operating systems and computer networks memory management · partitioning, paging, segmentation...
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Operating Systems and Computer Networks
Memory Management
Prof. Dr.-Ing. Axel HungerAlexander Maxeiner, M.Sc.
Institute of Computer EngineeringFaculty of Engineering
University Duisburg-Essen
Dr.-Ing. Pascal A. Klein
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
2Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
1 – Swapping|
2 – Segmentation Algorithms|
3 – Memory Allocation|
4 – Virtual Memory|
5 – Paging|
6 – Page Replacement Algorithms
Agenda
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
3Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Goals of Memory Management
Convenient abstraction for programming
Allocation of scarce memory resources among competing processes
maximize performance with minimal overhead
Mechanisms
Physical and virtual addressing
Partitioning, Paging, Segmentation
Page replacement algorithms
Memory Management
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
4Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Memory Hierarchy (1)
Power on
Power onvery short term
Power offshort term
Power offmid term
Power offLong term
Power onImmediate term
small sizesmall capacity
small sizesmall capacity
medium sizemedium capacity
medium sizelarge capacity
Large sizevery large capacity
Large sizevery large capacity
processor registersvery fast, very expensive
processor cachevery fast, very expensive
Random access memoryfast, affordable
hard drivesslow, very cheap
flash / USB memoryslower, cheap
Tape backupvery slow, affordable
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
5Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Memory Hierarchy (2)
Power on
Power onvery short term
Power offmid term
Power onImmediate term
small sizesmall capacity
small sizesmall capacity
medium sizemedium capacity
Large sizevery large capacity
processor registersvery fast, very expensive
processor cachevery fast, very expensive
Random access memoryfast, affordable
hard drivesslow, very cheap
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
6Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Allocation
Protection from each other, Protecting OS
Translating logical addresses to physical
Swapping(if physical memory is too small for all processes)
Virtual memory(memory abstraction)
Sharing of Memory
Program 1
Free Space
Program 3
Program 2
Free Space
OS
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
7Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Fetch strategy
• When AND how much to load at a time (e.g.: prefetching)
Placement (or allocation) strategy
• Determination WHERE data is to be placed
Replacement strategy
• Determination WHICH area can be removed
Variations• Fixed partitions
• Variable partitions
• Segmentation
• paging
Memory Strategies
Program 1
Free Space
Program 3
Program 2
Free Space
OS
Modern PCs
Early computers,
AND embedded, small, etc...
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
8Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
MOV REG1, 1000
set of physical addresses from 0 to ...
one program only
– How to execute more than one program?
No Memory Abstraction (1)
Program
OS
Program
OS
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
9Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Divide all physical memory into a fixed set ofcontiguous partitions
Place only one process at a time in any partition
Partition boundaries limit the available memory foreach process
Proces either entirely in main memory or not
No sharing between processes
Memory wasting:
– partition internal fragmentation
– Free partitions too small
Without virtual addressing – protection problems!
Without Swapping/virtual memory – size problems!
