This document discusses concepts related to virtual memory systems, including demand paging, page replacement algorithms, and techniques to prevent thrashing. Demand paging brings pages into memory only when needed, reducing I/O. When a page is accessed that is not in memory, a page fault occurs and the OS selects a frame using a page replacement algorithm like least recently used. If no frames are available, thrashing can occur where processes are constantly swapping pages in and out.
The French Revolution, which began in 1789, was a period of radical social and political upheaval in France. It marked the decline of absolute monarchies, the rise of secular and democratic republics, and the eventual rise of Napoleon Bonaparte. This revolutionary period is crucial in understanding the transition from feudalism to modernity in Europe.
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It is possible to hide or invisible some fields in odoo. Commonly using “invisible” attribute in the field definition to invisible the fields. This slide will show how to make a field invisible in odoo 17.
The Roman Empire A Historical Colossus.pdfkaushalkr1407
The Roman Empire, a vast and enduring power, stands as one of history's most remarkable civilizations, leaving an indelible imprint on the world. It emerged from the Roman Republic, transitioning into an imperial powerhouse under the leadership of Augustus Caesar in 27 BCE. This transformation marked the beginning of an era defined by unprecedented territorial expansion, architectural marvels, and profound cultural influence.
The empire's roots lie in the city of Rome, founded, according to legend, by Romulus in 753 BCE. Over centuries, Rome evolved from a small settlement to a formidable republic, characterized by a complex political system with elected officials and checks on power. However, internal strife, class conflicts, and military ambitions paved the way for the end of the Republic. Julius Caesar’s dictatorship and subsequent assassination in 44 BCE created a power vacuum, leading to a civil war. Octavian, later Augustus, emerged victorious, heralding the Roman Empire’s birth.
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June 3, 2024 Anti-Semitism Letter Sent to MIT President Kornbluth and MIT Cor...Levi Shapiro
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The US House of Representatives is deeply concerned by ongoing and pervasive acts of antisemitic
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The House of Representatives will not countenance the use of federal funds to indoctrinate students into hateful, antisemitic, anti-American supporters of terrorism. Investigations into campus antisemitism by the Committee on Education and the Workforce and the Committee on Ways and Means have been expanded into a Congress-wide probe across all relevant jurisdictions to address this national crisis. The undersigned Committees will conduct oversight into the use of federal funds at MIT and its learning environment under authorities granted to each Committee.
• The Committee on Education and the Workforce has been investigating your institution since December 7, 2023. The Committee has broad jurisdiction over postsecondary education, including its compliance with Title VI of the Civil Rights Act, campus safety concerns over disruptions to the learning environment, and the awarding of federal student aid under the Higher Education Act.
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1. Operating System Concepts Silberschatz and Galvin19999.1Operating System Concepts Silberschatz and Galvin19995.1Operating System Concepts Silberschatz and Galvin 19994.1
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O P E R A T I N G S Y S T E M S
Module 9 : Virtual Memory
• Basic Concepts
• Scheduling Criteria
• Scheduling Algorithms
• Multiple-Processor Scheduling
• Real-Time Scheduling
• Algorithm Evaluation
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Module 9: Virtual Memory
• Background
• Demand Paging
• Performance of Demand Paging
• Page Replacement
• Page-Replacement Algorithms
• Allocation of Frames
• Thrashing
• Other Considerations
• Demand Segmenation
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Background
• Virtual memory – 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.
– Need to allow pages to be swapped in and out.
• Virtual memory can be implemented via:
– Demand paging
– Demand segmentation
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Demand Paging
• 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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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 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: Least Recently Used
– block move
– auto increment/decrement location
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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.
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Performance of Demand Paging
• Page Fault Rate 0 p 1.0
– if p = 0 no page faults
– if p = 1, every reference is a fault
• Effective Access Time (EAT)
EAT = (1 – p) x memory access
+ p (page fault overhead
+ [swap page out ]
+ swap page in
+ restart overhead)
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Demand Paging Example
• Memory access time = 1 microsecond
• 50% of the time the page that is being replaced has been
modified and therefore needs to be swapped out.
• Swap Page Time = 10 msec = 10,000 msec
EAT = (1 – p) x 1 + p (15000)
1 + 15000P (in msec)
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Page Replacement
• 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.
• Page replacement completes separation between logical memory
and physical memory – large virtual memory can be provided on
a smaller physical memory.
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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.
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First-In-First-Out (FIFO) Algorithm
• 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
• FIFO Replacement – Belady’s Anomaly
– more frames less 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
44 3
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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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Least Recently Used (LRU) Algorithm
• Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
• 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.
1
2
3
5
4
4 3
5
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LRU Algorithm (Cont.)
• 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
– No search for replacement
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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.
• Second chance
– Need reference bit.
– Clock replacement.
– If page to be replaced (in clock order) has reference bit = 1.
then:
set reference bit 0.
leave page in memory.
replace next page (in clock order), subject to same rules.
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Counting Algorithms
• Keep a counter of the number of references that have been
made to each page.
• LFU Algorithm: replaces page with smallest count.
• MFU Algorithm: based on the argument that the page with the
smallest count was probably just brought in and has yet to be
used.
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Allocation of Frames
• Each process needs minimum number of pages.
• Example: IBM 370 – 6 pages to handle SS MOVE instruction:
– instruction is 6 bytes, might span 2 pages.
– 2 pages to handle from.
– 2 pages to handle to.
• Two major allocation schemes.
– fixed allocation
– priority allocation
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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.
si= size of process pi
S=∑si
m= total number of frames
ai= allocation for pi=
si
S
×m m=64
si=10
s2
=127
a1=
10
137
×64≈5
a2=
127
137
×64≈59
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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.
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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.
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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 added to the system.
• Thrashing a process is busy swapping pages in and out.
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Thrashing Diagram
• 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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Working-Set Model
working-set window a fixed number of page references
Example: 10,000 instruction
• 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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Keeping Track of the 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?
• Improvement = 10 bits and interrupt every 1000 time units.
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Page-Fault Frequency Scheme
• Establish “acceptable” page-fault rate.
– If actual rate too low, process loses frame.
– If actual rate too high, process gains frame.
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Other Considerations
• Preparing
• Page size selection
– fragmentation
– table size
– I/O overhead
– locality
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Other Consideration (Cont.)
• Program structure
– Array A[1024, 1024] of integer
– Each row is stored in one page
– One frame
– Program 1 for j := 1 to 1024 do
for i := 1 to 1024 do
A[i,j] := 0;
1024 x 1024 page faults
– Program 2 for i := 1 to 1024 do
for j := 1 to 1024 do
A[i,j] := 0;
1024 page faults
• I/O interlock and addressing
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Demand Segmentation
• Used when insufficient hardware to implement demand paging.
• OS/2 allocates memory in segments, which it keeps track of
through segment descriptors
• Segment descriptor contains a valid bit to indicate whether the
segment is currently in memory.
– If segment is in main memory, access continues,
– If not in memory, segment fault.