3 Job 4 Free 3 Monitor Job 1 Job 5 Job 3 Job 4 Free Free Monitor Job 5 Job 3 Job 4 Job 6 4 Free Monitor Job 7 Job 5 Job 3 Job 8 Job 6 5 Memory wasted by External Fragmentation CS 423: Operating Systems Design 19

Bad Parking Analogy Source: https://xkcd.com/562/ CS 423: Operating Systems Design 20

History: Storage Placement Strategy ■ Best Fit ■ Use the hole whose size is equal to the need, or if none is equal, the hole that is larger but closest in size. ■ Problem: Creates small holes that can't be used. ■ Worst Fit? ■ Use the largest available hole. ■ Problem: Gets rid of large holes making it difficult to run large programs. ■ First Fit ■ Use the first available hole whose size is sufficient to meet the need. ■ Problem: Creates average size holes. ■ Next Fit . ■ Minor variation of first fit: search from the last hole used. ■ Problem: slightly worse performance than first fit. CS 423: Operating Systems Design 21

History: Example Policies • Allocate 12K block • Red is allocated 8K • Green is free 12K 26K 8K 20K Last allocation 8K 20K CS 423: Operating Systems Design 22

History: Example Policies • Allocate 12K block • Red is allocated 8K • Green is free 12K 26K 8K Best Fit 20K Last allocation 8K 20K CS 423: Operating Systems Design 23

History: Example Policies • Allocate 12K block • Red is allocated 8K • Green is free 12K Worst Fit 26K 8K Best Fit 20K Last allocation 8K 20K CS 423: Operating Systems Design 24

History: Example Policies • Allocate 12K block • Red is allocated 8K • Green is free 12K Worst Fit 26K First Fit 8K Best Fit 20K Last allocation 8K 20K CS 423: Operating Systems Design 25

History: Example Policies • Allocate 12K block • Red is allocated 8K • Green is free 12K Worst Fit 26K First Fit 8K Best Fit 20K Last allocation 8K Next Fit 20K CS 423: Operating Systems Design 26

History: Summary Fixed Overlay Relocation Partitions • No multi- • Supports multi- • No internal programming programming fragmentation • Internal • Introduces support fragmentation external fragmentation CS 423: Operating Systems Design 27

Virtual Memory ■ Provide user with virtual memory that is as big as user needs ■ Store virtual memory on disk ■ Cache parts of virtual memory being used in real memory ■ Load and store cached virtual memory without user program intervention CS 423: Operating Systems Design 28

Paging Page Table VM Frame Memory 1 2 3 4 1 2 3 4 Virtual Memory Stored on Disk 1 2 3 4 5 6 7 8 29 CS 423: Operating Systems Design

Paging Request Page 3… Page Table VM Frame Memory 3 1 2 3 4 1 2 3 4 Virtual Memory Stored on Disk 1 2 3 4 5 6 7 8 30 CS 423: Operating Systems Design

Paging Request Page 1… Page Table VM Frame Memory 3 1 1 2 3 4 1 2 3 4 Virtual Memory Stored on Disk 1 2 3 4 5 6 7 8 CS 423: Operating Systems Design 31

Paging Request Page 6… Page Table VM Frame Memory 3 1 1 2 6 3 4 1 2 3 4 Virtual Memory Stored on Disk 1 2 3 4 5 6 7 8 CS 423: Operating Systems Design 32

Paging Request Page 2… Page Table VM Frame Memory 3 1 1 2 6 3 2 4 1 2 3 4 Virtual Memory Stored on Disk 1 2 3 4 5 6 7 8 CS 423: Operating Systems Design 33

Paging Request Page 8. Swap Page 1 to Disk First… Page Table VM Frame Memory 3 1 1 2 6 3 2 4 1 2 3 4 Virtual Memory Stored on Disk 1 2 3 4 5 6 7 8 CS 423: Operating Systems Design 34

Paging Request Page 8. … now load Page 8 into Memory. Page Table VM Frame Memory 3 1 8 2 6 3 2 4 1 2 3 4 Virtual Memory Stored on Disk 1 2 3 4 5 6 7 8 CS 423: Operating Systems Design 35

Page Mapping Hardware Virtual Memory Virtual Address (P,D) Page Table P P D 0 D 1 4 Contents(P,D) 0 1 P → F 1 0 Physical Memory 1 F D F Physical Address (F,D) D Contents(F,D) CS 423: Operating Systems Design 36