Assumption: Increased Size
- no Memory Abstraction
12K
2K
6K
OS: 2K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
10Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Swapping
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
11Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Issue: memory (RAM) is too small
„swap“ data on slower but bigger memory
Swapper decides which processes should be in main memory
Memory Abstraction – Swapping
Free Space
OS
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
12Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Assumption: 180K of Memory, OS: 20K
Swapping – Example
Free Space:180K
OS: 20K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
13Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Assumption: 180K of Memory, OS: 20K
Process A (30K) is created or swapped in
Swapping – Example
Free Space:150K
OS: 20K
Process A:30K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
14Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Assumption: 180K of Memory, OS: 20K
Process A (30K) is created or swapped in
Process B (50K) is created or swapped in
Swapping – Example
Free Space:100K
OS: 20K
Process A:30K
Process B:50K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
15Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Assumption: 180K of Memory, OS: 20K
Process A (30K) is created or swapped in
Process B (50K) is created or swapped in
Process C (20K) is created or swapped in
Swapping – Example
Free Space:80K
OS: 20K
Process A:30K
Process B:50K
Process C:20K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
16Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Assumption: 180K of Memory, OS: 20K
Process A (30K) is created or swapped in
Process B (50K) is created or swapped in
Process C (20K) is created or swapped in
Process D (20K) is created or swapped in
Swapping – Example
Free Space:60K
OS: 20K
Process A:30K
Process B:50K
Process C:20K
Process D:20K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
17Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Assumption: 180K of Memory, OS: 20K
Process A (30K) is created or swapped in
Process B (50K) is created or swapped in
Process C (20K) is created or swapped in
Process D (20K) is created or swapped in
Process A is swapped out to disk
Swapping – Example
Free Space:60K
OS: 20K
Process B:50K
Process C:20K
Process D:20K
Free Space:30K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
18Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Assumption: 180K of Memory, OS: 20K
Process A (30K) is created or swapped in
Process B (50K) is created or swapped in
Process C (20K) is created or swapped in
Process D (20K) is created or swapped in
Process A is swapped out to disk
Process C is swapped out to disk
What is the problem?
Swapping – Example
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free Space:30K
Free Space:20K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
19Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Memory Fragmentation
• Memory Compaction
Problem: CPU time
• Fixed size of partitions
Problem: growing memoryBigger partitions with room for grow
(here: heap)
• Variable size of partitions
Management with Bit-Mapsor Linked Lists
Trade-off: Space requirements vs. time for allocation
Swapping Issue
Free Space
OS
Free Space
Process C
Process D
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
20Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Memory division into allocation units
Corresponding bit for each allocation unit (0,1)telling whether it is free or not
Example: Allocation units of 10KiB
• For current 20 bits: 00000011 00111110 0011
Trade-off: • Bit-Map size depends on allocation unit
• searching for k consecutive 0-bits in map costly,
• fixed size
Managing free space
with Bit-Maps
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free Space:30K
Free Space:20K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
21Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Linked list of allocated andfree memory segments
Each record has
– Process ID/ Free (H: hole)
– Start address
– length
– pointer to next record
Managing free space
with Linked Lists
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free Space:30K
Free Space:20K
2H 3 5B 5 10H 2
12D 2 14H X6
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
22Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Update Process:
– When a process is swapped out, neighbouring blocks need to be examined
– doubly linked lists(containing a pointer to the next AND previousnode
Managing free space
with Linked Lists
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free Space:30K
Free Space:20K
2H 3 5P 5 10H 2
12P 2 14H X6
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
23Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Assuming a new process E requests 15K
Which hole should it use?
Available algorithms
First fit
Next fit
Best fit
Worst fit
Algorithms to allocate memory
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free Space:30K
Free Space:20K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
24Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
simplest algorithm
scanning along the list (from beginning) until itfinds sufficient hole
Breaking hole in two pieces:
• One for process
• One for unused memory (new hole)
Memory Allocation: First Fit
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free Space:30K
Free Space:20K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
25Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
simplest algorithm
scanning along the list (from beginning) until itfinds sufficient hole
Breaking hole in two pieces:
– One for process
– One for unused memory (new hole)
Fitting into 30K hole
Very fast – searches as little as possible
Memory Allocation: First Fit
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free: 15K
Free Space:20K
Process E:15K
start
stop
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
26Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Variation of First Fit
Keeps track of where it found a hole
Next time: start at position where it left off last time
Assumption that prospective fitting holes comeafter already found holes
However, slight worse performancethan first fit
Memory Allocation: Next Fit
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free Space:30K
Free Space:20K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
27Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Variation of First Fit
Keeps track of where it found a hole
Next time: start at position where it left off last time
Assumption that prospective fitting holes comeafter already found holes
However, slight worse performancethan first fit
Fitting into 30K hole
Memory Allocation: Next Fit
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free: 15K
Free Space:20K
Process E:15K
start
stop
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
28Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Variation of First Fit
Keeps track of where it found a hole
Next time: start at position where it left off last time
Assumption that prospective fitting holes comeafter already found holes
However, slight worse performancethan first fit
Fitting into 30K hole
Then, fitting into 20K hole
Memory Allocation: Next Fit
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free: 15K
Free: 5K
Process E:15K
start
stop
Process F:15K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
29Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Searches entire list (from beginning to end)
Takes smallest hole
Assumption of best memory spacial performance
Memory Allocation: Best Fit
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free Space:30K
Free Space: 20K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
30Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Searches entire list (from beginning to end)
Takes smallest hole
Assumption of best memory spacial performance
Fitting into 20K hole
Takes much CPU time (for searching)
Resulting in more memory wasting because ofnumerous tiny useless holes.
