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Today Memory Management Virtual Memory Segmentation, Paging - - PDF document

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Today Memory Management Virtual Memory Segmentation, Paging Project 5 Nov 30, 2018 Sprenkle - CSCI330 1 Review What abstraction does virtual memory provide? What requirements do we have for the VM, from the various

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SLIDE 1

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Today

  • Memory Management

Ø Virtual Memory Ø Segmentation, Paging

  • Project 5

Nov 30, 2018 Sprenkle - CSCI330 1

Review

  • What abstraction does virtual memory provide?
  • What requirements do we have for the VM, from

the various stakeholders, so far?

Nov 30, 2018 Sprenkle - CSCI330 2

Aaron Bauer, University of Washington “Understanding Human Problem Solving in Complex Digital Environments” Talk at 4 p.m. (Reception at 3:45 p.m.)

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Address Translation: Wish List

  • Map virtual addresses to

physical addresses

  • Allow multiple processes

to be in memory at once, but isolate them from each other

  • Determine which subset
  • f data to keep in

memory/move to disk

  • Allow the same physical

memory to be mapped in multiple process VASes

Nov 30, 2018 Sprenkle - CSCI330 3

Process 1 Process 3 OS Process 2 Process 1 Text Data Stack OS Heap libc code

Review: Where should process P be placed?

  • First fit

Ø Don’t spend time searching!

  • Best fit

Ø It fits tightly! Ø Maybe no other process will fit in that spot

  • Worst fit

Ø Leaves lots of space for another process

OS

Process P C B A Nov 30, 2018 Sprenkle - CSCI330 4

First Fit Best Fit Worst Fit

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SLIDE 3

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(External) Fragmentation

  • No matter where it ends up, the

remaining gaps get smaller

  • Large gaps are probably still usable,

small ones likely aren’t

  • Fragmentation: over time, we end up

with small gaps that become more difficult to use (eventually, wasted)

  • “External” because the gaps are between

allocated pieces

OS

C B A Nov 30, 2018 Sprenkle - CSCI330 5

(External) Fragmentation

  • Suppose we put it here, and later, P asks

for more memory?

  • What if there isn’t enough space…

Ø Move P? Ø Move everybody to compact the address space?

  • This seems bad. Lots of tough problems

(placement, fragmentation) with no clear solutions.

OS

P Nov 30, 2018 Sprenkle - CSCI330 6

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SLIDE 4

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Alternative Organization of PAS

  • Divide PAS into fixed-size pieces
  • Use memory translation to assign virtual

addresses to physical locations

  • Every physical location is an equally good

choice!

OS OS

Nov 30, 2018 Sprenkle - CSCI330 7

Address Translation: Wish List

  • Map virtual addresses to

physical addresses

  • Allow multiple processes to

be in memory at once, but isolate them from each other

  • Determine which subset of

data to keep in memory/move to disk

  • Allow the same physical

memory to be mapped in multiple process VASes

  • Make it easier to perform

placement in a way that reduces fragmentation

Nov 30, 2018 Sprenkle - CSCI330 8

Process 1 Process 3 OS Process 2 Process 1 Text Data Stack OS Heap libc code

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SLIDE 5

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OS Perspective

  • Primary challenge: Which physical memory do

we give to processes?

  • Other important considerations:

Ø Protection: OS is resource gatekeeper, must isolate itself (and processes) Ø Performance: OS should map memory for best performance, as long as it doesn’t violate protection

Nov 30, 2018 Sprenkle - CSCI330 9

Address Translation: Wish List

  • Map virtual addresses to

physical addresses

  • Allow multiple processes to be

in memory at once, but isolate them from each other

  • Determine which subset of

data to keep in memory/move to disk

  • Allow the same physical

memory to be mapped in multiple process VASes

  • Make it easier to perform

placement in a way that reduces fragmentation

Nov 30, 2018 Sprenkle - CSCI330 10

Process 1 Process 3 OS Process 2 Process 1 Text Data Stack OS Heap libc code

  • Protection: OS is resource gatekeeper, must isolate itself (and processes)
  • Performance: OS should map memory for best performance,

as long as it doesn’t violate protection

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6

Recall: Context Switching Performance

  • Even though it’s fast, context

switching is expensive:

