What is Process Scheduling?
Process scheduling in OS is an essential function of an operating system (OS) is an essential function that manages how different programs (processes) share the CPU. In a multitasking environment where several processes are generally executed simultaneously, the operating system has to choose which process will be granted CPU time and for how long. Such a procedure keeps the system running smoothly and makes sure that resources are utilized efficiently.
Why Process Scheduling is Important?
The objectives of process scheduling in an OS mainly revolve around maintaining system efficiency and providing fair access to execution time to all processes. The key points to mention are:
- Fairness: Every process should get CPU time without being ignored or delayed indefinitely. This prevents issues like starvation, where some processes never get a chance to run.
- Efficiency: The CPU should be kept busy almost all the time; thus, the periods of inactivity should be as short as possible, and the overall system performance should be improved.
- Responsiveness: Faster response times lead to a better user experience. The scheduling system aims to reduce delays so programs can run smoothly without long wait times.
- Resource Utilization: The OS must efficiently manage not just the CPU but also memory, input/output devices, and other system resources to provide optimal performance.
What is Process Scheduling in an Operating System?
Process scheduling in OS is the method by which the operating system decides which process gets to use the CPU and for how long. As modern computers are capable of running several processes simultaneously, the OS makes sure that each process is allotted a certain amount of CPU time, and at the same time, efficiency is maintained. This is the work of the Process Manager who, by organizing the execution of various processes, is able to avoid waiting times and make the most of the system's performance.
Components of Process Scheduling
1. Scheduler
The scheduler decides which process will be executed next. To decide, it uses certain scheduling algorithms like First-Come, First-Served (FCFS), Shortest Job Next (SJN), or Round Robin so that the decisions are both fair and efficient.
2. Dispatcher
Once the scheduler picks a process, the dispatcher transfers control from the current process to the next one. It handles:
- Context switching (saving and restoring process states)
- Mode switching (changing CPU mode)
- Jumping to the correct memory location to start execution
3. Queues in Process Scheduling
Processes in Process Scheduling In OS are stored in different queues based on their current execution state.
- Ready Queue: Holds processes that are prepared to execute but are waiting for CPU time.
- Blocked Queue: Contains processes that are waiting for external resources, such as input/output (I/O) operations.
- Running Process: The process currently being executed by the CPU.
Process States in Scheduling
A process moves through several states during its execution:
- Ready: The process is prepared to execute but is waiting for the CPU to be assigned.
- Running: The process is currently executing on the CPU.
- Blocked (or Waiting): The process is essentially on hold as it is waiting for an I/O operation (for instance reading a file) to be completed.
- Terminated (or Zombie): The process has finished executing but is still listed in the process table because the OS has not yet removed its entry.
Operations on Processes
Process scheduling in an operating system is practically dependent on the basic operations that can be done on processes during their lifecycle. Such operations are:
1. Process Creation
- A new process can be created by an existing process, typically using system calls like fork (in Unix-based systems) or spawn.
- The creating process is known as the parent process, while the new one is called the child process.
- The child process may inherit resources from its parent or have its own separate resources, depending on the system design.
- Creation is often triggered when a new program needs to run or a task requires a separate execution flow.
2. Process Termination
- A process finishes execution and is terminated using system calls such as exit.
- Termination can occur voluntarily (process completes its task) or involuntarily (killed by the OS or parent).
- When a process terminates, its resources are released and its entry is removed from the process table.
- If a parent process ends before its child, the child becomes an orphan process, which is then typically re-parented to a special system process.
3. Process Suspension and Reintroduction
- Sometimes, a process may be suspended (swapped out of memory) to free up resources or manage memory pressure—this is often handled by the medium-term scheduler.
- Suspended processes can later be reintroduced (swapped back in) to resume execution from where they left off.
- This operation is crucial for balancing CPU and memory usage, especially when managing many I/O-bound tasks.
4. Parent and Child Relationships
- Parent and child processes may communicate and synchronize their operations.
- The parent may be allowed to wait for the child to finish, or they can both operate separately.
