📝 CPU Scheduling Basic Concepts (54 MCQs)

📖 Explanation: A single-processor system can execute only one process at a time. All other processes must wait until the CPU becomes available and can be rescheduled. This fundamental constraint forms the basis for CPU scheduling decisions in operating systems.

📖 Explanation: Multiprogramming aims to maximize CPU utilization by ensuring that some process is always running. When one process waits for I/O, the operating system switches to another process, preventing the CPU from sitting idle and wasting valuable processing time.

📖 Explanation: When a process requires I/O, the operating system takes the CPU away from that process and schedules another ready process. This prevents CPU idle time and allows productive use of the processor while the first process waits for I/O completion.

📖 Explanation: The CPU-I/O burst cycle describes the repeating pattern where a process alternates between executing CPU instructions (CPU burst) and waiting for I/O operations to complete (I/O burst). This cycle continues throughout the process lifetime.

📖 Explanation: A CPU burst is a period during which a process executes instructions on the CPU. During this time, the process actively uses the processor to perform computations and logical operations before it may need to wait for I/O operations.

📖 Explanation: An I/O burst is a period during which a process waits for input/output operations to complete. During this time, the process cannot execute on the CPU, allowing the operating system to schedule another process to utilize the CPU productively.

📖 Explanation: After completing a CPU burst, a process typically initiates an I/O operation and enters an I/O burst phase. During this waiting period, the process cannot execute further instructions until the I/O operation completes, after which it may resume with another CPU burst.

📖 Explanation: CPU scheduling is fundamental because the CPU is one of the primary computer resources. Since all computer resources are scheduled before use, proper CPU scheduling ensures efficient system operation, high utilization, and fair distribution of processing time among competing processes.

📖 Explanation: In a simple system without multiprogramming, when a process waits for I/O, the CPU sits idle. This time is wasted with no useful work accomplished. Multiprogramming addresses this problem by using the waiting time productively by running other processes.

📖 Explanation: Keeping multiple processes in memory simultaneously enables the operating system to switch to another ready process when the current process must wait for I/O. This approach prevents CPU idle time and improves overall system throughput through better CPU utilization.

📖 Explanation: During a context switch, the operating system saves the state of the current process (registers, program counter, etc.), selects another ready process from the ready queue, and loads its saved state. This allows the CPU to execute a different process while the first process waits for I/O.

📖 Explanation: Multiprogramming improves CPU utilization by enabling the operating system to switch to another ready process whenever the currently executing process needs to wait for I/O. This prevents the CPU from remaining idle during I/O operations, allowing productive execution of other processes in the meantime.

📖 Explanation: During an I/O burst, the process has issued an I/O request and must wait for the operation to complete before it can execute further instructions. The process cannot proceed with CPU execution because it depends on data from the I/O operation that is not yet available.

📖 Explanation: CPU bursts and I/O bursts alternate sequentially throughout a process's lifetime. A process executes a CPU burst, then may issue an I/O request causing an I/O burst, followed by another CPU burst, and this pattern continues until the process completes or terminates.

📖 Explanation: The operating system uses a CPU scheduling algorithm to determine which process from the ready queue should be selected for execution next. The specific algorithm (such as FCFS, SJF, or Round-Robin) implements criteria that guide this selection process based on system objectives.

📖 Explanation: The alternating CPU-I/O burst pattern creates natural scheduling points whenever a process completes a CPU burst and requires I/O. At these points, the CPU becomes available, allowing the scheduler to decide which ready process should be executed next based on system scheduling policies.

📖 Explanation: When multiple processes reside in memory, the scheduler's role is to select which process from the ready state should be allocated the CPU for execution. This selection decision is based on the scheduling algorithm and aims to meet system performance objectives such as fairness, responsiveness, or throughput.

📖 Explanation: The CPU-I/O burst cycle creates opportunities for CPU scheduling when processes wait for I/O, improving CPU utilization. However, it also introduces context switching overhead when switching between processes. The scheduling algorithm must balance these factors to optimize overall system performance.

📖 Explanation: CPU-bound processes typically have long CPU bursts and short I/O bursts, meaning they primarily perform computations. I/O-bound processes have short CPU bursts and long I/O bursts, meaning they frequently perform I/O operations. Understanding this distinction helps in designing scheduling algorithms that appropriately balance these process types.

📖 Explanation: Multiprogramming directly addresses the problem of CPU idle time that occurs when a process waits for I/O operations. By keeping multiple processes in memory and switching to another ready process during I/O waits, the CPU remains busy, significantly improving CPU utilization compared to systems without multiprogramming.

