Virtual Memory Philipp Koehn 25 April 2018 Philipp Koehn Computer - - PowerPoint PPT Presentation

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Virtual Memory Philipp Koehn 25 April 2018 Philipp Koehn Computer - - PowerPoint PPT Presentation

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Virtual Memory Philipp Koehn 25 April 2018 Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018 Recall: Process Address Space 1 Kernel memory ffffffff User stack Stack pointer Memory-mapped region for shared

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SLIDE 1

Virtual Memory

Philipp Koehn 25 April 2018

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 2

1

Recall: Process Address Space

Kernel memory User stack Memory-mapped region for shared libraries Run time heap (created by malloc) Read/write segment (.data / .bss) Read-only code segment (.init, .text., .rodata)

Stack pointer Loaded from executable 400000 ffffffff

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 3

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Virtual Memory

  • Abstraction of physical memory
  • Purpose

– appearance of more available memory than physically exists (DRAM) – handles disk caching / loading – insulates memory of each process

  • Page table:

maps from virtual address to physical addresses

  • Memory management unit (MMU):

hardware implementation of address translation

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 4

3

Warning

  • This is going to get very complex
  • Closely tied with multi-tasking (multiple processes)
  • Partly managed by hardware,

partly managed by software

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 5

4

virtual addressing

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 6

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Physical Addressing

CPU

…

0: 1: 2: 3: 4: 5: 6: 7:

Physical address (PA) Main memory Data CPU chip

  • So far, assumed CPU addresses physical memory

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 7

6

Virtual Addressing

CPU MMU

…

0: 1: 2: 3: 4: 5: 6: 7:

Address translation Virtual address (VA) Physical address (PA) CPU chip Main memory Data

  • Memory management unit (MMU): maps virtual to physical addresses

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 8

7

Address Space

  • Virtual memory size:

N = 2n bytes, e.g., 256TB

  • Physical memory size:

M = 2m bytes, e.g., 16GB

  • Page (block of memory):

P = 2p bytes, e.g., 4KB

  • A virtual address can be encoded in n bits

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 9

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caching

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 10

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Caching... Again?

  • Yes, we already discussed caching

but: for on-chip cache of DRAM memory

  • Now

– caching between RAM and disk – driven by a large virtual memory address space – to avoid unnecessary and duplicate loading

  • Jargon

– previously "block", now "page" – now: "swapping" or "paging"

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 11

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Mapping

empty empty empty empty empty Physical memory unallocated cached uncached cached unallocated uncached cached unallocated unallocated unallocated

…

0: 1: 2: 3: 4: 5: 6: 7:

Virtual memory

8: 15:

Virtual pages (VP) stored on disk Physical pages (PP) cached in DRAM

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 12

11

State of Virtual Memory Page

  • Cached

– allocated page – stored in physical memory

  • Uncached

– allocated page – not in physical memory

  • Unallocated

– not used by virtual memory system so far

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 13

12

Page Table

  • Array of page table entries (PTE)
  • Valid bit

– set if PTE currently maps to physical address (cached) – not set otherwise (uncached or unallocated)

  • Mapped address

– if cached: physical address in DRAM – if not cached: physical address on disk

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 14

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Page Table

VP1 VP6 VP3 VP7 VP0 VP1 VP2 VP3 Physical memory DRAM null null

0: 1: 2: 3: 4: 5: 6: 7:

Page table Disk 1 1 1 1 VP4 VP5 VP6 VP7 Valid Address

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 15

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Page Hit

VP1 VP6 VP3 VP7 VP0 VP1 VP2 VP3 Physical memory DRAM null null

0: 1: 2: 3: 4: 5: 6: 7:

Page table Disk 1 1 1 1 VP4 VP5 VP6 VP7 Valid Address Virtual address

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Page Fault

VP1 VP6 VP3 VP7 VP0 VP1 VP2 VP3 Physical memory DRAM null null

0: 1: 2: 3: 4: 5: 6: 7:

Page table Disk 1 1 1 1 VP4 VP5 VP6 VP7 Valid Address Virtual address

  • Valid bit = 0
  • Page not in RAM

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Page Fault

VP1 VP6 VP3 VP7 VP0 VP1 VP2 VP3 Physical memory DRAM null null

0: 1: 2: 3: 4: 5: 6: 7:

Page table Disk 1 1 1 1 VP4 VP5 VP6 VP7 Valid Address Virtual address

  • Page is on disk

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 18

17

Page Fault

VP1 VP3 VP7 VP0 VP1 VP2 VP3 Physical memory DRAM null null

0: 1: 2: 3: 4: 5: 6: 7:

Page table Disk 1 1 1 1 VP4 VP5 VP6 VP7 Valid Address Virtual address

  • Make space in RAM
  • Pre-empt "victim" page
  • Typically out-dated cached page