- System calls such as wait give the opportunity for the parent to stop it will be a moment only until the child disappears, thus providing an orderly way to manage resources.
5. Special Process States
- Orphan processes: Created when a parent terminates before its child; the OS typically reassigns them to a system process.
- Zombie processes: Processes that have run to completion but still have a record in the process table (waiting for the parent to get their termination status).
Why These Operations Matter in Scheduling:
- Creation and termination affect the number of active processes and the workload for schedulers.
- Suspension and reintroduction help manage system resources and maintain performance, especially for I/O-bound tasks.
- Proper handling of parent-child relationships and process cleanup prevents resource leaks and ensures system stability.
What are the Categories of Scheduling?
1. Non-Preemptive Scheduling
Non-preemptive scheduling is a CPU scheduling method where, once a process starts executing, it continues running until it either completes or voluntarily releases the CPU (e.g., by waiting for I/O). The operating system does not interrupt a running process to switch to another.
Example: First-Come-First-Serve (FCFS)
- How It Works: The processes are carried out strictly in the sequence of their arrival in the queue.
- Advantage: It is simple to implement and does not require complex scheduling decisions.
- Disadvantage: It can cause a convoy effect, where shorter processes get stuck waiting behind longer ones, leading to inefficiency.
- Best Used For: Batch processing systems where execution order is not a major concern and process execution times are relatively similar.
2. Preemptive Scheduling
Preemptive scheduling in an operating system interrupts a process that is currently running and, most of the time, based on priority or fairness, assigns CPU time to another process. This improves system responsiveness, especially in real-time or multi-user environments.
Example: Round Robin (RR)
- Every single process is assigned a fixed time slice (or time quantum).
- If a process doesn’t finish within its time slice, it is sent to the back of the queue, and the next process gets CPU time.
- Round-Robin scheduling is best used for Time-sharing systems where fairness among multiple users is needed.
Example: Shortest Remaining Time First (SRTF)
- The process with the least remaining execution time gets the CPU first.
- If a new process arrives with a shorter remaining time, it preempts the currently running process.
- This is best used for Scenarios requiring minimal average waiting time, like embedded systems or real-time processing.
Differences Between Non-Preemptive and Preemptive Scheduling
| Feature |
Non-Preemptive Scheduling |
Preemptive Scheduling |
| Process Interruption |
In non-preemptive scheduling, a running process cannot be interrupted and continues execution until it finishes or voluntarily releases the CPU. |
When preemptive scheduling is used, the execution of a process that is currently running may be forcibly stopped so that the CPU can be given to a higher-priority or more urgent process. |
| Overhead |
This approach has low overhead because the operating system does not need to perform frequent context switching. |
This approach has higher overhead due to the frequent context switching required when processes are interrupted and resumed. |
| Risk of Starvation |
Starvation is possible because shorter or lower-priority processes may wait indefinitely behind long-running tasks. |
The risk of starvation is minimized because the scheduler can interrupt tasks and allocate CPU time based on priority or fairness. |
| System Responsiveness |
System responsiveness is lower because once a process starts execution, other processes must wait until it completes. |
System responsiveness is higher because the OS can quickly switch to more urgent tasks, making the system feel faster and more interactive. |
What are Scheduling Queues in an Operating System?
Process Scheduling in OS is a source of major inefficiencies that contribute to a negative performance feedback loop. The queues that Process Scheduling in OS refers to are the core of the whole mechanism. These queues guarantee that processes are performed in a regulated way; thus, the efficiency of the entire system is elevated. There are four principal scheduling queues:
1. Job Queue
It is a queue that contains all processes of a system, also those that are waiting for memory allocation in order to be executed. A freshly made process is initially placed in the job queue.
2. Ready Queue
This queue contains processes that are loaded into main memory (RAM) and are ready to execute but are waiting for CPU time. The CPU scheduler selects processes from the ready queue and assigns them CPU resources.
3. Device Queue
When a process needs to perform an input/output (I/O) operation, it moves to a device queue. Each I/O device (such as a printer, disk, or keyboard) has its own queue for processes waiting to use it.
