Lecture 8- Virtual Memory Final.pptx

Operating Systems: CSE 3204
ASTU
Department of CSE
January 4, 2023 1
Operating Systems
Lecture 8 - Virtual Memory
Chapter Three
Memory Management
Outline
Demand Paging
Page Replacement
Allocation of Frames
Thrashing

Background
• First requirement for execution: Instructions must be in physical memory
• One Approach: Place entire logical address in main memory.
• Overlays and dynamic loading may relax this criteria.
• But the size of the program is limited to size of main memory.
• Normally entire program may not be needed in main memory.
• Programs have error conditions.
• Arrays, lists, and tables may be declared by 100 by 100 elements, but
seldom larger than 10 by 10 elements.
• Assembler program may have room for 3000 symbols, although average
program may contain less than 200 symbols.
• Certain portions or features of the program are used rarely.
• Benefits of the ability to execute program that is partially in memory:
• User can write programs and software for entirely large virtual address
space.
• More programs can run at the same time.
• Less I/O would be needed to load or swap each user program into
memory.
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Background (Cont….)
 Virtual memory is a technique that allows the execution of processes that may
not be completely in memory.
• Programs are larger than main memory.
• VM abstract main memory into an extremely large, uniform array of
storage.
 Separation of user logical memory from physical memory.
• Only part of the program needs to be in memory for execution.
• Logical address space can therefore be much larger than physical address
space.
• Allows address spaces to be shared by several processes.
• Allows for more efficient process creation.
• Frees the programmer from memory constraints.
 Virtual memory can be implemented via:
• Demand paging
• Demand segmentation
 We only cover demand paging.
 For demand segmentation refer research papers. -> IBM OS/2, Burroughs’
computer systems
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Virtual Memory That is Larger Than Physical Memory
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Demand Paging
• Paging system with swapping.
• When we execute a process we swap into memory (next fig).
• For demand paging, we use lazy swapper.
• Never swaps a page into memory unless required.
• Bring a page into memory only when it is needed.
• Less I/O needed
• Less memory needed
• Faster response
• More users
• Page is needed  reference to it
• invalid reference  abort
• not-in-memory  bring to memory
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Transfer of a Paged Memory to Contiguous Disk Space
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Valid-Invalid Bit
 With each page table entry a valid–invalid bit is associated
(1  in-memory, 0  not-in-memory)
 Initially valid–invalid but is set to 0 on all entries.
 Example of a page table snapshot.
 During address translation, if valid–invalid bit in page table entry is 0  page
fault.
1
1
1
1
0
0
0
Frame # valid-invalid bit
page table
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Page table when some pages are not in main memory
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Page Fault
• If there is ever a reference to a page, first reference will trap to OS 
page fault
• OS looks at another table to decide:
• Invalid reference  abort.
• Just not in memory.
• Get empty frame.
• Swap page into frame.
• Reset tables, validation bit = 1.
• Restart instruction:
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Steps in Handling a Page Fault
1. Check the internal table, to
determine whether this
reference is valid or invalid.
2. If the reference is invalid,
then terminate.
3. Find free frame.
4. Schedule disk operation.
5. Modify the internal table to
indicate that page is in main
memory.
6. Restart the instruction.
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What happens if there is no free frame?
• Page replacement – find some page in memory, but not
really in use, swap it out.
• algorithm
• performance – want an algorithm which will result in
minimum number of page faults.
• Same page may be brought into memory several times.
• Theoretically some programs access several pages of new memory causing
multiple page faults per instruction.
• But analysis of program show locality of reference.
• Hardware support to demand paging
• Page table
• Secondary memory; high speed disk
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Hardware support to demand paging