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 19

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Page Fault

VP1 VP2 VP3 VP7 VP0 VP1 VP2 VP3 Physical memory DRAM null null

0: 1: 2: 3: 4: 5: 6: 7:

Page table Disk 1 1 1 1 VP4 VP5 VP6 VP7 Valid Address Virtual address

  • Load page into RAM

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Page Fault

VP1 VP2 VP3 VP7 VP0 VP1 VP2 VP3 Physical memory DRAM null null

0: 1: 2: 3: 4: 5: 6: 7:

Page table Disk 1 1 1 1 1 VP4 VP5 VP6 VP7 Valid Address Virtual address

  • Update page table entry

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 21

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Allocating Pages

  • What happens when we load a program?
  • We need to load its executable into memory
  • Similar:

create data objects when program is running ("allocating" memory)

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Allocating Page

VP1 VP6 VP3 VP7 VP0 VP1 VP2 VP3 Physical memory DRAM null null

0: 1: 2: 3: 4: 5: 6: 7:

Page table Disk 1 1 1 1 VP4 VP5 VP6 VP7 Valid Address

  • Identify space in virtual memory

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 23

22

Allocating Page

VP1 VP6 VP3 VP7 VP0 VP1 VP2 VP3 Physical memory DRAM null

0: 1: 2: 3: 4: 5: 6: 7:

Page table Disk 1 1 1 1 VP4 VP5 VP6 VP7 Valid Address

  • Map to data on disk

– do not actual load – just create page table entries – let virtual memory system handle loading ⇒ On-demand loading

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 24

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Process Memory

  • Nothing loaded at startup
  • Working set (or resident set)

– pages of a process that are currently in DRAM – loaded by virtual memory system on demand

  • Thrashing

– memory actively required by all processes larger than physically available – frequent swapping of memory to/from disk – very bad: slows down machine dramatically

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 25

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memory management

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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One Page Table per Process

PP10 VP1 VP2

0: 1: 2: 3:

Process 1 1 1 VP1 VP2

0: 1: 2: 3:

Process 2 1 1 PP2 PP7 Physical memory Shared page

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Process Address Space

Kernel memory User stack Memory-mapped region for shared libraries Run time heap (created by malloc) Read/write segment (.data / .bss) Read-only code segment (.init, .text., .rodata)

Stack pointer Loaded from executable 400000 ffffffff

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Simplified Linking

Data Code

400000

  • Each process has its code in address 0x400000
  • Easy linking:

Linker can establish fixed addresses

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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28

Simplified Loading

  • When loading process into memory...
  • Enter .data and .text section into page table
  • Mark them as invalid (= not actually in RAM)
  • Called memory mapping (more on that later)

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Simplified Sharing

Shared libraries Shared libraries

Physical memory

  • Shared libraries

used by several processes e.g., stdio providing printf, e.g., scanf, open, close, ...

  • Not copied multiple times

into RAM

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Simplified Memory Allocation

  • Process may need more memory (e.g., malloc call)

⇒ New entry in page table

  • Mapped to arbitrary pages in physical memory
  • Do not have to be contiguous

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Memory Protection

Physical memory

yes no no PP 6 yes yes no PP 4 VP 0 VP 1 yes yes yes PP 2 VP 2 READ WRT SUP Address yes no no PP 9 yes yes yes PP 6 VP 0 VP 1 yes yes no PP 11 VP 2 READ WRT SUP Address

Process 1 Process 2

PP 0 PP 2 PP 4 PP 6 PP 9 PP 11

  • Page may be kernel only:

SUP=yes

  • Page may be read-only (e.g., code)

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 33

32

address translation

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Address Space

  • Virtual memory size:

N = 2n bytes

  • Physical memory size:

M = 2m bytes

  • Page (block of memory):

P = 2p bytes

  • A virtual address can be encoded in n bits

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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SLIDE 35

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Address Translation

  • Task:

mapping virtual address to physical address – virtual address (VA): used by machine code instructions – physical address (PA): location in RAM

  • Formally

MAP: VA → PA ∪ 0 where: MAP(A) = PA if in RAM xxxxx = 0 otherwise

  • Note:

this happens very frequently in machine code

  • We will do this in hardware:

Memory Management Unit (MMU)

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Basic Architecture

page table base register

Virtual address Physical address

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Basic Architecture

page table base register

Valid Physical page number

Virtual address Physical address

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Basic Architecture

page table base register virtual page number page offset physical page number page offset

Valid Physical page number

Virtual address Physical address

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018

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Basic Architecture

page table base register virtual page number page offset physical page number page offset

Valid Physical page number valid = 0?

  • > page fault

Virtual address Physical address

Philipp Koehn Computer Systems Fundamentals: Virtual Memory 25 April 2018