Memory Allocation: Best Fit
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free Space:30K
Free: 5K
Process E:15K
start
stop
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
31Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Variation of best fit
Always takes the largest hole
Assumption to get around splitting into tinyholes, to be big enough for other processes
Memory Allocation: Worst Fit
Free Space:60K
OS: 20K
Process B:50K
Process D:20K
Free Space:30K
Free Space: 20K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
32Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Variation of best fit
Always takes the largest hole
Assumption to get around splitting into tinyholes, to be big enough for other processes
Fitting into 60K hole
Takes much CPU time (for searching)
Still memory wasting
Memory Allocation: Worst Fit
Free Space:45K
OS: 20K
Process B:50K
Process D:20K
Free Space:30K
Free Space: 20K
Process E:15K
start
stop
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
33Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Maintaining separate lists for processes AND holes
• Algorithms devote energy to inspect holes NOT processes
• BUT higher effort for deallocating (changing both lists)
Sorting of lists regarding size
enhancing speed of Best Fit/Worst Fit
In practice:
FF is usually better
NF is pointless when sorted lists are used
Memory Allocation:
Enhancing performance
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
34Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Memory encapsulated in units of powers of 2
Process requests rounded up to fit into units
Possible hole sizes: ..., 4K, 8K, 16K, 32K, 64K, 128K, ...
Buddy System
64K
32K
16K
8K4K4K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
35Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Example: Suppose a memory of 128K One hole of 128K
Process A requests 6K rounded to 8K (2K wasted)
Buddy System
128K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
36Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
128K
Example: Suppose a memory of 128K One hole of 128K
Process A requests 6K rounded to 8K (2K wasted)
Process B requests 5K rounded to 8K(3K wasted)
Buddy System
64K
32K
16K
8K
A: 8K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
37Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Example: Suppose a memory of 128K One hole of 128K
Process A requests 6K rounded to 8K (2K wasted)
Process B requests 5K rounded to 8K(3K wasted)
Process C requests 24K rounded to 32K(8K wasted)
Buddy System
64K
32K
16K
B: 8K
A: 8K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
38Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Example: Suppose a memory of 128K One hole of 128K
Process A requests 6K rounded to 8K (2K wasted)
Process B requests 5K rounded to 8K(3K wasted)
Process C requests 24K rounded to 32K(8K wasted)
Process D requests 3K rounded to 4K(1K wasted)
Buddy System
64K
C: 32K
16K
B: 8K
A: 8K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
39Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Example: Suppose a memory of 128K One hole of 128K
Process A requests 6K rounded to 8K (2K wasted)
Process B requests 5K rounded to 8K(3K wasted)
Process C requests 24K rounded to 32K(8K wasted)
Process D requests 3K rounded to 4K(1K wasted)
Process A exits
Buddy System
64K
C: 32K
8K
B: 8K
A: 8K
D: 4K4K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
40Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Example: Suppose a memory of 128K One hole of 128K
Process A requests 6K rounded to 8K (2K wasted)
Process B requests 5K rounded to 8K(3K wasted)
Process C requests 24K rounded to 32K(8K wasted)
Process D requests 3K rounded to 4K(1K wasted)
Process A exits
Process B exits
Buddy System
64K
C: 32K
8K
B: 8K
8K
D: 4K4K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
41Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Example: Suppose a memory of 128K One hole of 128K
Process A requests 6K rounded to 8K (2K wasted)
Process B requests 5K rounded to 8K(3K wasted)
Process C requests 24K rounded to 32K(8K wasted)
Process D requests 3K rounded to 4K(1K wasted)
Process A exits
Process B exits
Buddy System
64K
C: 32K
8K
16K
D: 4K4K
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
42Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Virtual Memory
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
43Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Issue: By using physical addresses all executions are absolutely adressedNOT relatively!