Process Scheduling Strategies
To decide the order in which processes are executed, operating systems use different scheduling strategies. Process Scheduling in OS ensures efficient task management by allocating CPU time based on priority, fairness, and system performance.
1. First-In-First-Out (FIFO)
Operations are carried out in the exact sequence of their arrival; the oldest process in the queue is given CPU time first. While this method is straightforward, it can result in lengthy waiting times for the processes that come later.
2. Priority Scheduling
Each process is assigned a priority level based on importance or urgency. Processes with higher priority are executed before lower-priority ones. If priorities are not managed well, low-priority tasks may be delayed indefinitely (a problem known as starvation).
3. Round Robin (RR)
Each process is given a fixed time slice (also called a time quantum) to execute. After its time expires, the process moves to the back of the ready queue, and the next process gets CPU time. This ensures fairness and prevents any single process from monopolizing the CPU.
Types of Process Schedulers
Process schedulers are responsible for managing how processes move through a system, ensuring efficient CPU usage and system responsiveness. Process Scheduling in OS revolves around three main types of schedulers: short-term, long-term, and medium-term ones. Each of them is a vital cog in the wheel of process management.
1. Long-Term Scheduler (Job Scheduler)
The long-term scheduler or the job scheduler is the one that controls the level of multiprogramming in a system. It selects which processes should be loaded into memory for execution. Since this scheduler works at a higher level, it runs infrequently when a new process is created or when system resources are freed up.
One of its essential functions is maintaining a balance between CPU-bound and I/O-bound processes. A well-balanced mix guarantees that the CPU remains efficiently utilized while preventing bottlenecks in input and output operations.
If too many CPU-bound processes are loaded, the CPU may become overwhelmed. On the other hand, if there are too many I/O-bound processes, the CPU may remain idle, waiting for input/output operations to complete.
2. Short-Term Scheduler (CPU Scheduler)
The short-term scheduler, or CPU scheduler, is responsible for selecting which process from the ready queue should be assigned to the CPU for execution. Unlike the long-term scheduler, it operates frequently, often every few milliseconds, so that processes are executed promptly.
This component is very important for the entire system to be fast and efficient. The scheduler decides how to distribute CPU time between different processes, thus it guarantees that tasks with high priority will be performed without delay and, at the same time, all processes will be treated fairly. Depending on which scheduling algorithm is used, for instance, First-Come, First-Served (FCFS), Round Robin or Priority Scheduling, the next process to be given CPU time is determined.
3. Medium-Term Scheduler
The responsibilities of the medium-term scheduler revolve around the overall control of process swapping in Process Scheduling in OS. It manages the going and coming of processes from memory to make the best use of the resources available. It is a very useful tool in a system memory-constrained environment, as it temporarily swaps out processes to the disk and returns them when they are required again.
This scheduler performs its functions at regular intervals and is very important for the elimination of thrashing, a condition in which overly frequent swapping leads to a system slowing down drastically. The medium-term scheduler is instrumental in keeping the system running at a steady pace ,and it makes sure that the processes in the foreground get sufficient resources to continue operating effectively.
Comparison of Process Schedulers
Process scheduling is based on three main schedulers, the Long-Term Scheduler, Short-Term Scheduler, and Medium-Term Scheduler, that are hierarchically controlling different stages of process execution. Even though they operate conjointly to keep the system running efficiently, each scheduler has a specific function that is related to CPU performance, multiprogramming levels, and the ratio of CPU-bound and I/O-bound processes.
