CMPT 300 Operating System I

1
CMPT 300 Operating System I

• Chapter 4Memory Management

2
Why Memory Management?

• Why money management?
• Not enough money. Same thing for memory
• Parkinsons law programs expand to fill the
  memory available to hold them
• 640KB memory are enough for everyone Bill
  Gates
• Programmers ideal an infinitely large,
  infinitely fast memory, nonvolatile
• Reality memory hierarchy

Registers
Cache
Main memory
Magnetic disk
Magnetic tape
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What Is Memory Management?

• Memory manager the part of the OS managing the
  memory hierarchy
• Keep track of memory parts in use/not in use
• Allocate/de-allocate memory to processes
• Manage swapping between main memory and disk
• Basic memory management every program is put and
  run in main memory as whole
• Swapping paging move processes back and forth
  between main memory and disk

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Outline

• Basic memory management
• Swapping
• Virtual memory
• Page replacement algorithms
• Modeling page replacement algorithms
• Design issues for paging systems
• Implementation issues
• Segmentation

5
Mono Programming

• One program at a time
• Share memory with OS
• OS loads the program from disk to memory
• Three variations

0xFFF
0
0
0
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Multiprogramming With Fixed Partitions

• Advantages of Multiprogramming?
• Scenario multiple programs at a time
• Problem how to allocate memory?
• Divide memory up into n partitions, one partition
  can at most hold one program (process)
• Equal partitions vs. unequal partitions
• Each partition has a job queue
• Can be done manually when system is up
• A job arrives, put it into the input queue for
  the smallest partition large enough to hold it
• Any space in a partition not used by a job is lost

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Example Multiprogramming With Fixed Partitions
800K
700K
B
400K
A
200K
100K
0
Multiple input queues
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Single Input Queue
800K

• Disadvantage of multiple input queues
• Small jobs may wait, while a queue with larger
  memory is empty
• Solution single input queue

700K
250K
B
400K
A
10K
200K
100K
0
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How to Pick Jobs?

• Pick the first job in the queue fitting an empty
  partition
• Fast, but may waste a large partition on a small
  job
• Pick the largest job fitting an empty partition
• Memory efficient
• Smallest jobs may be interactive ones, need best
  service, slow
• Policies for efficiency and fairness
• Have at least one small partition around
• A job may not be skipped more than k times

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A Naïve Model for Multiprogramming

• Goal determine the number of processes in main
  memory to keep the CPU busy
• Multiprogramming improves CPU utilization
• If on average, a process computes 20 of the time
  it sitting in memory ? 5 processes can keep CPU
  busy all the time
• Assume all processes never wait for I/O at the
  same time.
• Too optimistic!

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A Probabilistic Model

• A process spends a fraction p of its time waiting
  for I/O to complete
• 0ltplt1
• At once n processes in memory
• CPU utilization 1 pn
• Probability that all n processes are waiting for
  I/O pn
• Assume processes are independent to each other
• Not true in reality. A process has to wait
  another process to give up CPU
• Using queue theory.

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CPU Utilization

• 1 pn

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Memory Management for Multiprogramming

• Relocation
• When program is compiled, it assumes the starting
  address is 0. (logical address)
• When it is loaded into memory, it could start at
  any address. (physical address)
• How to map logical address to physical address?
• Protection
• A programs access should be confined to proper
  area

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Relocation Protection

• Logical address for programming
• Call a procedure at logical address 100
• Physical address
• When the procedure is in partition 1 (started
  from physical address 100k), then the procedure
  is at 100K100
• Relocation problem translation between logical
  address and physical address
• Protection a malicious program can jump to space
  belonging to other users
• Generate a new instruction on the fly that can
  reads or writes any word in memory

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Relocation/Protection Using Registers

• Base register start of the partition
• Every memory address generated adds the content
  of base register
• Base register 100K, CALL 100 ? CALL 100K 100
• Limit register length of the partition
• Addresses are checked against the limit register
• Disadvantage perform addition and comparison on
  every memory reference