Better memory management with logicaladdresses
Independancy of actual physical location ofdata in physical memory
OS determines logical location of data
CPU instructions using virtual addresses(don‘t care about location)
Translated by hardware into physical address
Virtual Addressing
Program 1
Free Space
Program 3
Program 2
Free Space
OS
addressspace
addressspace
addressspace
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
44Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Software size much bigger than available RAM
Each program‘s address space broken up into chunks
Pages
Page: contiguous range of addresses mapped onto physicalmemory
Not all pages have to be in physical memory
If OS notices, a program part is not in physical memory, it needs to be loaded and re-executed
In Multiprogramming: While program is waiting for pieces ofitself to be read in, CPU can execute another process
Virtual Memory
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
45Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Page: Fixed size unit in virtual memory
Page frame: corresponding units in physical memory
Same size (here: 4KB)
Transparent to the program
Protection – a program cannot leave Virtual Address Space
Example: 64KB virtual address space, 32KB physical space, 4KB page size
• How many pages/page frames?
Answer: 16 virtual pages, 8 page frames
Paging
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
46Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Relationship between
virtual and physical memory
X
X
X
7X
X
6
2
X
4
5
X
3
0
1
virtual
address
space
0KB-4KB
4KB-8KB
...
60KB-64KB
28KB-32KB
0KB-4KB
4KB-8KB
...
page
frame
Addresses No.
4.096 - 8.191
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
47Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Relationship between
virtual and physical memory
X
X
X
7X
X
6
2
X
4
5
X
3
0
1
virtual
address
space
0KB-4KB
4KB-8KB
...
60KB-64KB
28KB-32KB
0KB-4KB
4KB-8KB
...
page
frame
Addresses No.
0 – 4.095
MOV REG, 0
MMU checks VM
page „0“ corresponds to page frame 1
Address is changed to „4096“
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
48Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Relationship between
virtual and physical memory
X
X
X
7X
X
6
2
X
4
5
X
3
0
1
virtual
address
space
0KB-4KB
4KB-8KB
...
60KB-64KB
28KB-32KB
0KB-4KB
4KB-8KB
...
page
frame
Addresses No.
8.192 – 12.287
MOV REG, 8192
MMU checks VM
page „8192“ corresponds to page frame 3
Address is changed to „12288“
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
49Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Relationship between
virtual and physical memory
X
X
X
7X
X
6
2
X
4
5
X
3
0
1
virtual
address
space
0KB-4KB
4KB-8KB
...
60KB-64KB
28KB-32KB
0KB-4KB
4KB-8KB
...
page
frame
Addresses No.
20.480 – 24.576
MOV REG, 20500
MMU checks VM
page „20500“ corresponds to page
frame 4 with an offset of 20 Byte
(20.480 + 20)
Address is changed to „16404“
(16.384 + 20)
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
50Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Memory Management Unit (MMU)
p: page number
d: offset
r: page frame
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
51Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Virtual address has two parts: virtual page number and offset
Virtual page number is an index into a page table
Page table determines page frame number
Present/Absent bit if page is in physical memory
What happens, if program references an unmapped address? (page fault)
Paging – translating addresses
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
52Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Relationship between
virtual and physical memory
X
X
X
7X
X
6
2
X
4
5
X
3
0
1
virtual
address
space
0KB-4KB
4KB-8KB
...
60KB-64KB
28KB-32KB
0KB-4KB
4KB-8KB
...
page
frame
Addresses No.
