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Questions and Answers
What is the return value of the compare_and_swap function when the expected value is not equal to the current value at *value?
What is the return value of the compare_and_swap function when the expected value is not equal to the current value at *value?
- The original value of *value (correct)
- 0, indicating a failed swap
- The value of *value after the swap
- The new_value that was passed in
In the provided solution using compare_and_swap, what does the condition while (compare_and_swap(&lock, 0, 1) != 0) signify?
In the provided solution using compare_and_swap, what does the condition while (compare_and_swap(&lock, 0, 1) != 0) signify?
- The variable lock has been initialized to 1
- The function compare_and_swap has successfully swapped values
- The lock is available for entering the critical section
- The lock is being held by another process (correct)
What atomic operation is being demonstrated in the compare_and_swap instruction?
What atomic operation is being demonstrated in the compare_and_swap instruction?
- Performing concurrent updates without locking
- Updating a variable only if its value matches an expected value (correct)
- Checking the value of a variable and returning it
- Returning the new value without changing the original
How does the compare_and_swap function contribute to solving the critical-section problem?
How does the compare_and_swap function contribute to solving the critical-section problem?
What does the term 'atomic' mean in the context of operations like compare_and_swap?
What does the term 'atomic' mean in the context of operations like compare_and_swap?
What is the primary purpose of a memory barrier in a computer architecture?
What is the primary purpose of a memory barrier in a computer architecture?
Which type of memory model ensures that a memory modification by one processor is immediately observable to all other processors?
Which type of memory model ensures that a memory modification by one processor is immediately observable to all other processors?
Which synchronization tool is described as a method to solve the critical-section problem using a simple lock?
Which synchronization tool is described as a method to solve the critical-section problem using a simple lock?
What happens during the execution of a memory barrier instruction?
What happens during the execution of a memory barrier instruction?
Which scenario would likely require the use of semaphores for synchronization?
Which scenario would likely require the use of semaphores for synchronization?
In the context of a weakly ordered memory model, what can be a consequence?
In the context of a weakly ordered memory model, what can be a consequence?
What is one of the primary evaluations of synchronization tools according to their contention scenarios?
What is one of the primary evaluations of synchronization tools according to their contention scenarios?
What is a unique characteristic of monitors within synchronization tools?
What is a unique characteristic of monitors within synchronization tools?
What does the test_and_set instruction do?
What does the test_and_set instruction do?
Which of the following correctly describes the properties of the test_and_set instruction?
Which of the following correctly describes the properties of the test_and_set instruction?
In the provided solution using test_and_set(), what is the initial value of the lock variable?
In the provided solution using test_and_set(), what is the initial value of the lock variable?
What is the primary purpose of using the test_and_set instruction in multithreading?
What is the primary purpose of using the test_and_set instruction in multithreading?
What is an outcome of the while loop in the solution using test_and_set()?
What is an outcome of the while loop in the solution using test_and_set()?
What is the primary role of a memory barrier in a multi-threaded environment?
What is the primary role of a memory barrier in a multi-threaded environment?
In the provided example, what does the memory barrier accomplish for Thread 1?
In the provided example, what does the memory barrier accomplish for Thread 1?
Why is disabling interrupts considered inefficient in multiprocessor systems?
Why is disabling interrupts considered inefficient in multiprocessor systems?
What is one advantage of hardware support for critical sections?
What is one advantage of hardware support for critical sections?
Which of the following statements is true regarding the synchronization in the example given?
Which of the following statements is true regarding the synchronization in the example given?
What does the function increment() ensure when used with an atomic variable?
What does the function increment() ensure when used with an atomic variable?
Which of the following statements correctly describes the role of mutex locks?
Which of the following statements correctly describes the role of mutex locks?
What operation can be considered complex and inaccessible to application programmers?
What operation can be considered complex and inaccessible to application programmers?
In the increment() function, what is the purpose of the compare_and_swap operation?
In the increment() function, what is the purpose of the compare_and_swap operation?
What is a primary characteristic of semaphores compared to mutex locks?
What is a primary characteristic of semaphores compared to mutex locks?
Which of the following best describes the lifecycle of operations using a mutex lock?
Which of the following best describes the lifecycle of operations using a mutex lock?
Why are calls to acquire() and release() required to be atomic in mutex locks?
Why are calls to acquire() and release() required to be atomic in mutex locks?
Which programming technique is most closely associated with the function definition of increment()?
Which programming technique is most closely associated with the function definition of increment()?
Flashcards
Race Condition
Race Condition
A situation where the outcome of an operation depends on the unpredictable timing of multiple threads accessing shared resources.
