this is chapter om operating system course

1. COURSE NAME : OPERATING SYSTEM
COURSE CODE : 381CCS-3
CHAPTER 7 : VIRTUAL MEMORY
SUBJECT COORDINATOR: DR. ANAND DEVADURAI
SUBJECT TEACHER: MRS. MARIYAM AYSHA BIVI
DEPARTMENT OF COMPUTER SCIENCE
KING KHALID UNIVERSITY
ABHA, KSA.
REFERENCE DETAIL FOR CHAPTER 6
Topic Text Reference Chapter No. Page No
Virtual Memory
“Operating System
Concepts”, 10th Edition,
Abraham SilberSchatz,
Peter Baer Galvin,
Greg Gagne, Wiley, 2018
Chapter 10
389-399
401-412

2. 2
 Background 399 (501 to 511)
 Demand Paging 10.2
 Basic Concept
 Free-Frame list
 Performance of Demand paging
 Page Replacement 10.4
 Basic Page Replacement (401 - 412 : 513 to 524)
 FIFO Page Replacement
 Optimal Page Replacement
 LRU Page Replacement
 Counting-Based Page Replacement
CHAPTER 7 : VIRTUAL MEMORY

3. 7.1 BACKGROUND
3
 Code needs to be in memory to execute, but entire program rarely used
• Error code, unusual routines, large data structures
 Entire program code not needed at same time
 Virtual memory
• It is a scheme that allows a process to execute while it is only partially
in memory,
 Advantages of Virtual memory
• Program no longer constrained by limits of physical memory
• Logical address space will be larger than physical address space Process
size can exceed the physical memory size
• Less I/O needed
• Less memory needed
• Faster response
• More programs running concurrently
• Increased CPU utilization and throughput
 Disadvantages of Virtual memory
• Many page faults when a process is started but should decrease as
more pages are brought in

4. 7.1 BACKGROUND(CONT…)
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 Virtual address space: logical/virtual) view of how process is stored in memory
 start at address 0, contiguous addresses until end of space
 Physical memory is organized in page frames, assigned to a process may not be
contiguous. Memory Management Unit must map logical to physical
 The heap grow upward used for dynamic memory allocation and the stack grow
downward in memory through successive function calls.
 Virtual address spaces that include holes are known as sparse address spaces..
Figure 7.1 Virtual address space
of a process in memory Figure 7.2 Shared library using virtual memory.

5. 7.2 DEMAND PAGING
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 Load pages only as they are needed is known as Demand Paging, commonly
used in virtual memory systems.
7.2.1 BASIC CONCEPT
 With each page table entry a valid–invalid bit is associated
(v  in-memory – memory resident, i  not-in-memory)
 Initially valid–invalid bit is set to i on all entries
 During MMU address translation, if valid–invalid bit in is i  page fault
Figure 7.3 Page table when some pages are not in main memory.

6. 6
STEPS IN HANDLING A PAGE FAULT
1. If there is a reference to a page, first reference to that page will trap to operating
system Page fault
2. Operating system looks at another table to decide:
Invalid reference  abort, Just not in memory
3. Find free frame
4. Swap page into frame via scheduled disk operation
5. Reset tables to indicate page now in memory, Set validation bit = v
6. Restart the instruction that caused the page fault
Figure 7.3 Page table when some pages are not in main memory.

7. 7.2.2 FREE-FRAME LIST
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 When a page fault occurs, the operating system must bring the desired page from
secondary storage into main memory.
 Most operating systems maintain a free-frame list -- a pool of free frames for
satisfying such requests.
 Operating system typically allocate free frames using a technique known as zero-
fill-on-demand -- the content of the frames zeroed-out before being allocated.
 When a system starts up, all available memory is placed on the free-frame list.
7.2.2 PERFORMANCE OF DEMAND PAGING
 Page Fault Rate 0  p  1,if p = 0 no page faults, if p = 1, every reference is a fault
 Effective Access Time (EAT) = (1 − p) × ma + p × page fault time.
 Example: If one access out of 1,000 causes a page fault, Memory access time =
200 nanoseconds, Average page-fault service time = 8 milliseconds Compute EAT
EAT = (1 – p) x 200 + p (8 milliseconds)
= (1 – p) x 200 + p x 8,000,000
= 200 – 200 p + 8,000,000 p =
= 200 + p x 7,999,800
= 200 + (1/1000) x 7,999,800 = 8,199.8 milli Seconds
EAT = 8,199.8 /1000 = 8.2 microseconds.

8. 7.3 PAGE REPLACEMENT
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 Prevent over-allocation of memory by modifying page-fault service routine to
include page replacement
 Use modify (dirty) bit to reduce overhead of page transfers – only modified
pages are written to disk
7.3.1 BASIC PAGE REPLACEMENT
1. Find the location of the desired page on disk
2. Find a free frame:
- If there is a free frame, use it
- If no free frame, use a page replacement
algorithm to select a victim frame
- Write victim frame to disk if dirty
3. Bring the desired page into the (newly) free
frame; update the page and frame tables
4. Continue the process by restarting the
instruction that caused the trap

9. 7.3.2 FIFO PAGE REPLACEMENT ALGORITHM
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 When a page must be replaced, the oldest page is chosen.
 Reference string: 7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1
 3 frames (3 pages can be in memory at a time per process)
 Page Fault = 15
 Can vary by reference string: consider 1,2,3,4,1,2,5,1,2,3,4,5
• Adding more frames can cause more page faults!
 Belady’s Anomaly
 How to track ages of pages?
• Just use a FIFO queue

10. 7.3.3 OPTIMAL PAGE REPLACEMENT ALGORITHM
1
0
 Replace page that will not be used for longest period of time
• 9 is optimal for the example
 How do you know this?
• Can’t read the future
 Used for measuring how well your algorithm performs
 Page Fault = 9

11. 7.3.4 LEAST RECENTLY USED (LRU) ALGORITHM
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 Replace page that has not been used in the most amount of time
 Associate time of last use with each page
 Page Fault = 12, Better than FIFO but worse than OPT
 Generally good algorithm and frequently used
7.3.5 COUNTING BASED PAGE REPLACEMENT ALGORITHM
 Lease Frequently Used (LFU) Algorithm:
• Replaces page with smallest count
 Most Frequently Used (MFU) Algorithm:
• Based on the argument that the page with the smallest count was
probably just brought in and has yet to be used