1. Memory management
Igor Radovanović

2. Quiz about an Operating system
What is an OS?
What is an OS composed of?
Why does anyone need an OS?
When is an OS efficient?
Why are some OS popular and some not?
If you are to develop an OS, what must you make sure before you can expect that it can be widely used?
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3. … … … REMINDER: OS provides abstract machines Abstract Machines Idea
Analogy: car driving
Idea
Result
Program
Physical
Machine
Idea
Program
Result … … …
Idea
Simplicity-flexibility tradeoff (teller machine)
Program
Result
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4. REMINDER: OS as resource manager
Process: An executing program
Resource: Anything that is needed for a process to run
Memory
Space on a disk
The CPU
“An OS creates resource abstractions”
“An OS manages resource sharing”
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5. REMINDER: Resource sharing
Space- vs. time-multiplexed sharing
To control sharing, must be able to isolate resources
OS usually provides mechanism to isolate, then selectively allows sharing
How to isolate resources?
How to be sure that sharing is acceptable?
Concurrency
Appearance
True concurrency
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6. REMINDER: Logical OS Organization
Process, thread
and
resource manager
File manager
Network manager
Memory manager
Device manager
Processor(s)
Main memory
Devices
All modules coordinate
Examples:
in Virtual Memory
PM –MM to coordinate scheduling activity with memory allocation
For performance improvement: FM –DM & FM-MM
Information from storage device read prior to its being requested by a thread
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7. Memory manager functionalities
Today’s memory manager:
Supports multiprogramming by allocating memory space to many processes simultaneously
Resource manager for the spaced-multiplexed main memory
Robust isolation mechanism to protect process’s allocated memory
Ability for processes to share parts of allocated memory
Ideally:
Keeps programs and data in main memory ONLY while they are being used by CPU
Restore the info to secondary memory soon after it has been created
Automatically move information back and forth between the primary and secondary memory
Virtual memory abstraction
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8. Memory manager functionalities (cnt’d)
Classic memory managers
Abstraction
Main memory appears to be larger than the physical machine mem.
Memory as a large array of contiguously addressed bytes
Abstract set of addresses to reference physical primary memory
Allocation
Allocate primary memory to processes as requested
Isolation
Exclusive use of memory by processes
Sharing
Two types:
Main memory shared by many processes
Block of primary memory shared by many processes
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9. Memory manager non-functional requirements
Performance requirements
Minimize main memory access time
Maximize main memory size
Main memory must be cost-effective
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10. Storage Hierarchies Analogy with the office storage hierarchy
Worker’s desktop
File folder
File cabinet
Warehouse
Upper-layers: info more frequently used
Lower layers: info less frequently used
Less Frequently Used Information
More Frequently Used Information
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11. Memory Hierarchy (cnt’d)
CPU register memory
fast access (1 clk cycle)
but limited in size
Primary (executable memory)
direct access (a few clk cycles),
relatively large,
access time is of less importance
Secondary memory (storage devices)
I/O operations required for access,
very large,
data stored for longer period of time,
access speed is slow (3 orders of a magnitude than the CPU register memory)
CPU Registers
Less Frequently Used Information
Primary Memory
(Main Memory)
e.g. RAM
More Frequently Used Information
Secondary Memory
e.g. Disk or Tape
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12. Contemporary Memory Hierarchy & Dynamic Loading\
CPU Registers
L1 Cache Memory
(Executable)
Primary
L2 Cache Memory
“Main” Memory
Rotating Magnetic Memory
Larger storage
Faster access
Optical Memory
Secondary
Sequentially Accessed Memory
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13. Exploiting the Hierarchy
Upward moves are (usually) copy operations
Require allocation in upper memory
Image exists in both higher & lower memories
Updates are first applied to upper memory
Downward move is (usually) destructive
Destroy image in upper memory
Update image in lower memory
Place frequently-used info high and infrequently-used info low in the hierarchy
Reconfigure as process changes phases (process switch)
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14. The Address Space Abstraction
Instead of a set of physical, processes are allocated a set of logical primary memory addresses (process address space)
Memory manager has to bind logical primary memory addresses to physical primary memory address
Hardware Main (Primary) Memory
Process Address Space
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15. Virtual and Physical address
Decouple addresses (names) used by a process to identify objects from addresses used by memory to identify storage locations
Objects in different processes
can have the same name (address),
but, must be stored at different locations
Process A
0=Δ =◊ =□
Process B
Δ ◊ □ Δ ◊ □
processor
memory
address (name)
object
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16. Managing the address space
Compile time
Isolated addresses of relocatable object modules
Link time
Addresses of the absolute module stored in the file in the secondary memory
Program instructions
Data
Stack
Load time
Binding logical to physical addresses
Static binding
Source
code
Symbolic addresses (count)
Library
code
Other
objects C
Secondary
memory
Absolute address
Link
Edit
Reloc
Object
code
Primary
memory
Relative address
Process
address
space
Loader
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17. Early systems Single-user configuration Fixed partitions
Dynamic partitions
Relocatable Dynamic partitions
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18. Single-user configuration
Each program loaded in its entirety into memory and allocated as much contiguous memory space as needed
if a program does not fit the memory it cannot be executed
Hardware needed
register to store base address
accumulator to track size of program as it is loaded into memory (limit)
Once in main memory, program stays there until the end of execution
No multiprogramming supported
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19. Early systems Single-user configuration Fixed partitions
Dynamic partitions
Relocatable Dynamic partitions
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20. Fixed partitions Advantage: Disadvantage: Design issue: Partition size
attempt at multiprogramming using fixed partitions
1 process per partition
Issue: protection of the task’s memory space
Disadvantage:
entire program was stored contiguously in memory during entire execution
Problem: Internal fragmentation
Design issue: Partition size
Choosing the right partition size was a problem
Too small – long turnaround time
Too big – wasted memory
Partition allocation: first-fit
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21. Fixed partition -an example-
Process List :
P1 30K
P2 50K
P3 30K
P4 25K
Conclusion: Process 3 has to wait even though 70K of free space is available
After Process Entry
Process 1 (30K)
Partition 1
Process 4 (25K)
Partition 2
Partition 3
Process 2 (50K)
Partition 4
wasted memory
Original State
100K
Partition 1
25K
Partition 2
25K
Partition 3
50K
Partition 4
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22. Early systems Single-user configuration Fixed partitions
Dynamic partitions
allocation
deallocation
Relocatable Dynamic partitions
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23. Dynamic partitions
Partition size not fixed but determined based on the size of a process
Advantage:
less memory waste
Disadvantages:
Memory waste not solved completely
Performance deteriorates as new processes enter the system
External fragmentation
Programs still have to be stored contiguously
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24. Dynamic partitions -an example-
First-fit allocation scheme
Conclusion:
Job 8 has to wait even though there is 35K of free memory space available
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25. Dynamic partition -allocation schemes-
Allocation schemes issues: decrease time, increase utilization
First-fit
Allocate the first partition that is big enough
Advantage:
Faster in making the allocation
Disadvantage:
Waste of memory
Best-fit
The smallest partition fitting the requirements
Advantages:
Produces the smallest leftover partition
Makes best use of memory
Takes more time
Analogy: lending library Q:
How to organize free/busy list in both cases?