Software support to demand paging
• Additional software is also required.
• Restarting of instruction after page fault.
• Page fault could occur any time during execution.
• Ex: Add the contents of A and B and replace the result in C
• 1.Fetch and decode the instruction
• Fetch A
• Fetch B
• Add A and B
• Store the sum in C.
• If the page is faulted if we try to store C, we have to restart the
instruction.
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Software support to demand paging…
• Difficulty occurs if the instruction modifies several different locations.
• In IBM 360/370 MVC (Move character) instruction, we can move 256
bytes from one location to another.
• source and destination may overlap
• If the page fault occurs after partial moving, we can not redo the
instruction, if regions overlap.
• Solution:
• 1. Use micro code to access both ends of blocks
• If page fault is going to occur, it will occur.
• 2. Use temporary registers to hold the values of temporary registers
• If a page fault occurs old values are written back to memory,
restoring the memory state to before the instruction was started.
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Hardware support and software support to
demand paging…
• Also, similar difficulty occurs in machines that use special addressing modes.
Uses register as a pointer
• Auto-increment: increment after using
• auto-decrement. Decrements before using
• MOV (R2)+, -(R3)
• If the page fault occurs while storing in R3, we have to restart the
instruction by restoring the values of R2 and R3.
• if the instruction modifies several different locations.
• Solution: use status register to record the register number and amount
modified so that OS can undo the effect of partially executed instruction that
causes a page fault.
• EVERY THING SHOULD BE TRANSAPARENT TO USER
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Performance of Demand Paging
• Let p be the probability of page fault.
• Page Fault Rate 0  p  1.0
• if p = 0 no page faults , effective access time=memory access time.
• if p = 1, every reference is a fault
• Effective Access Time (EAT)
EAT = (1 – p) x memory access+ p (page fault overhead)
 Major operations during page fault:
 Trap to OS; save user registers and process state; issue a disk read; wait
for interrupt from disk; wait for the CPU; restore the process status;
Demand Paging Example
 Memory access time = 100 nanoseconds, Page fault service time =
25milliseconds
Effective access time (EAT) = (1 – p) x 100 + p (25 msec)
• =100+24,999,900 x p
• EAT is directly proportional to page-fault rate.
• It is important to keep the page-fault rate low.
• Otherwise EAT increases and slowing the process execution dramatically.
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Advantages of VM: Process Creation
• Virtual memory allows other benefits during process creation:
a. Copy-on-Write(COW):- allows both parent and child processes to
initially share the same pages in memory.
If either process modifies a shared page, only then the page copied.
• COW allows more efficient process creation as only modified pages are
copied.
• Free pages are allocated from a pool of zeroed-out pages.
b. Memory-Mapped Files:- allows file I/O to be treated as routine
memory access by mapping a disk block to a page in memory.
• A file is initially read using demand paging. A page-sized portion of the
file is read from the file system into a physical page. Subsequent
reads/writes to/from the file are treated as ordinary memory accesses.
• Simplifies file access by treating file I/O through memory rather than
read() write() system calls.
• Also allows several processes to map the same file allowing the pages in
memory to be shared.
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Memory Mapped Files
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Page Replacement
• Prevent over-allocation of memory by modifying page-fault
service routine to include page replacement.
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 there is no free frame, use a page replacement
algorithm to select a victim frame.
3. Read the desired page into the (newly) free frame. Update the
page table.
4. Restart the process.
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Page Replacement
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Reducing overhead: modify bit
 Use modify (dirty) bit to reduce overhead of page transfers – only modified pages
are written to disk.
 Modify bit can be used to reduce the overhead with the help of hardware.
 It is set indicating the page has been modified.
 While replacing a page
• If the modify bit is set, we must write that page to disk.
• If it is not set we can avoid overwriting it, if is not overwritten.
Page Replacement Algorithms
• Page replacement completes separation between logical memory and physical
memory – large virtual memory can be provided on a smaller physical memory.
• Enormous virtual memory can be provided on a smaller physical memory.
• Two major problems are solved to implement demand paging
• Frame allocation algorithm
• If multiple processes exist in memory we have to decide the number frames
for each process
• Page replacement algorithm
• We have to select a frame that is to be replaced.
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Page Replacement Algorithms
 Want lowest page-fault rate.
 Evaluate algorithm by running it on a particular string of memory references
(reference string) and computing the number of page faults on that string.
 In all our examples, the reference string is :1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5.
 Address reference divided by page size.
 If the address reference is 0432 and page size is 100 then the reference
number is 0432/100=4
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Graph of Page
Faults Versus
The Number of
Frames

First-In-First-Out (FIFO) Algorithm
 Oldest page is replaced; FIFO queue; replace the head of the queue
• Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
 3 frames (3 pages can be in memory at a time per process)
 4 frames
 more frames  more page faults
1
2
3
1
2
3
4
1
2
5
3
4
9 page
faults
1
2
3
1
2
3
5
1
2
4
5 10 page faults
4
4 3
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 This most unexpected result is known as Belady's anomaly: For
some page-replacement algorithms, the page-fault rate may
increase as the number of allocated frames increases.

Optimal Algorithm
• Replace page that will not be used for longest period of time.
• 4 frames example: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
• How do you know this?
• Used for measuring how well your algorithm performs.
1
2
3
4
6 page faults
4 5
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Optimal Page Replacement

Least Recently Used (LRU) Algorithm
• The page that has not been used for longest period of time is replaced.
• Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
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1
2
3
5
4
4 3
5
LRU Page
Replacement

LRU Algorithm Implementation
 Counter implementation
• Every page entry has a counter; every time page is referenced through
this entry, copy the clock into the counter.
• When a page needs to be changed, look at the counters to determine
which are to change.
• Issues:
• Search pf page table to find LRU page, Overflow of clock,..
 Stack implementation – keep a stack of page numbers in a double link
form:
• Page referenced:
• move it to the top
• requires 6 pointers to be changed
• Update is expensive
• No search for replacement
• Top is the most recently used page and bottom is the LRU page.
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Use Of A Stack to Record The Most Recent Page
References
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Performance issue: Stack and Counters
 The updating of stack or clock must be done on every memory reference.
 If we use interrupt for every reference, to allow software to update data
structures, it would slow every reference by a factor of 10.
 Few systems tolerate such degradation in performance.
 Sol: Systems follow LRU approximation implemented through hardware.