Critical Section
Critical Section
A programming construct that allows multiple threads to access shared resources safely, ensuring only one thread is in the critical section at a time.
Strongly Ordered Memory Model
Strongly Ordered Memory Model
A mechanism that guarantees that a memory modification by one processor is immediately visible to all other processors.
Weakly Ordered Memory Model
Weakly Ordered Memory Model
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Memory Barrier
Memory Barrier
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Mutex Lock
Mutex Lock
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Semaphore
Semaphore
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Monitor
Monitor
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Test-and-Set Instruction
Test-and-Set Instruction
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Compare-and-Swap Instruction
Compare-and-Swap Instruction
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test_and_set()
test_and_set()
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Lock
Lock
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Compare-and-Swap (CAS)
Compare-and-Swap (CAS)
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Atomic Variable
Atomic Variable
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Compare-and-Swap Solution for Critical Section
Compare-and-Swap Solution for Critical Section
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Disabling Interrupts
Disabling Interrupts
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Hardware Instructions
Hardware Instructions
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Synchronization Hardware
Synchronization Hardware
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Atomic Operation (e.g., increment())
Atomic Operation (e.g., increment())
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Critical Section (CS)
Critical Section (CS)
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Mutex Lock Solution
Mutex Lock Solution
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Study Notes
Chapter 6: Synchronization Tools
- This chapter covers synchronization tools in operating systems.
- The outline includes background, the critical-section problem, Peterson's solution, hardware support for synchronization, mutex locks, semaphores, monitors, liveness, and evaluation.
- The chapter objectives include describing the critical-section problem and illustrating race conditions.
- It illustrates hardware solutions to critical section problems using memory barriers, compare-and-swap operations, and atomic variables.
Memory Barriers
- Memory models define the memory guarantees a computer architecture makes to applications.
- Models can be strongly ordered, where a processor's memory modification is immediately visible to all other processors or weakly ordered, where it might not be immediately visible.
- A memory barrier forces any memory change to propagate to other processors.
Memory Barrier Instructions
- When a memory barrier instruction is executed, the system ensures all previous loads and stores are complete before subsequent ones.
- Reordering of instructions is handled by the memory barrier, ensuring memory operations complete before future operations.
Memory Barrier Example
- An example slide shows the memory barrier code to ensure Thread 1 outputs 100, with a boolean flag to synchronize.
Synchronization Hardware
- Many systems provide hardware support for critical section code implementation.
- Uniprocessors can often disable interrupts to prevent preemption, but is generally inefficient on multiprocessors.
- Hardware instructions and atomic variables are the two forms of hardware support.
Hardware Instructions
- Special hardware instructions allow testing and modifying a word in memory atomically.
- Two examples of such instructions are test-and-set and compare-and-swap.
The Test-and-Set Instruction
- This instruction atomically tests and sets a boolean target.
- Initially, target is false.
- The instruction returns the original value of the target.
- It sets the target to true.
Solution Using Test-and-Set()
- A shared boolean variable lock, initially false, is used
- The while loop ensures that the critical section is executed by only one process at a time.
The Compare-and-Swap Instruction
- The compare-and-swap instruction is another hardware instruction.
- Input parameters include the variable pointer, the expected value, and a new value.
- Operates atomically, the new_value gets set to the variable only if the expected value matches the variable's contents.
- Returns the original value of the variable.
Solution using compare_and_swap
- A shared integer lock, initially 0, is used
- The
whileloop withcompare_and_swapensures that the critical section is executed by only one process at a time.
Atomic Variables
- Atomic variables provide atomic updates to basic data types.
- An example is the increment() function that uses compare-and-swap to increment an integer atomically, preventing interruption in the incrementing process.
Mutex Locks
- Mutex locks provide a simpler way to manage critical sections.
- A boolean variable (lock) indicates lock availability.
- Processes first acquire the lock, carry out critical section operations, and then release the lock.
acquireandreleasecalls have to be atomic.
Solution to CS Problem Using Mutex Locks
- A general structure showing the usage.
Semaphore
- Semaphores are a powerful synchronization tool.
- Their value is a single integer variable.
- Wait() and signal() (originally P() and V()) are the operations.
Semaphore (Cont.)
- Counting semaphores allow the range of values to be unrestricted.
- Binary semaphores have a range only between 0 and 1.
- Semaphores are used to solve complex synchronization problems.
Semaphore Usage Example
- Shows how semaphores,
mutexorsynchcan solve critical-section problems
Semaphore Implementation
- Busy waiting is used in implementation, but could be inefficient for frequently accessed critical sections.
Problems with Semaphores
- Incorrect use of semaphore operations ('signal...' 'wait...').
- Omitting either
waitorsignalcan lead to issues.
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