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26. First-fit allocation -an example-
P1 10K
P2 20K
P3 30K*
P4 10K
Memory Memory Process Process Internal
location block size number size Status fragmentation
K P K Busy K
K P K Busy K
K P K Busy K
K Free
Total Available:115K Total Used: 40K
location organization
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27. Best-fit allocation -an example-
P1 10K
P2 20K
P3 30K
P4 10K
Memory Memory Process Process Internal
location block size number size Status fragmentation
K P K Busy K
K P K Busy None
K P K Busy None
K P K Busy K
Total Available: 115K Total Used: 70K
size organization
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28. Smaller free space - sliver -
Exercise
It is required to put a process that occupies 200 spaces in the memory. The table of free memory spaces is given below.
Show how this table will look like after allocating 200 spaces using the best-fit algorithm.
Smaller free space sliver -
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29. Best-Fit vs. First-Fit First-Fit Best-Fit Next-fit
Increases memory use
Memory allocation takes less time
Reduces internal fragmentation
Memory waste not solved
Best-Fit
Worse memory utilization than First-it
Results in a smaller “free” space (sliver)
Memory waste not solved
More complex algorithm
Searches entire table before allocating memory
Other allocation schemes:
Worst-fit
Next-fit
Slightly worse memory utilization than First-Fit
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30. Early systems Single-user configuration Fixed partitions
Dynamic partitions
allocation
deallocation
Relocatable Dynamic partitions
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31. Release of memory space -deallocation-
Deallocation for fixed partitions is simple
Memory Manager resets status of memory block to “free”.
Deallocation for dynamic partitions tries to combine free areas of memory whenever possible
Is the block adjacent to another free block?
If yes, combine the 2 blocks
Is the block between 2 free blocks?
If yes, combine those 3 blocks
Is the block isolated from other free blocks?
If yes, put it in the table entry
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32. Deallocation -an example-
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33. Early systems Single-user configuration Fixed partitions
Dynamic partitions
Relocatable Dynamic partitions
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34. Relocatable Dynamic partitions
Solves problem of both internal and external fragmentation
Compaction and reallocation (increased throughput)
no memory waste
Memory Manager relocates programs to gather all empty blocks and compact them to make 1 memory block
Memory compaction (garbage collection, defragmentation) performed by OS to reclaim fragmented sections of memory space
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35. Compaction Steps
Relocate every program in memory so they’re contiguous
Adjust every address, and every reference to an address, within each program to account for program’s new location in memory
Must leave alone all other values within the program (e.g., data values) Q:
When should compaction be done?
When a certain percent of memory becomes busy (e.g. 75%),
When there are processes pending, or
After a prescribed amount of time
Note: Compaction cannot be done if the relocation is static
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36. Compaction -an example-
Memory before and after compaction
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37. Memory compaction (cnt’d)2 3 3 2 3 2 2 3 4 5 11 11 2 5 20 5 5 5 9 9 9
(a)
(b)
(c)
(d)
(a) initial state; (b) complete compaction
(c) partial compaction; (d) minimal data movement
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38. Memory compaction (cnt’d)
Schemes are very costly
Require each word of memory to be read from and written into memory
May take several seconds
Few contemporary systems implement memory compaction
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39. Relocation Introduces overhead Q:
Who keeps track of how each job has moved from its original storage area?
What lists have to be updated? A:
Special-purpose registers
Bounds register
Stores highest (or lowest) location of memory accessible by each program
Relocation register
Contains value to be added to each address referenced in the program
Both free and the busy lists are updated
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40. Relocation -an example-
Job loaded into memory location 30K (30720)
It requires memory block of 32K (32768)
At memory location an instruction: LOAD 4, ANSWER
Answer is the value 37
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41. Conclusions Single-user configuration Fixed partitions
Dynamic partitions
Relocatable Dynamic partitions
Comments:
All require that the entire program
be loaded into memory
be stored contiguously
remain in memory until a process is completed
Severe restrictions to the process size
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