  • 1. time spent is 100% overhead
  • 2. must invalidate other processes’

resources (caches, memory mappings)

  • 3. kernel must execute – it must be

accessible in memory

  • Solution to #3:

Ø keep kernel mapped in every process VAS Ø protect it to be inaccessible

0x0 0xFFFFFFFF Operating system Stack Text Data Heap

Nov 30, 2018 Sprenkle - CSCI330 11

Hardware

  • Hardware and OS are symbiotic, often influence

each other

Ø Example: atomic instructions

  • Memory management is another important area
  • f collaboration
  • Hardware goals:

Ø Make translation fast Ø Give OS storage for and control over mappings

Nov 30, 2018 Sprenkle - CSCI330 12

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SLIDE 7

7

Address Translation: Wish List

  • Map virtual addresses to physical

addresses

  • Allow multiple processes to be in

memory at once, but isolate them from each other

  • Determine which subset of data to

keep in memory/move to disk

  • Allow the same physical memory to

be mapped in multiple process VASes

  • Make it easier to perform

placement in a way that reduces fragmentation

  • Map addresses quickly with a little

HW help

Nov 30, 2018 Sprenkle - CSCI330 13

Process 1 Process 3 OS Process 2 Process 1 Text Data Stack OS Heap libc code Combination of hardware and OS, working together. In hardware, MMU: Memory Management Unit

How does each benefit from having a logical memory abstraction?

  • The user

Ø Memory size, protection

  • The programmer

Ø Shared libraries

  • The compiler

Ø Placement of data

  • The OS / OS designer

Ø Memory placement, fragmentation

  • The hardware / hardware designer

Ø Just makes it more complex BUT can help OS

Nov 30, 2018 Sprenkle - CSCI330 14

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SLIDE 8

8

PROJECT 5

Awkward transition

Nov 30, 2018 Sprenkle - CSCI330 15

Project 5

  • Combines processes and memory management
  • proc.c

Ø Implementations of process management and memory management

Nov 30, 2018 Sprenkle - CSCI330 16

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proc.h Data Structures: memoryMap

USED USED FREE FREE USED FREE FREE FREE

1 2 3 4 5 6 7

memoryMap

Sprenkle - CSCI330 17

  • Allow one process to be loaded into each

segment

Ø Valid Segments: 0x2000, … 0x9000 Ø (0x0000 reserved for interrupt vector, 0x1000 reserved for kernel)

  • Memory segment map:

Ø Each index corresponds to one memory segment.

  • segment = (index+2)*0x1000
  • index = (segment/0x1000)-2

Ø Marked as:

  • FREE or USED

Nov 30, 2018

proc.h Data Structures: PCB

  • Process Control Block:

struct truct PCB PCB char name[7] int state int segment int stackPointer struct PCB *next struct PCB *prev

DEFUNCT STARTING RUNNING READY BLOCKED Constants:

Sprenkle - CSCI330 18 Nov 30, 2018

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proc.h Data Structures: pcbPool

  • PCB Pool

1 2 3 4 5 6 7

pcbPool struct PCB struct PCB

. . .

struct truct PCB PCB char name[7]: "\0" int state: DEFUNCT int segment: 0x0000 int stackPointer 0x0000 struct PCB *next NULL struct PCB *prev NULL

Sprenkle - CSCI330 19 Nov 30, 2018

proc.h Data Structures

  • struct PCB *running

1 2 3 4 5 6 7

pcbPool struct PCB struct PCB

. . .

struct truct PCB PCB char name[7]: "uprog2\0" int state: RUNNING int segment: 0x3000 int stackPointer 0xFF00 struct PCB *next NULL struct PCB *prev NULL

Sprenkle - CSCI330 20

The currently running process

Nov 30, 2018

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SLIDE 11

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proc.h Data Structures

  • Ready Queue

Ø Doubly-linked list

1 2 3 4 5 6 7

pcbPool struct PCB state: READY next: NULL prev: struct PCB

. . .

struct PCB state: READY next: prev: NULL

. . .

readyHead readyTail

struct PCB state: READY next: prev:

