Multithreading: Concepts and Benefits
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What is the primary purpose of scheduler activations in the context of multithreading?

  • To eliminate the need for kernel threads.
  • To manage thread-specific data within the user space.
  • To enable communication between the kernel and the thread library, allowing applications to maintain the appropriate number of kernel threads. (correct)
  • To provide direct communication between threads, bypassing the kernel.

In the Windows XP threading model, what constitutes the 'context' of a thread?

  • The register set, stacks (user and kernel), and private data storage area. (correct)
  • The list of shared libraries and the heap memory region.
  • The program counter and the system call table.
  • The thread ID and its priority level.

How does the clone() system call in Linux facilitate thread creation?

  • It creates a new process with a unique process ID and a separate memory space, but shares file descriptors with the parent.
  • It allows a child task to share the address space of the parent task, effectively creating a thread. (correct)
  • It duplicates the entire process, including all threads and memory, with copy-on-write semantics.
  • It creates a completely isolated process with its own memory space.

Which of the following is NOT a typical objective of CPU scheduling in an operating system?

<p>To eliminate race conditions and deadlocks completely. (B)</p> Signup and view all the answers

Consider a scenario where multiple threads in a user-level threads implementation are mapped to a single kernel thread (M:1 model). Which statement is most accurate regarding their ability to leverage multi-core processors?

<p>Only one thread within the process can execute at a time because the operating system schedules the single kernel thread onto a single core; parallelism is not achieved regardless of the number of cores available. (A)</p> Signup and view all the answers

Which of the following is NOT a valid approach for delivering signals in a multithreaded program?

<p>Deliver the signal to the operating system kernel. (D)</p> Signup and view all the answers

What is the primary difference between asynchronous and deferred thread cancellation?

<p>Asynchronous cancellation immediately terminates the target thread, while deferred cancellation allows the target thread to check for cancellation points. (B)</p> Signup and view all the answers

What is the purpose of a signal handler?

<p>To process signals that have been delivered to a process. (C)</p> Signup and view all the answers

In Java, which mechanism is typically used to implement deferred cancellation?

<p>Using the <code>interrupt()</code> method and checking the interruption status. (A)</p> Signup and view all the answers

Which of the following best describes a 'cancellation point'?

<p>A specific location in a thread's execution where it checks if it should terminate. (A)</p> Signup and view all the answers

What is the potential drawback of asynchronous thread cancellation?

<p>It can lead to resource leaks if not handled carefully. (A)</p> Signup and view all the answers

Consider a scenario where a multithreaded process receives a synchronous signal. To which thread is the signal delivered?

<p>The thread that performed the operation that caused the signal. (C)</p> Signup and view all the answers

What is the behavior of the exec() system call with respect to threads?

<p>It replaces the entire process, including all threads, with a new program. (A)</p> Signup and view all the answers

Suppose a thread calls fork() in an environment where fork() duplicates all threads. If the original process had three threads, and exec() is NOT called immediately afterward in the child process, how many threads will the child process initially have?

<p>Three (C)</p> Signup and view all the answers

A multithreaded program uses a custom signal handler. If the program needs to ensure that all signals (synchronous and asynchronous) are handled by a specific thread, what is the most appropriate strategy?

<p>Assign a specific thread to receive all signals for the process. (D)</p> Signup and view all the answers

What is the primary purpose of the 'turn' variable in the provided synchronization algorithm?

<p>To ensure that processes enter the critical section in a specific order. (B)</p> Signup and view all the answers

In the context of critical section solutions, what does the term 'atomic' refer to?

<p>A section of code that executes as a single, uninterruptible unit. (D)</p> Signup and view all the answers

Why is disabling interrupts not a feasible solution for the critical-section problem in a multi-processor environment?

<p>Disabling interrupts on one processor does not prevent other processors from accessing the critical section. (C)</p> Signup and view all the answers

What is the purpose of the TestAndSet instruction in the context of mutual exclusion?

<p>To atomically test the value of a lock and, if it's free, acquire it. (C)</p> Signup and view all the answers

Consider the TestAndSet implementation. What is the significance of returning the original value of *target?

<p>It allows the process to know whether it successfully acquired the lock. (D)</p> Signup and view all the answers

In the 'Swap' based mutual exclusion, what is accomplished by the line swap(&lock, &key)?