📖 Explanation: The duration of a CPU burst depends on the specific computational requirements of the process and various system conditions. Some processes have short CPU bursts while others have long bursts, and this variation affects scheduling decisions and system performance analysis.

📖 Explanation: Increasing the number of processes in memory generally improves CPU utilization by providing more ready processes to execute during I/O waits. However, beyond a certain point, the overhead of context switching and scheduling may begin to reduce effective utilization, creating a trade-off between degree of multiprogramming and system performance.

📖 Explanation: Instructions like load, store, add, and index are typical CPU instructions that are executed during a CPU burst. These operations involve the CPU performing computational tasks using registers and memory, and they represent the productive work done by the CPU during the execution phase of a process.

📖 Explanation: The CPU scheduler's primary function is to select processes from the ready queue and allocate the CPU to them for execution. This selection decision implements the system's scheduling policy and directly affects performance metrics such as throughput, response time, and CPU utilization.

📖 Explanation: A process transitions from a CPU burst to an I/O burst by issuing an I/O request, such as reading from a file or writing to a device. This request causes the process to enter a waiting state while the I/O operation is performed by the appropriate device controller.

📖 Explanation: When a process transitions from a CPU burst to an I/O burst, the CPU becomes available. In a multiprogramming system, the operating system reassigns the CPU to another ready process from the ready queue, ensuring the CPU continues productive work instead of sitting idle during the I/O operation.

📖 Explanation: Keeping multiple processes in memory simultaneously is essential for multiprogramming because it ensures there is always a ready process available to run when the currently executing process must wait for I/O. Without multiple processes in memory, the CPU would have to remain idle during I/O waits, significantly reducing utilization.

📖 Explanation: As the degree of multiprogramming increases, the CPU may switch between processes more frequently, leading to increased context switching overhead. Each context switch requires saving and restoring process states, consuming CPU time that could otherwise be used for productive work. This creates a trade-off between improved utilization and scheduling overhead.

📖 Explanation: I/O wait refers to the period during which a process has issued an I/O request and is waiting for the operation to complete. During this time, the process cannot execute further instructions and must remain in a waiting state until the I/O operation is finished by the appropriate device.

📖 Explanation: The typical sequence alternates between CPU bursts and I/O bursts: a CPU burst of instruction execution, followed by an I/O burst for I/O operation, followed by another CPU burst, and so on. This alternating pattern continues until the process terminates, with scheduling opportunities occurring at each transition.

📖 Explanation: Processes in the ready state are loaded into memory and are prepared to execute. They are not currently running on the CPU but are available for execution. The scheduler selects processes from this ready state to run on the CPU according to the scheduling algorithm being used.

📖 Explanation: When a process makes an I/O request, the operating system manages the transition: it initiates the I/O operation, marks the process as waiting, and selects another ready process from the ready queue to execute on the CPU. This prevents CPU idle time during the I/O operation and maintains system productivity.

📖 Explanation: Scheduling is required for CPU resources because multiple processes compete for CPU time and the resource must be allocated among them. The scheduling algorithm determines how the CPU is allocated, balancing competing goals such as fairness, efficiency, responsiveness, and throughput to meet system design objectives.

📖 Explanation: All computer resources in an operating system are scheduled before use, and the CPU is one of the primary resources. Scheduling decisions for the CPU are therefore central to resource management, with CPU scheduling algorithms determining how this critical resource is allocated among competing processes to achieve system objectives.

📖 Explanation: CPU-bound processes are characterized by longer CPU bursts and shorter I/O bursts. These processes spend most of their time performing computations rather than I/O operations. Examples include scientific computations, image processing, and data analysis applications that require significant CPU processing.

📖 Explanation: I/O-bound processes are characterized by shorter CPU bursts and longer I/O bursts. These processes frequently require I/O operations and spend more time waiting for I/O than executing CPU instructions. Examples include user interactive applications, database queries, and web servers that frequently access storage or network resources.

📖 Explanation: CPU scheduling differs from scheduling other resources because of the frequency of CPU allocation decisions and the context switching overhead involved. The CPU is allocated and deallocated many times per second, requiring efficient scheduling algorithms and low-overhead context switching mechanisms to maintain system performance.

📖 Explanation: Process state transitions between running, ready, and waiting states occur at the boundaries of CPU-I/O bursts. A process moves from running to waiting when it initiates an I/O burst, and from waiting to ready when the I/O completes. It moves from ready to running when the scheduler selects it for CPU execution.