Sprenkle - CSCI330 21 Nov 30, 2018

proc.h Data Structures: running

  • Initially the running process will be the idle

process struct PCB *running

struct truct PCB PCB char name[7]: "IDLE\0" int state: READY int segment: 0x1000 int stackPointer 0x???? struct PCB *next NULL struct PCB *prev NULL

idleProc

Sprenkle - CSCI330 22

If no processes in the ready queue, run idle process

Nov 30, 2018

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SLIDE 12

12

VMàPHYSICAL ADDRESS TRANSLATION

Equally awkward translation back

Nov 30, 2018 Sprenkle - CSCI330 23

Address Translation: Wish List

  • Map virtual addresses to physical

addresses

  • Allow multiple processes to be in

memory at once, but isolate them from each other

  • Determine which subset of data to

keep in memory/move to disk

  • Allow the same physical memory to

be mapped in multiple process VASes

  • Make it easier to perform

placement in a way that reduces fragmentation

  • Map addresses quickly with a little

HW help

Nov 30, 2018 Sprenkle - CSCI330 24

Process 1 Process 3 OS Process 2 Process 1 Text Data Stack OS Heap libc code Combination of hardware and OS, working together. In hardware, MMU: Memory Management Unit

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SLIDE 13

13

Simple (Unrealistic) Translation Example

  • Process P2’s virtual addresses

don’t align with physical memory’s addresses

  • Consider: P2 wants to access

address 0x1000

  • Determine offset from

physical address 0 to start of P2

Ø store in base

P3 P1 P2 P2 P2max Phymax base +

Nov 30, 2018 Sprenkle - CSCI330 25

base

Generalizing

  • Problem: process may not fit in one contiguous

region

Nov 30, 2018 Sprenkle - CSCI330 26

P2 P2max Phymax Base? + P2 P2 P2

… …

Base? + Base? +

?

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SLIDE 14

14

Generalizing

  • Problem: process may not fit in
  • ne contiguous region
  • Solution: keep a table (one per

process)

Ø Keep details for each region in a row Ø Store additional metadata (ex. permissions)

  • Questions:

Ø How many regions should there be (and what size)? Ø How to determine which table entry we should use?

Nov 30, 2018 Sprenkle - CSCI330 27

P2 P2max Phymax P2 P2 P2

… …

?

Perm Base R, X R R, W

Defining Regions - Two Approaches

  • Segmentation:

Ø Partition address space and memory into logical segments Ø Segments have varying sizes

  • Paging:

Ø Partition address space and memory into pages Ø Pages are a constant, fixed size

Nov 30, 2018 Sprenkle - CSCI330 28

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SLIDE 15

15

Why would you use each approach? Pros/Cons

  • Segmentation:

Ø Partition address space and memory into segments Ø Segments have varying sizes

  • Paging:

Ø Partition address space and memory into pages Ø Pages are a constant, fixed size

Nov 30, 2018 Sprenkle - CSCI330 29

Consider how memory would be requested and used Do the pros/cons change based on whose perspective?

Fragmentation

Internal

  • Process asks for memory,

doesn’t use it all

  • Possible reasons:

Ø Process was wrong about needs Ø OS gave it more than it asked for

  • internal: within an allocation

External

  • Over time, we end up

with small gaps that become more difficult to use

Ø eventually, wasted

  • external: unused

memory between allocations

OS Used Memory allocated to process Unused

Nov 30, 2018 Sprenkle - CSCI330 30

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SLIDE 16

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Which scheme is better for reducing internal and external fragmentation?

A.Segmentation is better than paging for both

forms of fragmentation.

B.Segmentation is better for internal

fragmentation, while paging is better for external fragmentation.

C.Paging is better for internal fragmentation,

while segmentation is better for external fragmentation.

D.Paging is better than segmentation for both

forms of fragmentation.

Nov 30, 2018 Sprenkle - CSCI330 31

Which scheme is better for reducing internal and external fragmentation?

A.Segmentation is better than paging for both

forms of fragmentation.

B.Segmentation is better for internal

fragmentation, while paging is better for external fragmentation.

C.Paging is better for internal fragmentation,

while segmentation is better for external fragmentation.

D.Paging is better than segmentation for both

forms of fragmentation.