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Outline

• Basic memory management
• Swapping
• Virtual memory
• Page replacement algorithms
• Modeling page replacement algorithms
• Design issues for paging systems
• Implementation issues
• Segmentation

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In Time-sharing/Interactive Systems

• Not enough main memory to hold all currently
  active processes
• Intuition excess processes must be kept on disk
  and brought in to run dynamically
• Swapping bring in each process in entirely
• Assumption each process can be held in main
  memory, but cannot finish at one run
• Virtual memory allow programs to run even when
  they are only partially in main memory
• No assumption about program size

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Swapping
Hole
Time ?
Swap A out
Swap B out
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Swapping V.S. Fixed Partitions

• The number, location and size of partitions vary
  dynamically in swapping
• Flexibility, improve memory utilization
• Complicate allocating, de-allocating and keeping
  track of memory
• Memory compaction combine holes in memory into
  a big one
• More efficient in allocation
• Require a lot of CPU time
• Rarely used in real systems

20
Enlarge Memory for a Process

• Fixed size process easy
• Growing process
• Expand to the adjacent hole, if there is a hole
• Otherwise, wait or swap some processes out to
  create a large enough hole
• If swap area on the disk is full, wait or be
  killed
• Allocate extra space whenever a process is
  swapped in or move

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Handling Growing Processes
Processes with one growing data segment
Processes with growing data and stack segments
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Memory Management With Bitmaps

• Two ways to keep track of memory usage
• Bitmaps and free lists
• Bitmaps
• Memory is divided into allocation units
• One bit per unit 0-free, 1-occupied

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Size of Allocation Units

• 4 bytes/unit ? 1 bit in map for 32 bits of memory
  ? bitmap takes 1/33 of memory
• Trade-off between allocation unit and memory
  utilization
• Smaller allocation unit ? larger bitmap
• Larger allocation unit ? smaller bitmap
• On average, half of the last unit is wasted
• When bring a k unit process into memory
• Need find a hole of k units
• Search for k consecutive 0 bits in the entire map

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Memory Management With Linked Lists

• Two types of entries hole(H)/process(P)

Address 20
Length 6
List is kept sorted by address.
Starts at 20
Process
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Updating Linked Lists

• Combine holes if possible
• Not necessary for bitmap

Before process X terminates
After process X terminates
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Allocate Memory for New Processes

• First fit find the first hole fitting
  requirement
• Break the hole into two pieces P smaller H
• Next fit start search from the place of last fit
• Empirical evidence Slightly worse performance
  than first fit
• Best fit take the smallest hole that is adequate
• Slower
• Generate tiny useless holes
• Worst fit always take the largest hole

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Using Distinct Lists

• Distinct lists for processes and holes
• List of holes can be sorted on size
• Best fit becomes faster
• Problem how to free a process?
• Merging holes is very costly
• Quick fit grouping holes based on size
• Different lists for different sizes
• E.g., List 1 for 4KB holes, List 2 for 8KB holes.
• How about a 5KB hole?
• Speed up the searching
• Merging holes is still costly

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Outline

• Basic memory management
• Swapping
• Virtual memory
• Page replacement algorithms
• Modeling page replacement algorithms
• Design issues for paging systems
• Implementation issues
• Segmentation

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Why Virtual Memory?

• If the program is too big to fit in memory
• Split the program into pieces overlays
• Swapping overlays in and out
• Problem programmer does the work of splitting
  the program into pieces.
• Virtual memory OS takes care of everything
• Size of program could be larger than the physical
  memory available.
• Keep the parts currently used in memory
• Put other parts on disk

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Virtual and Physical Addresses

• Virtual addresses (VA) are used/generated by
  programs
• Each process has its own VA.
• E.g, MOV REG, 1000 1000 is VA
• Physical addresses (PA) are used in execution
• MMU maps VA to PA