12.288 – 16.383
MOV REG, 12288
MMU checks VM
page fault!
page fault!
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
53Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Relationship between
virtual and physical memory
X
X
X
7X
X
6
2
X
4
5
X
3
0
1
virtual
address
space
0KB-4KB
4KB-8KB
...
60KB-64KB
28KB-32KB
0KB-4KB
4KB-8KB
...
page
frame
Addresses No.
12.288 – 16.383
MOV REG, 12288
MMU checks VM
page fault!
Choose a rarely used page frame (in
phys. mem)
Save content on hard drive
page fault!
e.g.
...001010100011..
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
54Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Relationship between
virtual and physical memory
X
X
X
7X
X
X
2
X
4
5
6
3
0
1
virtual
address
space
0KB-4KB
4KB-8KB
...
60KB-64KB
28KB-32KB
0KB-4KB
4KB-8KB
...
page
frame
Addresses No.
12.288 – 16.383
MOV REG, 12288
MMU checks VM
page fault
Choose a rarely used page frame (in
phys. mem)
Save content on hard drive
Load requested page into phys. Mem
page fault!
e.g.
...111010100100..
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
55Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Relationship between
virtual and physical memory
X
X
X
7X
X
X
2
X
4
5
6
3
0
1
virtual
address
space
0KB-4KB
4KB-8KB
...
60KB-64KB
28KB-32KB
0KB-4KB
4KB-8KB
...
page
frame
Addresses No.
12.288 – 16.383
MOV REG, 12288
MMU checks VM
page fault
Choose a rarely used page frame (in
phys. mem)
Save content on hard drive
Load requested page into phys. Mem
Restart trapped
instruction
page fault!
e.g.
...111010100100..
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
56Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Internal MMU operation
0 0 01 0 0 0 0 0 0 0 0 0 01 0
000
000
000111
000
101
000
000
000
011
100
000
110
001
010
0
0
0
10
1
0
0
0
1
1
1
1
1
10
1
2
3
4
5
6
7
8
9
10
11
12
13
14
000 015
110
1 1 0 0 0 0 0 0 0 0 0 0 01 0
4 Bit 16 pages
3 Bit page
frame
12 Bit Offset 12 Bit 4096 byte
4 Bit page
number
present bit
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
57Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Strongly machine dependent
M/R – bit protocol access to the page
– M-bit is set when a program writes to a page
to know if it needs to be saved before physical space canbe reused
Structure of a page table entry
M R P Prot Page frame number
M – modified bit
R – reference bit
P – present/absent bit
Protection bits
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
58Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Strongly machine dependent
M/R – bit protocol access to the page
– R-bit is set when read/write access occurs
for decision if space should be used for overwriting
Structure of a page table entry
M R P Prot Page frame number
M – modified bit
R – reference bit
P – present/absent bit
Protection bits
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
59Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Strongly machine dependent
P – bit shows if page exists or not
Structure of a page table entry
M R P Prot Page frame number
M – modified bit
R – reference bit
P – present/absent bit
Protection bits
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
60Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Strongly machine dependent
Prot – bits are setting rights which operations are allowed on a page
– E.g.: read, write, execute
Structure of a page table entry
M R P Prot Page frame number
M – modified bit
R – reference bit
P – present/absent bit
Protection bits
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
61Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Easy to allocate memory
• Memory comes from a free list of fixed size chunks
• Allocating a page is just removing it from the list
• External fragmentation not a problem
Easy to swap out chunks of a program
• All chunks are the same size
• Use present bit to detect references to swapped pages
• Pages are a convenient multiple of the disk block size
Paging advantages
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
62Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Memory reference overhead
• 2 references per address lookup (page table, then memory)
Solution – use a hardware cache of lookups
Memory requirements to hold page tables can be significant
• 32 bit address space á 4KB pages = 220
• 4 bytes/PTE = 4MB per page table
• 25 processes = 100MB just for page tables!