<p>It attempts to acquire the lock by atomically swapping the process's 'key' value with the lock's value. (A)</p> Signup and view all the answers

What is the main purpose of the waiting array in the bounded-waiting TestAndSet algorithm?

<p>To prevent starvation by ensuring that all processes eventually get a chance to enter the critical section. (A)</p> Signup and view all the answers

In semaphore operations, under what condition the process will be blocked?

<p>When the semaphore value is negative (A)</p> Signup and view all the answers

Suppose a system uses the bounded-waiting TestAndSet algorithm for critical section access. A process Pi leaves the critical section. What determines which process (if any) enters the critical section next?

<p>The first process in the cyclic ordering (i+1, i+2, ..., n-1, 0, ..., i-1) that is in the entry section (waiting[j] == true). (C)</p> Signup and view all the answers

Consider a scenario where multiple processes are contending for a critical section protected by the TestAndSet lock. Due to a programming error, a process releases the lock (sets it to false) before actually entering the critical section. What is the most likely consequence?

<p>Multiple processes might enter the critical section simultaneously, violating mutual exclusion. (A)</p> Signup and view all the answers

In a multilevel feedback queue scheduling algorithm, what mechanism is typically employed to prevent starvation?

<p>Moving processes that have waited too long in a lower-priority queue to a higher-priority queue (Aging). (B)</p> Signup and view all the answers

What is the primary difference between local and global thread scheduling?

<p>Local scheduling determines which thread runs on an available LWP, while global scheduling determines which kernel thread runs next. (C)</p> Signup and view all the answers

What is asymmetric multiprocessing?

<p>A system where one processor handles all scheduling decisions, I/O processing, and other system activities. (B)</p> Signup and view all the answers

What is 'processor affinity' in the context of multiprocessor scheduling?

<p>The attempt by an OS to keep a process running on the same processor to avoid cache invalidation. (B)</p> Signup and view all the answers

Which of the following is a disadvantage of using simulations to evaluate scheduling algorithms?

<p>Simulations can be costly, requiring hours of computer time. (D)</p> Signup and view all the answers

What is the key difference between 'push migration' and 'pull migration' in load balancing?

<p>Push migration is initiated by a busy processor, while pull migration is initiated by an idle processor. (D)</p> Signup and view all the answers

Consider a system with three queues (Q0, Q1, Q2) in a multilevel feedback queue. Q0 uses RR with a time quantum of 8ms, Q1 uses RR with a time quantum of 16ms, and Q2 uses FCFS. A job that requires 40ms of CPU time arrives. How many times will this job be preempted, and what queue will it be in, before its completion?

<p>2 preemptions, Q2 (D)</p> Signup and view all the answers

A real-time operating system (RTOS) supports hard affinity. A critical process, P, is bound to CPU core 0. Due to an unexpected hardware interrupt, core 0 becomes temporarily unavailable. What is the MOST likely immediate outcome for process P?

<p>Process P enters a blocked state, awaiting the availability of CPU core 0. (B)</p> Signup and view all the answers

What is the primary goal of addressing the critical-section problem in concurrent programming?

<p>To ensure the consistency of shared data by controlling access. (B)</p> Signup and view all the answers

Which of the following is NOT a requirement for a solution to the critical-section problem?

<p>Deadlock prevention (C)</p> Signup and view all the answers

In the context of the critical-section problem, what does 'progress' ensure?

<p>That a process will eventually enter its critical section if it requests to do so. (D)</p> Signup and view all the answers

What does the 'bounded waiting' condition in the critical-section problem prevent?

<p>Starvation of processes wanting to enter the critical section. (B)</p> Signup and view all the answers

What is a 'race condition' in the context of concurrent processes?

<p>A situation where the outcome of execution depends on the order in which data access occurs. (B)</p> Signup and view all the answers

In Peterson's solution, what is the role of the flag array?

<p>To signal that a process intends to enter the critical section. (C)</p> Signup and view all the answers

Assuming LOAD and STORE instructions are atomic, why is atomicity important in the context of Peterson's solution and preventing race conditions?

<p>It guarantees that once an instruction starts, it completes without interruption, preventing intermediate states from causing issues. (A)</p> Signup and view all the answers

Suppose two processes, P1 and P2, are using Peterson's solution to synchronize access to a shared resource. Both processes set their flag to true and then set the turn variable to each other's process ID almost simultaneously. What could be the outcome?