📖 Explanation: Effective CPU scheduling can significantly improve system throughput by minimizing CPU idle time. When a process waits for I/O, scheduling another process maintains CPU utilization, allowing more processes to complete within a given time period. Good scheduling algorithms balance this improved utilization against context switching overhead.

📖 Explanation: The primary benefit of keeping multiple processes in memory simultaneously is that it enables quick switching to another ready process when the currently executing process must wait. This keeps the CPU busy and improves utilization. Without multiple processes in memory, the CPU would idle during I/O waits.

📖 Explanation: The three main process states in CPU scheduling are Running (currently executing on CPU), Ready (prepared to execute but waiting for CPU allocation), and Waiting (blocked waiting for an event such as I/O completion). The scheduler manages transitions between these states through scheduling decisions.

📖 Explanation: CPU scheduling decisions are typically made at CPU-I/O burst boundaries, when a process completes a CPU burst and initiates an I/O operation. At these points, the CPU becomes available, and the scheduler selects the next process from the ready queue to execute, making efficient use of the CPU.

📖 Explanation: CPU scheduling algorithm design must balance multiple objectives: maximizing CPU utilization to keep the processor busy, maintaining fairness to prevent starvation, ensuring responsiveness for interactive users, and providing reasonable throughput. These often conflicting goals require careful algorithm design and parameter tuning.

📖 Explanation: CPU scheduling directly determines the waiting time of processes in the ready queue. Different scheduling algorithms result in different waiting times for processes, affecting both fairness and performance. The scheduler's choice of which process to run next determines how long each process must wait before receiving CPU service.

📖 Explanation: Frequent context switches enable better CPU utilization by allowing the system to switch away from processes during I/O waits. However, each context switch carries overhead: saving and restoring process states, flushing caches, and performing scheduling calculations. The optimal frequency balances improved utilization against the performance impact of the overhead.

📖 Explanation: When a process requests an I/O operation, it moves from the running state to the waiting state because it cannot continue execution until the I/O operation completes. While in the waiting state, the process is removed from the CPU, allowing the scheduler to allocate the CPU to another process from the ready queue.

📖 Explanation: When an I/O operation completes, the process moves from the waiting state to the ready state. It does not immediately begin execution; instead, it joins the ready queue and waits for the scheduler to select it for CPU execution. This transition makes the process eligible for CPU allocation when the scheduler makes its next selection.

📖 Explanation: A ready process is loaded into memory and is prepared to execute but is not currently using the CPU. A running process is currently executing on the CPU. The scheduler selects a ready process to become running, and when a running process must wait for an event, it leaves the running state. These state distinctions are fundamental to scheduling.

📖 Explanation: The dispatcher is responsible for giving control of the CPU to the process selected by the scheduler. It performs the context switch, loading the saved state of the selected process, and transferring control to that process. The dispatcher handles the mechanics of process switching once the scheduler has made the selection decision.

📖 Explanation: CPU burst length is an important consideration in scheduling decisions. Processes with shorter CPU bursts may be prioritized to improve system responsiveness and reduce waiting times for interactive users. Some scheduling algorithms attempt to predict future CPU burst lengths to make more informed scheduling decisions, aiming to optimize performance metrics such as turnaround time and waiting time.

📖 Explanation: Effective CPU scheduling requires consideration of process characteristics. Different processes have different CPU-I/O burst patterns, priorities, and requirements. The scheduler must consider these characteristics when allocating CPU time to achieve system objectives such as fairness, responsiveness, and throughput. Understanding process behavior patterns is essential for scheduler design.

📖 Explanation: When new processes arrive, they are typically placed in the ready queue where they join existing processes waiting for CPU time. The scheduler's selection algorithm determines when these newly arrived processes get CPU service. This interaction between process arrivals and scheduling decisions affects system performance and fairness.

📖 Explanation: Preemption in CPU scheduling refers to the ability of the scheduler to forcibly remove a running process from the CPU before it voluntarily releases the CPU. This allows the scheduler to implement time-sharing algorithms, ensure fairness, respond to higher-priority processes, and prevent any single process from monopolizing the CPU. Preemptive scheduling is essential for interactive and real-time systems.

📖 Explanation: Scheduling algorithms directly affect system responsiveness by determining how quickly interactive processes receive CPU attention. Preemptive scheduling with appropriate algorithms can ensure that user processes are scheduled promptly, providing acceptable response times. Poor scheduling decisions can result in sluggish system response and poor user experience.