The table below summarizes their main characteristics, benefits, and drawbacks.
| Feature |
Long-Term Scheduler (Job Scheduler) |
Short-Term Scheduler (CPU Scheduler) |
Medium-Term Scheduler (Swapping Scheduler) |
| Main Role |
The long-term scheduler decides which processes should be admitted into the system for execution, controlling the degree of multiprogramming. |
The short-term scheduler selects which ready process should be executed next by the CPU, ensuring fast and efficient task switching. |
The medium-term scheduler manages swapping by temporarily removing processes from memory and bringing them back when resources are available. |
| Frequency of Execution |
It runs infrequently because it only makes decisions when new jobs enter the system. |
It runs very frequently, often every few milliseconds, because it constantly selects the next process for the CPU. |
It runs periodically when memory needs to be optimized or when the system experiences a heavy load. |
| Key Function |
It balances the mix of CPU-bound and I/O-bound processes to maintain smooth overall system performance. |
It allocates CPU time using scheduling algorithms to maximize responsiveness and CPU efficiency. |
It reduces memory pressure by swapping processes in and out, preventing thrashing and maintaining stability. |
| Impact on System Performance |
Its decisions affect long-term stability and overall multiprogramming levels. |
Its decisions directly impact CPU performance, user responsiveness, and context-switch rate. |
Its decisions improve memory utilization and ensure active processes have enough resources to run efficiently. |
| Relation to Preemptive vs Non-Preemptive Scheduling |
It is not directly involved in preemptive or non-preemptive decisions because it operates at a higher level. |
It is heavily involved in preemptive scheduling because it decides when to interrupt a running process. |
It is not involved in preemption decisions but influences process availability through swapping. |
| Type of Processes Managed |
It manages all new processes entering the system before they are ready to run. |
It manages only the processes in the ready queue that are prepared to execute. |
It manages suspended processes that have been removed from memory due to resource constraints. |
| Advantage |
It prevents system overload by controlling how many processes can enter the ready queue at once. |
It improves responsiveness by ensuring that the CPU always works on the most appropriate process. |
It optimizes memory usage and prevents thrashing during heavy multitasking or virtual machine operations. |
| Limitation |
Poor decisions can lead to too many CPU-bound or I/O-bound tasks, reducing system efficiency. |
Frequent context switching can increase overhead and reduce overall throughput. |
Swapping creates latency and can slow down the system if performed too often. |
| Used In |
It is used in job scheduling systems and environments requiring controlled multiprogramming. |
It is used in interactive systems, real-time computing, and environments that require rapid task switching. |
It is used in virtual memory systems, virtual machines, and systems with limited physical memory. |
The performance of process scheduling in OS is evaluated by several essential criteria. These criteria are tools that show how effectively a scheduling algorithm manages the system's resources and satisfies the users' needs:
1. CPU Utilization
- This is a measure of how the CPU is kept busy, in the most effective manner, with work.
- High CPU utilization is indicative of a situation where the processor is kept busy for most of the time, and thus it does not remain idle or wasted for long periods.
2. Throughput
- The number of processes completed per unit of time.
- Higher throughput indicates the system is processing more work efficiently.
3. Turnaround Time
- The entire time from process submission to its completion (the waiting, execution, and I/O times are also included).
- Lower turnaround time means processes finish faster.
4. Waiting Time
- The sum of the waiting times of a process in the ready queue, where it is waiting for the CPU to be given to it.
- Good scheduling minimizes waiting time to improve process flow.
5. Response Time
- The time interval from the submission of a process up to the point when the first response is produced (not completion).
- Especially important in interactive systems; lower response times mean better user experience.
6. Fairness
- Ensures all processes get a reasonable share of CPU time and are not indefinitely postponed (prevents starvation).
- Fairness plays an essential role in multi-user and time-sharing systems.
How Criteria Relate to Algorithms:
- FCFS Scheduling: Very basic, but can lead to very high turnaround and waiting times, especially if a few short processes are waiting behind a long one.
- Round Robin: Improves fairness and response time by giving each process a time slice.
- SJF (Shortest Job First): Minimizes average waiting and turnaround times but may cause starvation for long processes.
- Preemptive Priority-Based Scheduling: The response time of high-priority jobs is enhanced, however, there is a risk that low-priority ones will be starved.
Why These Criteria Matter:
A good scheduling algorithm balances these criteria to maximize efficiency, minimize delays, and provide a fair and responsive system for all users.
