Assignment 3 Virtual Memory

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Assignment 3 Virtual Memory

• In your third assignment you will implement a
  virtual memory simulator.
• VMs goal is to give the user the ability to
  write programs without the concern of physical
  memory size in her computer.
• The simulator will enable simulation of paging
  hardware and page-replacement software and
  testing of various page replacement strategies.

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The main questions

• Which page replacement algorithm to use?
• how to maintain the page tables?
• Before we can answer these questions we must
  review our hardware.

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The main components

• Swapper - very simple swapper device,
  simulating a paging disk. It reads/writes pages
  from/to a specific page address.
• Fast memory - the physical memory and some
  info.It has the ability to read/write a byte or
  a page from/to a specific address. For the same
  price, it includes also a table with the
  following info on each page ID ,Dirty bit,
  Reference bit
• MMU the hardware translator from logical to
  physical addresses. Has limited amount of space
  to store information. When a page is not in
  physical memory, the MMU will trap to the page
  replacement manager.

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And two more

• Page Replacement Manager - acts as the OS in
  time of a trap from the MMU. When called to duty,
  it chooses a page from physical memory and
  replaces it with the requested page.
• VM The object that the user has to interface.
  All other components are transparent to the user.
  It provides read/write from/to any address in the
  virtual address space, and requests from the
  system some statistical data (e.g. hit ratio).

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Back to our questions

• Which page replacement algorithm to use?
• Answer you will have to design a LRU
  approximation algorithm with the given hardware
  in the fast memory.
• For comparison, also a FIFO algorithm.

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• How to maintain the page table?
• Answer Use a 2-level page table. The first
  level is stored in the MMU cache memory. The 2nd
  level tables are page sized each and are located
  in the physical memory.Important 2nd level
  tables may be swapped in and out of memory.

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A Typical configuration.
Physical Memory
Kernel space
Swap device
User space
Kernel space
User space
User Page no 1
First Level Table in MMU.
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What happens if

• The user wishes to write to user page no 6.
• The user wishes to write to user page no 5, while
  the next candidate to be swapped out is user page
  no 6.
• The user wishes to write to user page no 3, while
  the next candidate to be swapped out is user page
  no 7.

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The scenario

• User wishes to Read/Write a character from/to
  address v_adr in the virtual memory that belongs
  to a virtual page number pg.
• The virtual memory queries the MMU for the
  physical address of v_adr .
• The MMU first checks (in the first level table)
  if the second level page (that contains the entry
  for pg) is in physical memory. If it is, go to 6.
• Notify the Page Replacement manager that a page
  fault occurred provide the required information.
  
• The Page Replacement Manager chooses a page p
  from the second level pages section in the
  physical memory and replaces the requested page
  with p. Then it updates both entries in the first
  level table. Go to 3.

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• Look for the physical address of pg in the
  appropriate second level page table entry. If it
  is in physical memory, then return correct
  physical address of v_adr and go to 9.
• The MMU notifies the Page Replacement Manager
  that a page fault occurred.
• The Page Replacement Manager chooses page sp from
  the user pages section of the physical memory and
  replaces the requested page with sp. Then it
  updates both entries in the appropriate second
  level pages (But the second level page containing
  the entry of sp might not be in physical memory.
  In that case we have another page fault that has
  to be taken care of). Go to 6
• The VM receives from the MMU the physical address
  of v_adr and reads/writes from/to that physical
  address.

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VMInterface

• virtual bool initialize(char fileName) 0
• virtual bool write(int address,char data) 0
• virtual const char read(int address) const 0
• There is more..

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For evaluating your assignment

• virtual void pf_history() 0 for each page
  fault, displays on screen a record serial
  number, type(kernel/user), Page In, Page out
• virtual double hit_ratio() 0
• virtual void showMemoryTable()
• virtual void showPhysicalAddress(int adr)0
• virtual void showFirstLevelPageTable()0
• virtual void showSecondLevelPageTable(int i)0
• Important these methods are for evaluation only
  and will not change the simulators configuration.

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Assignment 3 part 2

• Write a short program in C that on execution
  will do the following
•  
• Do
• 1. Display Menu
• 1. Increase Working set.
• 2. Decrease working set
• 3. Exit.
• 2. Read input
• 3. Execute code that will cause Increase/
  Decrease Working set size.
• While (input! Exit)