Virtual Memory Concepts, Lecture Slide - Computer Science, Slides of Computer System Design and Architecture

Download Virtual Memory Concepts, Lecture Slide - Computer Science and more Slides Computer System Design and Architecture in PDF only on Docsity!

Virtual Memory: Concepts

15-­‐213: Introduc0on to Computer Systems

th

Lecture, Oct. 14, 2010

Instructors:

Randy Bryant and Dave O’Hallaron

Today

 Address spaces

 VM as a tool for caching

 VM as a tool for memory management

 VM as a tool for memory protec;on

 Address transla;on

A System Using Virtual Addressing

 Used in all modern servers, desktops, and laptops

 One of the great ideas in computer science

M-­‐1:

Main memory

MMU

Physical address (PA) Data word 8: ...

CPU

Virtual address (VA) CPU Chip

Address Spaces

 Linear address space: Ordered set of con0guous non-­‐nega0ve integer

addresses:

 Virtual address space: Set of N = 2

n

virtual addresses

{0, 1, 2, 3, …, N-­‐1}

 Physical address space: Set of M = 2

m

physical addresses

{0, 1, 2, 3, …, M-­‐1}

 Clean dis;nc;on between data (bytes) and their aVributes (addresses)

 Each object can now have mul;ple addresses

 Every byte in main memory:

one physical address, one (or more) virtual addresses

Today

 Address spaces

 VM as a tool for caching

 VM as a tool for memory management

 VM as a tool for memory protec;on

 Address transla;on

VM as a Tool for Caching

 Virtual memory is an array of N con;guous bytes stored

on disk.

 The contents of the array on disk are cached in physical

memory ( DRAM cache )

 These cache blocks are called pages (size is P = 2

p

bytes)

PP 2m-­‐p-­‐

Physical memory

Empty Empty Uncached

VP 0

VP 1

VP 2n-­‐p-­‐

Virtual memory

Unallocated Cached Uncached Unallocated Cached Uncached

PP 0

PP 1

Empty Cached 0 N-­‐ M-­‐ 0

Virtual pages (VPs)

stored on disk

Physical pages (PPs)

cached in DRAM

Page Tables

 A page table is an array of page table entries (PTEs) that

maps virtual pages to physical pages.

 Per-­‐process kernel data structure in DRAM

null null

Memory resident

page table

(DRAM)

Physical memory

(DRAM)

VP 7

VP 4

Virtual memory

(disk)

Valid

Physical page

number or

disk address

PTE 0

PTE 7

PP 0

VP 2

VP 1

PP 3

VP 1

VP 2

VP 4

VP 6

VP 7

VP 3

Page Hit

 Page hit: reference to VM word that is in physical memory

(DRAM cache hit)

null null

Memory resident

page table

(DRAM)

Physical memory

(DRAM)

VP 7

VP 4

Virtual memory

(disk)

Valid

Physical page

number or

disk address

PTE 0

PTE 7

PP 0

VP 2

VP 1

PP 3

VP 1

VP 2

VP 4

VP 6

VP 7

VP 3

Virtual address

Handling Page Fault

 Page miss causes page fault (an excep0on)

null null

Memory resident

page table

(DRAM)

Physical memory

(DRAM)

VP 7

VP 4

Virtual memory

(disk)

Valid

Physical page

number or

disk address

PTE 0

PTE 7

PP 0

VP 2

VP 1

PP 3

VP 1

VP 2

VP 4

VP 6

VP 7

VP 3

Virtual address

Handling Page Fault

 Page miss causes page fault (an excep0on)

 Page fault handler selects a vic0m to be evicted (here VP 4)

null null

Memory resident

page table

(DRAM)

Physical memory

(DRAM)

VP 7

VP 4

Virtual memory

(disk)

Valid

Physical page

number or

disk address

PTE 0

PTE 7

PP 0

VP 2

VP 1

PP 3

VP 1

VP 2

VP 4

VP 6

VP 7

VP 3

Virtual address

Handling Page Fault

 Page miss causes page fault (an excep0on)

 Page fault handler selects a vic0m to be evicted (here VP 4)

 Offending instruc0on is restarted: page hit!

null null

Memory resident

page table

(DRAM)

Physical memory

(DRAM)

VP 7

VP 3

Virtual memory

(disk)

Valid

Physical page

number or

disk address

PTE 0

PTE 7

PP 0

VP 2

VP 1

PP 3

VP 1

VP 2

VP 4

VP 6

VP 7

VP 3

Virtual address

Locality to the Rescue Again!

 Virtual memory works because of locality

 At any point in ;me, programs tend to access a set of ac;ve

virtual pages called the working set

 Programs with beger temporal locality will have smaller working sets

 If (working set size < main memory size)

 Good performance for one process aier compulsory misses

 If ( SUM(working set sizes) > main memory size )

 Thrashing: Performance meltdown where pages are swapped (copied)

in and out con0nuously

19

VM as a Tool for Memory Management

 Key idea: each process has its own virtual address space

 It can view memory as a simple linear array

 Mapping func0on scagers addresses through physical memory

 Well chosen mappings simplify memory alloca0on and management

Virtual Address Space for Process 1: Physical Address Space (DRAM)

N-­‐

(e.g., read-­‐only library code) Virtual Address Space for Process 2:

VP 1

VP 2

N-­‐

VP 1

VP 2

PP 2

PP 6

PP 8

M-­‐

Address

translaHon

20

VM as a Tool for Memory Management

 Memory alloca;on

 Each virtual page can be mapped to any physical page

 A virtual page can be stored in different physical pages at different 0mes

 Sharing code and data among processes

 Map virtual pages to the same physical page (here: PP 6)

Virtual Address Space for Process 1: Physical Address Space (DRAM)

N-­‐

(e.g., read-­‐only library code) Virtual Address Space for Process 2:

VP 1

VP 2

N-­‐

VP 1

VP 2

PP 2

PP 6

PP 8

M-­‐

Address

translaHon