Nov 30, 2018 Sprenkle - CSCI330 32

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Segmentation vs. Paging

  • A segment is good logical unit of information

Ø Can be sized to fit any contents Ø Easy to share large regions (e.g., code, data) Ø Protection requirements correspond to logical data segment

  • A page is good physical unit of information

Ø Simple physical memory placement Ø No external fragmentation Ø Constant sizes make it easier for hardware to help

Nov 30, 2018 Sprenkle - CSCI330 33

Logical View of Segmentation

1 3 2 4 1 4 2 3 user space physical memory space

Nov 30, 2018 Sprenkle - CSCI330 34

Since segments vary in length, memory allocation is a dynamic storage allocation problem

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18

Paging Model of Logical and Physical Memory

Nov 30, 2018 Sprenkle - CSCI330 35

Generalizing

  • Problem: process may not fit in
  • ne contiguous region
  • Solution: keep a table (one per

process)

Ø Keep details for each region in a row Ø Store additional metadata (ex. permissions)

  • Interesting questions:

Ø How many regions should there be (and what size)? Ø How to determine which table entry we should use?

Nov 30, 2018 Sprenkle - CSCI330 36

P2 P2max Phymax P2 P2 P2

… …

?

Perm Base R, X R R, W

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SLIDE 19

19

For Both Segmentation and Paging…

  • Each process has a table to track memory

address translations

  • When a process attempts to read/write to

memory:

Ø use high order bits of virtual address to determine which row to look at in the table Ø use low order bits of virtual address to determine an

  • ffset within the physical region

Nov 30, 2018 Sprenkle - CSCI330 37

Address Translation

Virtual Address

Upper bits Lower bits

Physical Address Phy Loc Meta Perm … Physical Memory Table

Nov 30, 2018 Sprenkle - CSCI330 38

Which row? Offset into region

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Performance Implications

Virtual Address

Upper bits Lower bits

Physical Address Phy Loc Meta Perm … Physical Memory Table

Nov 30, 2018 Sprenkle - CSCI330 39

Which row? Offset into region Without VM: Go directly to address in memory With VM: Do a lookup in memory to determine which address to use

Concept: level of indirection

Defining Regions - Two Approaches

  • Segmentation:

Ø Partition address space and memory into logical segments Ø Segments have varying sizes

  • Paging:

Ø Partition address space and memory into pages Ø Pages are a constant, fixed size

Nov 30, 2018 Sprenkle - CSCI330 40

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SLIDE 21

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Segment Table

  • One table per process
  • Where is the table located in

memory?

Ø Segment table base register (STBR) Ø Segment table size register (STSR)

  • Table entries: Segment metadata

Ø V: valid bit

  • does it contain a mapping?

Ø Base: segment location in physical memory Ø Bound: segment size in physical memory Ø Permissions

Bound Base V Perm …

STBR STSR

Nov 30, 2018 Sprenkle - CSCI330 41

Segment Address Translation

  • Physical address:

base of s + i

Virtual Address

Segment s … Offset i Physical Address

Nov 30, 2018 Sprenkle - CSCI330 42

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SLIDE 22

22

Check if Segment s is within Range

Nov 30, 2018 Sprenkle - CSCI330 43

Virtual Address

Segment s … Offset i Physical Address STBR STSR

s < STSR

Check if Segment Entry s is Valid

Nov 30, 2018 Sprenkle - CSCI330 44

Virtual Address

Segment s … Offset i Physical Address STBR STSR

V == 1

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Check if Offset i is within Bounds

Nov 30, 2018 Sprenkle - CSCI330 45

Virtual Address

Segment s … Offset i Physical Address STBR STSR

i < Bound

Translate Address

Nov 30, 2018 Sprenkle - CSCI330 46

Virtual Address

Segment s … Offset i Physical Address STBR STSR

+

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Pros and Cons of Segmentation

Pros

  • Each segment can be

Ø located independently Ø separately protected Ø grown/shrunk independently

  • Small segment table size

Ø ~256 Bytes à 1GB memory

Cons

  • Variable-size allocation

Ø Difficult to find holes in physical memory Ø External fragmentation

Nov 30, 2018 Sprenkle - CSCI330 47

Looking Ahead

  • Project 5 due next Friday

Nov 30, 2018 Sprenkle - CSCI330 48