<p>One process will enter and execute the critical section, while the other waits. (B)</p> Signup and view all the answers

Consider a scenario where multiple processes are competing for access to a critical section, and one process is consistently granted access while others are perpetually blocked. Which condition is violated in this scenario?

<p>Bounded waiting. (A)</p> Signup and view all the answers

A system uses a shared counter to track the number of available resources. Multiple processes increment or decrement this counter. If the increment and decrement operations are not atomic, and a race condition occurs, what is the most severe consequence?

<p>The system might misreport the number of available resources, leading to resource exhaustion or over-allocation and potential system instability. (A)</p> Signup and view all the answers

Flashcards

Thread-specific data in Java

Data that is unique to each thread in Java applications.

Scheduler Activations

A mechanism allowing communication from the kernel to the thread library to manage kernel threads.

Context of a thread

The register set, stacks, and private storage area associated with a thread in operating systems.

Linux threads

Referred to as tasks in Linux, created using the clone() system call.

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CPU Scheduling Algorithms

Methods for determining how CPU time is allocated among processes in multi-programmed systems.

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Time Slice

The period a queue gets to schedule its processes on the CPU.

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Multilevel Feedback Queue

A scheduling system where processes can move between different priority queues.

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Processor Affinity

Preference for keeping a process on a specific processor to optimize cache usage.

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Load Balancing

The method of evenly distributing work among processors.

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Deterministic Modeling

A method to predict algorithm performance based on a fixed workload.

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Queuing Models

Mathematical models used to predict wait times and process utilization.

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Simulation

Programming a model to imitate the behavior of a computer system.

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Co-operating Process

A process that can be affected by or can affect other processes.

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Critical Section

A segment of code where shared resources are accessed.

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Mutual Exclusion

Ensures that only one process accesses the critical section at a time.

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Progress Requirement

A condition that guarantees a waiting process will eventually enter the critical section.

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Bounded Waiting

Limits the number of times a process can be bypassed while waiting.

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TestAndSet Instruction

An atomic operation that tests a value and sets it, preventing race conditions.

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Semaphore

A synchronization tool to control access to shared resources.

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Wait Operation

Decreases semaphore value and may block the process if the value is zero.

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Signal Operation

Increases semaphore value and wakes up a waiting process if necessary.

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Lock

A simple signaling mechanism used to ensure mutual exclusion.

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Atomic Operation

An operation that completes in a single step, undisturbed by other operations.

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Runnable Interface

An interface that should be implemented by any class whose instances are intended to be executed by a thread.

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Critical-section Problem

A situation where multiple processes access shared data, risking inconsistency.

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fork() System Call

Creates a duplicate process; can reproduce all or only the invoking thread based on context.

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exec() System Call

Replaces the process image with a new process; all threads in the process are replaced.

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Race Condition

When the execution outcome depends on the order of access to shared data.

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Thread Cancellation

Terminating a thread before it has finished executing, either asynchronously or deferred.

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Count Variable

An integer tracking full buffers in producer-consumer scenarios.

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Asynchronous Cancellation

One thread immediately terminates the target thread regardless of its state.

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Progress Condition

Only processes not in their remainder section can enter the critical section.

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Deferred Cancellation

The target thread checks periodically if it should terminate, allowing it to finish necessary tasks.

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Signal Handling

The process of reacting to notifications about events in a UNIX system, using a signal handler.

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Synchronous Signals

Signals delivered to the same process that caused them, like division by zero.

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Atomic Transaction

An operation that completes entirely or not at all, ensuring consistency.

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Peterson's Solution

A software method to prevent race conditions in two processes.

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Asynchronous Signals

Signals generated by external events affecting the running process.

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Thread Interruption

A method used in Java to check a thread's status and potentially cancel its execution.

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Entry and Exit Sections

Parts of the critical section; entry requests permission, exit completes the interaction.

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Study Notes

Overview of Threads

  • A thread is a flow of control within a process.
  • Multithreaded processes have multiple flows of control within the same address space.
  • Traditional processes have only one thread of control.
  • A thread is a lightweight process, a unit of CPU utilization.
  • It includes a thread ID, program counter, register set, and stack.
  • Threads belonging to the same process share the code section, data section, and other OS resources.

Multithreaded Processes vs Single-threaded

  • If a process has multiple threads, it can perform more than one task simultaneously.
  • A single-threaded process can only perform one task at a time.

Motivation for Multithreading

  • Multithreading is more efficient than using many processes.
  • RPC (Remote Procedure Call) servers use multithreading.
  • When a server receives a message, it uses a separate thread to service it.
  • This allows the server to handle many concurrent requests efficiently.