Solution – multilevel page tables
Paging disadvantages
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
63Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Observation, programs have most access on just a few pages
Hardware Implementation – TLB
Part of MMU
Small list of most important pages of a process(not more than 64)
e.g. variables used in a loop
Virtual page number, M-bit, Protect-bit, page frame, Validity
Access:
• check TLB, if needed page can be found
• If NOT: fetch entry from page table, write entry to TLB (overwrite older entries)
Translation Lookaside Buffer (TLB)
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
64Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
To be used for big virtual address spaces
Dividing a page table into mulitple tables
Multilevel Page Table
P=1
P=0
P=0
...
P=1
1
2
0
1023
...
...
...
• Two 4MB tables
vs. of 1024*1024 entries
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
65Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Page fault! Load page into physical memory! Which page schould be replaced?
Answer: pages which are rarely used!
Possible Algorithms:
– Not Recently Used (NRU)
– Least Recently Used (LRU)
– First In First Out (FIFO)
– Second Chance (SC)
Page Replacement Algorithms
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
66Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Using status bits – referenced (R) AND modified (M)
R-bit cleared periodically by OS
M-bit cleared when hard disk updated
Clearing random Entry of smallest availabe class:
Best entry class 0.
here: entry 1 or 3 is cleared!
Not Recently Used Algorithm (NRU)
M R P Prot Page framePage Index Offset0 1 1 111 10015 100010101011 0 1 111 10114 000001000010 0 1 111 01113 000110000011 0 1 111 00012 011100010010 0 1 111 10101 000000010011 1 1 111 01010 00110000001
Class 0: R=0, M=0Class 1: R=0, M=1Class 2: R=1, M=0Class 3: R=1, M=1
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
67Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Oldest page entry (that loaded first)
Sorted list ordered by loading time
Clear oldest entry no matter if referenced or modified
here: entry 5
First In First Out Algorithm (FIFO)
M R P Prot Page framePage Index Offset0 1 1 111 10015 100010101011 0 1 111 10114 000001000010 0 1 111 01113 000110000011 0 1 111 00012 011100010010 0 1 111 10101 000000010011 1 1 111 01010 00110000001
Loaded12
25612411626145
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
68Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Enhancement of FIFO (by using R-Bit)
Linked list ordered by loading time
Clear oldest entry if not referenced
If entry is referenced – clear R-bit and continue searching
here: entry 5 and 0 will be unreferenced AND loading time reset, then, entry 2 will be cleared and used
Second Chance Algorithm (SC)
M R P Prot Page framePage Index Offset0 1 1 111 10015 100010101011 0 1 111 10114 000001000010 0 1 111 01113 000110000011 0 1 111 00012 011100010010 0 1 111 10101 000000010011 1 1 111 01010 00110000001
Loaded12
25612411626145
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
69Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Clears page that has been unused for the longest time
Linked list ordered by access time
based on asusmption, that pages used in the last few instructions will beused in the next few future instructions
Problem: Entries have to be updated every accesslinked-list must be reordered ;-(
here: entry 2 is cleared. (updated on hard drive first)
Least Recently Used Algorithm (LRU)
M R P Prot Page framePage Index Offset0 1 1 111 10015 100010101011 0 1 111 10114 000001000010 0 1 111 01113 000110000011 0 1 111 00012 011100010010 0 1 111 10101 000000010011 1 1 111 01010 00110000001
Loaded12
25612411626145
Access245256163116267245
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
70Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Questions?
Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen
71Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering
OSCN – Memory Management
Tanenbaum, Andrew S., “Modern Operating Systems”, 3rd edition, Pearson Education Inc, Amsterdam, Netherlands, 2008.
Tanenbaum, Andrew S., “Moderne Bertriebssysteme”, 3rd edition, Pearson Education Inc, Amsterdam, Netherlands, 2009.
Lee, Insup, „CsE 380 Computer Operating Systems“, Lecture Notes, University of Pennsylvania, 2002.
Snoeren, Alex C., „Lecture 10: Memory Management“, LectureNotes, UC San Diego, 2010.
Resources