Ever notice how your computer smoothly runs many apps? It’s all thanks to clever CPU management. This is called CPU scheduling. It’s the OS’s secret weapon for efficiency. Let’s explore how it works. Ready to boost your understanding of processing power?
Why Essentials of CPU Scheduling
Essentially, CPU scheduling is the operating system’s most important function of determining which one of the many waiting processes gets to use the Central Processing Unit (CPU) at a specific moment. Consider an orchestra conductor; the scheduler distributes the time of the CPU among the vying processes.
Since a CPU can execute one instruction stream at any instant, such management is crucial to create the illusion of parallel operation—multitasking. CPU scheduling directly affects system responsiveness, throughput, and user satisfaction in general.
All of the fundamental CPU scheduling objectives are the same: to utilise the CPU to its fullest so as not to leave the processor idle, to minimise response time for interactive tasks so that the user perceives continuity, and justice in preventing a process from dominating the CPU and keeping others out for an extended period.
The choice and implementation of specific scheduling algorithms in os are central to achieving these goals.
The Indispensable Role of Scheduling Algorithms in Operating Systems
Consider a scenario devoid of sophisticated scheduling. Processes might simply run sequentially upon arrival, potentially leading to frustrating delays for short, interactive tasks held back by longer, CPU-intensive ones. This is where scheduling algorithms in an operating system become essential. They introduce a structured and intelligent way to allocate the CPU’s finite time.
In modern multiprogramming environments, where many processes reside in memory concurrently, the need for these algorithms is paramount. Without well-defined scheduling algorithms in the operating system, the CPU could sit idle while processes wait for I/O, or some processes might unfairly dominate processing time. Effective CPU scheduling ensures that if a process is waiting, another one may utilise the CPU; hence, increasing the usage of resources and efficiency of the system as a whole. This intricate management by CPU scheduling is a cornerstone of contemporary operating systems.
Core Concepts in CPU Scheduling
To speak about various scheduling approaches appropriately, one should be acquainted with these terms:
- Arrival Time: The precise point of time at which a process is put into the ready queue to wait on the CPU for execution.
- Burst Time: CPU time required by a process to finish its execution. This is often estimated or can be based on past behaviour.
- Completion Time: Actual time when a process completes its execution and stops the system.
- Turnaround Time: Total time spent by a process in the system, from arrival to finish (Completion Time – Arrival Time). This gives an estimate of the total time a user waits before the task is over.
- Waiting Time: The cumulative time a process spends waiting in the ready queue for its turn on the CPU (turnaround time minus burst time). Lower waiting times are generally desirable for a better user experience.
- Response Time: Particularly crucial in interactive systems, this is the time from the sending of a request to the creation of the first answer. Perceived interactivity depends on this.
Understanding these metrics allows for a quantitative comparison of the effectiveness of different scheduling algorithms in OS.
Guiding Principles for Designing CPU Scheduling Algorithms
Crafting a robust CPU scheduling algorithm involves a delicate balancing act among various performance objectives. Some basic design goals are as follows:
- Maximise CPU Utilisation: The goal is to keep the CPU always engaged, with high throughput and optimum utilisation of resources.
- Maximise Throughput: It is the number of processes that run their computation at an instant of time. Higher throughput indicates higher system productivity.
- Minimise Turnaround Time: Minimising the total sum of the time required by every process in the system raises user satisfaction, particularly for batch processing.
- Minimise Waiting Time: Having processes in the ready queue as briefly as possible enhances responsiveness, especially for interactive programs.
- Minimise Response Time: In interactive systems, having the wait until the first output as small as possible is of paramount importance for an acceptable user response.
- Ensure Fairness: The algorithm must attempt to give each process a fair amount of CPU time to prevent indefinite postponement or starvation.
Also, depending upon the type of operating system and the purposes for which it is being planned, the relative importance of these factors differs. Real-time systems, for instance, depend much on response time and deadline fulfilment; batch systems’ throughput may be very important.
Categorising CPU Scheduling Approaches
CPU scheduling algorithms are classified at large into two major categories:
Preemptive Scheduling: Under this approach, the operating system may preempt an executing process and put it into the ready queue. This is usually carried out when a more important process enters or when a process has exceeded its assigned time slice. Round Robin and preemptive priority scheduling are some of them.