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Paging

• Virtual address space is divided into pages
• Memories are allocated in the unit of page
• Page frames in physical memory
• Pages and page frames are always the same size
• Usually, from 512B to 64KB
• Pages gt Page frames
• On a 32-bit PC, VA could be as large as 4GB, but
  PA lt 1GB
• In hardware, a present/absent bit keeps track of
  which pages are physically present in memory.
• Page fault an unmapped page is requested
• OS picks up a little-used page frame and write
  its content back to hard disk
• Fetch the wanted page into the page frame just
  freed

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Paging An Example
Pages

• Page 0 04095
• VA 0? page 0? page frame 2? PA 8192
• 04095? 8192--12287
• VA 8192? page 2? page frame 6? PA 24567
• VA 8199? page 2, offset 7? page frame 6, offset
  7? PA 24567724574
• VA32789? page 8? unmapped? page fault

Page frames
8
2
1
0
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The Magic in MMU
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Page Table

• Map virtual pages onto page frames
• VA is split into page number and offset.
• Each page number has one entry in page table.
• Page table can be extremely large
• 32 bits virtual addresses, 4kb/page? 1M pages.
  How about 64 bits VA?
• Each process needs its own page table

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Typical Page Table Entry

• Entry size usually 32 bits
• Page frame number goal of page mapping
• Present/absent bit page in memory?
• Protection what kinds of access permitted
• Modified Has the page been written? (If so, need
  to write back to disk later) Dirty bit
• Referenced Has the page been referenced?
• Caching disable read from the disk?

Present/absent
Caching disabled
Modified
Referenced
Protection
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Fast Mapping

• Virtual to physical mapping must be fast
• several page table references/instruction
• Unacceptable to store the entire page table in
  main memory
• Have to seek for hardware solutions

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Two Simple Designs for Page Table

• Use fast hardware registers for page table
• Single physical page table in MMU an array of
  fast registers one entry for each virtual page
• Requires no memory reference during mapping
• Load registers at every process switching
• Expensive if the page table is large
• Cost of hardware and overhead of context
  switching
• Put the whole table in main memory
• Only one register pointing to the start of table
• Fast switching
• Several memory references/instruction
• Pure memory solution is slow, pure register
  solution is expensive, so

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Translation Lookaside Buffers (TLBs)

• Observation Most programs tend to make a large
  number of references to a small number of pages
• Put the heavily read fraction in registers
• TLB/associative memory

check
Page table
TLB
Virtual address
found
Not found
Physical address
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Outline

• Basic memory management
• Swapping
• Virtual memory
• Page replacement algorithms
• Modeling page replacement algorithms
• Design issues for paging systems
• Implementation issues
• Segmentation

40
Page Replacement

• When a page fault occurs, and all page frames are
  full
• Choose one page to remove, if modified (called
  dirty page), update its disk copy
• Better choose an unmodified page
• Better choose a rarely used page
• Many similar problems in computer systems
• Memory cache page replacement
• Web page cache replacement in web server
• Revisit page table entry

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Typical Page Table Entry

• Entry size usually 32 bits
• Page frame number goal of page mapping
• Present/absent bit page in memory?
• Protection what kinds of access permitted
• Modified Has the page been written? (If so, need
  to write back to disk later) Dirty bit
• Referenced Has the page been referenced?
• Caching disable read from the disk?

Present/absent
Caching disabled
Modified
Referenced
Protection
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Optimal Algorithm

• Label each page in the main memory with number of
  instructions will be executed before next
  reference
• E.g, a page labeled by 1 means this page will
  be referenced by the next instruction.
• Remove the page with highest label
• Put off page faults as long as possible
• Unrealizable!
• Why? SJF process scheduling, Bankers Algorithm
  for deadlock avoidance
• Could be used as a benchmark

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Remove Not Recently Used Pages

• R and M are initially 0
• Set R when a page is referenced
• Set M when a page is modified
• Done by hardware
• Clear R bit periodically by software (OS)
• Four classes of pages when a page fault
• Class 0 (R0M0) not referenced, not modified
• Class 1 (R0M1) not referenced, modified
• Class 2 (R1M0) referenced, not modified
• Class 3 (R1M1) referenced, modified
• NRU removes a page at random from the lowest
  numbered nonempty class