Benefits of Multithreading

  • Responsiveness: A program can continue executing even if parts of it are busy.
  • Resource sharing: Threads share the memory and resources of their process, making it more efficient for resource use.
  • Economy: Creating & maintaining process resources is costly. Threads are significantly cheaper to create than processes.
  • Scalability: In multiprocessor architectures, each thread can run on a separate processor to increase concurrency.

Multithreading Models

  • User-level threads: Visible to the programmer; not to the kernel.
  • Kernel-level threads: Supported directly by the OS kernel.

One-to-One Model

  • Each user thread is mapped to a kernel thread.
  • More concurrency than the many-to-one model
  • Multiple threads can run in parallel on multiprocessors.
  • Creating a user thread also requires creating a kernel thread, which may be inefficient when compared to many-to-many models.

Many-to-Many Model

  • Many user-level threads are multiplexed onto a smaller/equal number of kernel threads.
  • Developers can create many user-level threads as necessary.
  • Kernel threads can run in parallel on multiprocessors.
  • When a thread performs a blocking system call, the kernel can schedule another thread for execution

Thread Libraries

  • Provide an API for creating and managing threads.
  • Three main thread libraries are in use today:
    • POSIX Pthreads
    • Win32 threads
    • Java threads

Thread States

  • New
  • Runnable
  • Blocked
  • Dead

Threading Issues

  • Fork() and exec() System Calls: How the OS manages thread creation and termination and how to handle situations like:
    • fork() - the creation of a new process
    • exec() - the replacement of the current process
  • Cancellation: The task of terminating a thread before it has completed.
  • Asynchronous Cancellation: One thread immediately terminates the target thread.
  • Deferred Cancellation: The target thread periodically checks if it should terminate.

Signal Handling

  • Signals are used in UNIX to notify a process of an event.
  • Signals follow a pattern: event occurs, signal generated, and delivered to the process via a signal handler.
  • Signal handling in multithreaded programs can be complex: signals can be delivered to a specific thread or every thread of the process.

Thread Pools

  • A thread pool that holds threads and when a task arrives, a thread is taken from the pool.
  • If a thread is finished with a job, it returns to the pool.
  • Advantages of thread pools:
    • Faster to service a request with an existing thread
    • Less overhead in thread creation and termination.

CPU Scheduling

  • CPU scheduling is the task of selecting a waiting process from the ready queue and allocating the CPU to it.
  • CPU scheduling algorithms: FCFS, SJF, Priority, Round Robin, Multilevel feedback queue.
  • Metrics to evaluate CPU scheduling: CPU utilization, throughput, turnaround time, waiting time, and response time.

Multilevel Queue Scheduling

  • Partitioned into queues, with processes permanently assigned to a specific queue based on characteristics like memory size, priority, or process type.
  • Each queue has a different scheduling algorithm.
  • Scheduling among queues is typically done with fixed-priority preemption.

Multiprocessor Scheduling

  • CPU scheduling is more complex with multiple available CPUs.
  • Load balancing attempts to keep the workload across all processors.
  • Process affinity: to keep processes on a given processor.

Deadlocks

  • A deadlock occurs when two or more processes are waiting indefinitely for an event that can only be caused by one of the waiting processes.
  • The conditions necessary for a deadlock to occur are:
    • Mutual Exclusion: Only one process can access a resource at a time
    • Hold and Wait: A process must hold at least one resource and be waiting for others.
    • No Preemption: A resource cannot be taken away from a process that is using it.
    • Circular Wait: A set of waiting processes wait for resources held by other processes in the set.
  • Resource allocation graph shows all resources, allocatable resources and the relationships between requests & assignments.

Synchronization

  • Race condition: The final result of multiple processes competing for the same data depends on the order of execution.
  • Solutions:
    • Semaphores: Synchronization tool to control access to shared variables such that only one process can access a shared variable at a time.
    • Monitors: High-level synchronization mechanisms that group related procedures, variables and data structures. Conditions are used to control the sequence of operations inside the monitor.
  • Example synchronization problems: Dining Philosophers problem, Readers/Writers problem.

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Operating System Concepts PDF

Description

Explore multithreading, where processes have multiple flows of control within the same address space. Multithreading is more efficient than using many processes and allows servers to handle concurrent requests efficiently. Threads belonging to the same process share resources.

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