LRU Approximation Algorithms
• Reference bit
• With each page associate a bit, initially = 0
• When page is referenced bit set to 1.
• Replace the one which is 0 (if one exists). We do not know the order,
however.
• Additional ordering:
• By maintaining 8-bit byte for each page.
• Shifting can be used to record the history (interrupt to OS for every 100
msec).
• Second chance
• Need one reference bit.
• Clock replacement.
• If page to be replaced (in clock order) has reference bit = 1. then:
• set reference bit 0; arrival time is set to current time.
• leave page in memory.
• replace next page (in clock order), subject to same rules.
• Similar to FIFO if all bits are set.
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Second-Chance (clock) Page-Replacement
Algorithm
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Enhanced-second chance algorithm
• Use reference bit and modify bit as an ordered pair.
• (0,0) neither recently used nor modified – best page to replace.
• (0,1) not recently used but modified- not quite good; to be written before
replacement.
• (1,0) recently used but clean – it probably used again soon.
• (1,1) recently used and modified- it probably will be used again soon; to
be written before replacement.
• Each page is one of four classes.
• Keep a counter of the number of references that have been made to each
page.
• LFU(least frequently used) Algorithm: replaces page with smallest count.
• MFU (MOST frequently used)Algorithm: based on the argument that the
page with the smallest count was probably just brought in and has yet to be
used.
• MFU and LFU are not used
• Implementation is expensive
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Other algorithms: Counting Algorithms

Allocation of Frames
• Each process needs minimum number of pages allocation.
• If there is a single process, entire available memory can be allocated.
• Multi-programming puts two or more processes in memory at same
time.
• We must allocate minimum number of frames to each process.
• Two major allocation schemes.
• fixed allocation
• priority allocation
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1. Fixed Allocation
• Equal allocation – e.g., if 100 frames and 5 processes, give each 20 pages.
• Proportional allocation – Allocate according to the size of process.
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2. Priority Allocation:- Use a proportional allocation scheme using priorities rather
than size.
• If process Pi generates a page fault,
– select for replacement one of its frames.
– select for replacement a frame from a process with lower priority number.

Global vs. Local Allocation
• Global replacement – process selects a replacement frame from the
set of all frames; one process can take a frame from another.
• Local replacement – each process selects from only its own set of
allocated frames.
• With local replacement # of frames does not change.
• Performance depends on the paging behavior of the process.
• Free frames may not be used
• With global replacement, a process can take a frame from another
process.
• Performance depends not only paging behavior of that process, but
also paging behavior of other processes.
• In practice global replacement is used.
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Thrashing
• If a process does not have “enough” pages, the page-fault rate is very high.
This leads to:
• low CPU utilization.
• operating system thinks that it needs to increase the degree of
multiprogramming.
• another process is added to the system.
• Thrashing is High paging activity.
• Thrashing  a process is spending more time in swapping pages in and
out.
• If the process does not have number of frames equivalent to number of
active pages, it will very quickly page fault.
• Since all the pages are in active use it will page fault again.
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Thrashing
• Why does paging work?
Locality model
• Process migrates from one locality to another.
• Localities may overlap.
• Why does thrashing occur?
 size of locality > total memory size
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Causes of thrashing
 OS monitors CPU utilization
 If it is low, increases the degree of MPL
 Consider that a process enters new execution phase and starts faulting.
 It takes pages from other processes
 Since other processes need those pages, they also fault, taking pages
from other processes.
 The queue increases for paging device and ready queue empties
 CPU utilization decreases.
 Solution: provide process as many frames as it needs.
 But how we know how many frames it needs ?
 Locality model provides hope.
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Locality model
 Locality is a set of pages that are actively used together.
 A program is composed of several different localities which may
overlap.
 Ex: even when a subroutine is called it defines a new locality.
 The locality model states that all the programs exhibit this memory
reference structure.
 This is the main reason for caching and virtual memory!
 If we allocate enough frames to a process to accommodate its
current locality, it fault till all pages are in that locality are in
the MM. Then it will not fault.
 If we allocate fewer frames than current locality, the process
will thrash.
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Working-Set Model
 Based on locality->Define a parameter ;
   working-set window  a fixed number of page references
Example: 10,000 instruction
 Most recent references are examined
 WSSi (working set of Process Pi) = total number of pages referenced in the most
recent  (varies in time)
 if  too small will not encompass entire locality.
 if  too large will encompass several localities.
 if  =   will encompass entire program.
 D =  WSSi  total demand frames
 if D > m  Thrashing
 Policy: if D > m, then suspend one of the processes.
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Working Set
 OS monitors the WS of each process allocates to that working set enough
frames equal to WS size.
 If there are enough extra frames, another process can be initiated.
 If D>m, OS suspends a process, and its frames are allocated to other
processes.
 The WS strategy prevents thrashing by keeping MPL as high as possible.
 However, we have to keep track of working set.
Keeping track of Working Set
 Approximate with interval timer + a reference bit, -> Example:  = 10,000
 Timer interrupts after every 5000 time units.
 Keep in memory 2 bits for each page.
 Whenever a timer interrupts copy and sets the values of all reference bits
to 0.
 If one of the bits in memory = 1  page in working set.
 Why is this not completely accurate?
 We can not tell when the reference was occurred.
 Accuracy can be increased by increasing frequency of interrupts which
also increases the cost.
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