Common CPU Scheduling Algorithms
Process Scheduling in OS is dependent on various algorithms that are designed to determine how processes are to be given CPU time in order to have a smooth and efficient performance. The following are some of the most commonly used scheduling algorithms:
1. First-Come, First-Served (FCFS)
This algorithm treats the input of processes as it is, executing each in the order of the arrival time, which is similar to the way a line at the ticket counter works. It is very specific, easy to implement, and in theory, works well when the execution time of the tasks is roughly the same. However, it can lead to inefficiency when shorter tasks get stuck behind longer ones, causing unnecessary delays, a problem known as the convoy effect.
2. Shortest Job Next (SJN)
This is also called Shortest Job First (SJF); this approach prioritizes tasks that require the least processing time. This reduces the overall waiting time, making it an efficient scheduling technique. On the other hand, the biggest limitation or stumbling block it faces is that it requires detailed info on the execution time of each task, which is, most of the time, completely unpredictable away from theoretical scenarios.
3. Round Robin (RR)
With this technique, each process is allotted the CPU for a fixed time slice or period called a time quantum and after that, the process goes to the end of the queue. It is ensured by this that a particular process does not get to use the system resources fully for a long time, and thus, RR is considered to be the perfect multitasking technique. Nevertheless, if the time slice is too shor,t the frequent switching between tasks (context switching) can lead to efficiency loss.
4. Priority Scheduling
Here, each task is assigned a priority, and higher-priority tasks execute before lower-priority ones. This provides critical operations running without delay. However, if lower-priority tasks are constantly surpassed by higher-priority ones, they may experience indefinite delays, a problem known as starvation. To counter this, priority ageing techniques can be used to gradually increase the priority of long-waiting tasks.
5. Multilevel Feedback Queue (MLFQ)
This powerful yet complicated algorithm divides the jobs into many categories of different levels of importance and based on how long they have been executed it alters their priority. Those tasks that are shorter or are interactive processes are given a higher priority, and on the contrary, longer ones are pushed to lower-priority queues gradually. The model is highly efficient due to its equilibrium between fairness and efficiency, nonetheless, the difficulty lies in the implementation because of its sophisticated management requirements.
Comparison of CPU Scheduling Algorithms
| Algorithm |
Fairness |
Responsiveness |
Complexity |
Starvation Risk |
| FCFS |
Low |
Low |
Low |
High |
| SJN |
Medium |
Medium |
Medium |
Low |
| RR |
High |
High |
Medium |
Low |
| Priority |
Low |
High |
Medium |
High |
| MLFQ |
High |
High |
High |
Low |
What is Context Switching?
Context switching is the procedure by which the operating system pauses one running process and resumes another. It is the mechanism that makes multitasking possible. Whenever the CPU shifts its attention from one process to another, the OS must save the current process state and load the next one. This entire activity is called a context switch.
Context switching happens frequently in modern operating systems, especially in preemptive scheduling, where processes can be interrupted to ensure fairness, responsiveness, and efficient CPU utilization.
Why Does Context Switching Happen?
Context switching can occur due to several reasons:
1. Hardware Interrupts
Devices such as keyboards, disks, or network cards send interrupts that require the CPU to give them attention immediately. The CPU, therefore, ceases the work it is doing, records its state, and processes the interrupt. After it is done, it goes back to normal execution.
2. Multitasking and Scheduling Decisions
Schedulers may decide to suspend a process so that another one, often one with a higher priority or shorter burst time, can run. This is key to preemptive scheduling strategies like Round Robin and SRTF.
3. Switching Between User Mode and Kernel Mode
When a process requests access to system resources (like memory or I/O), it switches to kernel mode. To do this, the CPU state must be saved and restored, which is part of the context switching overhead.
Steps Involved in Context Switching
Context switching involves a series of steps to ensure a smooth transition between processes:
- Saving the Current Process State: The CPU saves information about the running process, including its program counter, register values, and other necessary data. This ensures that the process can resume from the exact point it was paused.
