Operating System Concepts: Multilevel Queue Scheduling

CFS Scheduler

• Target Latency: The CFS scheduler aims to maintain a desired average response time for interactive tasks.
• Increasing Active Tasks: More active tasks lead to a higher target latency. This is because each task gets less CPU time, resulting in slower responses.
• Decay Factor: This adjusts the target latency dynamically. A higher decay factor means quicker adjustments to changes in the number of active tasks.

Windows Scheduling

• Scheduling Algorithm: Windows uses a priority-based preemptive scheduling algorithm.
• Priority Classes: They group processes into categories, allowing system-critical tasks to run first, followed by user-critical tasks, and then background processes.
• Waiting on Resources: Waiting on a resource (like a file lock) suspends the process. When the resource becomes available, the process is moved back to the ready queue.

Multiprocessing Systems

• Symmetric Multiprocessing (SMP): All processors can access system data structures directly. This allows for efficient resource sharing and parallel execution.
• Asymmetric Multiprocessing (AMP): One processor (typically the master) manages all system data structures. The other processors (slaves) depend on the master for data access.
• Processor Affinity: Allows a process to be bound to a particular CPU core for optimal performance, especially for processes that are known to have high affinity to a specific core.

Solaris Scheduling

• Dispatch Table: Stores information about all runnable processes, facilitating efficient process switching.
• Dispatcher Module: Responsible for choosing the next process to run and switching control to it.
• Multiple Threads at Same Priority: Solaris uses a fair share policy to ensure that threads of equal priority get a fair share of CPU time.

Multiple-Processor Scheduling

• Load Balancing: Distributes processes evenly across available processors, maximizing processor utilization and minimizing response times.
• Push Migration: Transferring a process from a heavily loaded processor to a less loaded one, proactively balancing the load.
• Global Priority: A single priority value for each process, used to compare its priority with other processes across all processors.

Dispatch Latency

• Definition: The time taken to select and switch to the next process for execution.

Multicore Processor Advantages

• Increased Performance: Multiple cores offer parallel processing capabilities, leading to faster execution of tasks.
• Improved Responsiveness: By distributing tasks among multiple cores, responsiveness can be improved, as other tasks can run while one task is waiting for I/O.

CPU Scheduling Criteria

• Maximizing CPU Utilization: This aims to keep the CPU busy as much as possible, reducing idle time.
• Minimizing Waiting Time: This focuses on keeping processes waiting for the CPU for as short a time as possible.
• Minimizing Response Time: This aims to provide quick responses to interactive tasks, ensuring a pleasant user experience.

Scheduling Algorithms

• FCFS (First-Come, First-Served): Processes are served in the order they arrive in the ready queue. It's simple but can lead to long wait times for shorter tasks.
• SJF (Shortest-Job-First): Selects the process with the shortest estimated burst time next. It's optimal for minimizing average waiting time but requires accurate burst time estimates.
• Convoy Effect (FCFS): A long process can block shorter processes behind it in the FCFS queue, leading to inefficient resource usage.
• Thread Scheduling: In multicore systems, separate threads can be assigned to different cores, allowing for concurrent execution and improved performance.

Real-Time Systems

• Hard Real-Time: Requires tasks to be completed within strict deadlines, with no tolerance for delays.
• Soft Real-Time: Deadlines are important but occasional missed deadlines are tolerated.

Load Balancing (Multiple-Processor Scheduling)

• Pull Migration: A process is moved to a less loaded processor when it requests additional resources.

Key Differences

• FCFS vs. SJF: FCFS prioritizes arrival order, while SJF prioritizes estimated burst time.

Multiprocessor Systems

• Symmetric Multiprocessing: All processors are equal and share the same resources, offering better scalability than AMP systems.

Scheduling Criteria

• Minimizing Turnaround Time: This aims to reduce the total time spent by a process in the system, from arrival to completion.
• Maximizing Throughput: This aims to run as many processes as possible in a given time period, increasing the overall efficiency of the system.

Short-Term Scheduler

• Role: Responsible for selecting the next process to be loaded into memory and executed, making decisions based on scheduling criteria.

Test your knowledge on multilevel queue scheduling, a CPU scheduling algorithm that allows processes to move between queues. Learn about time slicing, foreground and background processes, and how aging is implemented. Based on Operating System Concepts, 9th Edition by Silberschatz, Galvin, and Gagne.