- Loading the Next Process State: The state of the next scheduled process is recovered from memory and loaded into the CPU, allowing it to continue execution.
- Updating Scheduling Queues: The operating system updates scheduling information, moving the paused process to the appropriate queue (such as the ready queue or waiting queue) and selecting the next process to run.
Why Context Switching Matters
Context switching is essential for:
- Multitasking: letting many processes run seemingly at the same time
- Responsiveness: switching very quickly to tasks that require user interaction
- Fairness: giving equal opportunities of CPU time to different processes
- Parallelism on single-core CPUs: making it appear as if multiple programs run concurrently
However, it is not free; each context switch adds overhead and slows down the system slightly. Those schedulers that are efficient are always trying to avoid unnecessary switches in order to keep up the system's performance.
Bottom Line
Context switching is the bridge between scheduling decisions and actual CPU execution. It enables multitasking, supports preemptive scheduling, and keeps systems responsive, but comes with performance overhead, making efficient scheduling essential.
Conclusion
Process scheduling in OS is the main function that permits multiple processes to run smoothly by effectively managing CPU time. The OS, through various scheduling strategies and coordination among different schedulers, ensures fairness, responsiveness, and resource utilization at the optimum level. In a stable and predictable multitasking environment, concepts such as context switching, ready queues, and preemption are of vital importance. Understanding these mechanisms equips learners with a solid base for systems programming, OS design, and technical interviews.
Key Points to Remember
- Choosing a scheduling algorithm is a trade-off; improving waiting time may worsen response time, and increasing fairness may reduce throughput. No single algorithm is perfect for all workloads.
- CPU-bound and I/O-bound processes must be balanced; otherwise, the CPU may sit idle or become overloaded. Good schedulers maintain this balance automatically.
- Starvation and convoy effects are real system issues, not just theory; poor scheduling choices can slow down an entire system even if the hardware is powerful.
- Time quantum in Round Robin determines system behavior; too small increases context switching overhead, too large makes RR behave like FCFS.
- Scheduling influences energy usage and battery life in modern systems; unnecessary context switching or poor CPU utilization drains power significantly.
Frequently Asked Questions
1. What is process scheduling in an operating system?
Process scheduling is the function of the OS that is responsible for changing the order of the processes to be executed on the processor. In order to offer multitasking, the technique deals with the usage of different process states, and scheduling algorithms, such as round-robin or Priority Scheduling, are applied.
2. What are the main types of schedulers in an OS?
- Long-Term Scheduler: Selects processes to enter the system.
- Short-Term Scheduler: Assigns CPU time to ready processes.
- Medium-Term Scheduler: Moves processes between memory and disk to free up resources.
3. What are scheduling queues, and why are they important?
Scheduling queues are groups of processes that share the same state:
- Job Queue is a queue where all new processes are kept.
- Ready Queue is a term representing the collection of processes that are in the wait state and are waiting for the processor's time.
- Device Queue is a type of queue where program/s that currently cannot perform some action as they await the required resource and/or data, are held.
4. What is context switching, and how does it affect performance?
Context switching happens when the CPU switches between processes, saving and loading their states. As the situation occurs in multitasking, the switching times become performance killers because of the overhead.
5. How does preemptive scheduling differ from non-preemptive scheduling?
- Preemptive: A process can be interrupted to let a higher-priority task run (e.g., Round Robin).
- Non-Preemptive: A process runs until it finishes or releases the CPU (e.g., FCFS).
6. What are the advantages of process scheduling?
Process scheduling increases the efficiency of the CPU, guarantees that all processes will be treated fairly, raises the system's responsiveness, and makes the CPU busy most of the time, it reducing the CPU idle tim,e which actually can be done better by some other peripherals.
7. What challenges exist in designing scheduling algorithms?
The difficulties deal with the necessity of the solution to be just and efficient at the same time, the problem of overhead created by the context switch, the issue of the so-called starvation of the fanatical low-priority processes, and the problem of forecasting sudden changes in real-time workloads. The Multilevel Feedback Queue method represents one of the advanced solutions for such problems.