Non-Preemptive Scheduling: Here, the process is given the CPU and keeps going till it either completes its CPU burst or blocks deliberately (e.g., for I/O wait). First-Come, First-Served (FCFS) and non-preemptive Shortest Job First (SJF) are among them.
Let us discuss some of the most commonly used CPU scheduling algorithms.
A Better Insight into the Most Popular CPU Scheduling Algorithms
The landscape of scheduling algorithms within an operating system consists of numerous tried and tested methods with varying strengths and weaknesses:
First-Come, First-Served (FCFS)
FCFS is the most intuitive to comprehend conceptually CPU scheduling algorithm. Processes are served strictly in the order of their arrival in the ready queue. It operates like a single queue where the first to join is the first to be served.
- Mechanism: The process that requests the CPU first is granted it first.
- Advantages: Easy to understand and straightforward to implement.
- Disadvantages: Can suffer from the convoy effect, where a long-running process at the front of the queue delays all subsequent shorter processes. It’s non-preemptive, which can lead to poor response times for short interactive tasks if a long job is running.
Shortest Job First (SJF)
The SJF algorithm prioritises the process with the smallest predicted next CPU burst time. The rationale is to minimise the average waiting time by processing shorter tasks quickly.
- Mechanism: The scheduler selects the process with the shortest expected CPU burst time to run next.
- Advantages: Provably optimal in terms of minimising the average waiting time for a given set of processes.
- Disadvantages: Requires accurate knowledge of the future CPU burst time, which is often not available. If longer jobs keep arriving, shorter jobs might always be prioritised, leading to starvation of the longer ones. It can be implemented in both preemptive and non-preemptive forms.
Shortest Remaining Time First (SRTF)
SRTF is the preemption form of the SJF algorithm. 2.If a new process arrives with an arrival time less than the current time of the executing process, the scheduler switches over the executing process and allocates the CPU to the new process.
- Mechanism: The process having the shortest remaining CPU burst time is always executed.
- Advantages: Offers lower average waiting times compared to non-preemptive SJF because shorter jobs are handled as soon as they become the shortest remaining.
- Disadvantages: Still requires knowledge of the CPU burst times. It can also lead to starvation of longer processes as short jobs continuously arrive. The overhead of frequent context switching due to preemption can also be a concern.
Round Robin (RR)
The round robin algorithm is specifically designed for time-sharing systems. Every process is allocated a particular amount of CPU time called a time quantum. A process, if not finished within its time quantum, is preempted and is put at the end of the ready queue.
- Mechanism: Every process is assigned a certain amount of CPU time. After the time slice has elapsed, the CPU is scheduled for the subsequent process in the ready queue.
- Pros: Offers a more balanced allocation of CPU time to all processes for improved responsiveness for interactive systems. No process can monopolise the CPU for too long.
- Disadvantages: The performance heavily depends on the size of the time quantum. A very small quantum results in frequent context switches, increasing overhead, and potentially decreasing CPU efficiency. A very large quantum makes RR behave more like FCFS.
Priority Scheduling
In priority scheduling, a priority is given to every process, and the highest-priority process is chosen to execute. Priorities may be allocated statically (during creation time) or dynamically (as a process execution function).
- Mechanism: The process with the highest priority is given the CPU. If multiple processes have the same priority, they are usually scheduled using another algorithm (e.g., FCFS).
- Advantages: Allows critical processes to receive preferential treatment.
- Disadvantages: Can lead to starvation of low-priority processes if there is a continuous influx of high-priority processes. The easiest way to solve this problem is ageing, where with time, the priority of an awaiting process increases. Priority scheduling is either preemptive or non-preemptive.
Multilevel Queue Scheduling
Multilevel Queue (MLQ) scheduling divides the ready queue into several distinct queues, each with its scheduling algorithm. Processes are permanently assigned to one queue based on some property (e.g., process type). There must also be scheduling between the queues themselves, often based on fixed priority.
- Mechanism: Multiple ready queues are maintained, each potentially using a different scheduling algorithm. Jobs are allocated to a queue depending on their type. Scheduling between queues is normally fixed priority (e.g., system processes take precedence over interactive processes, which take precedence over batch processes).
- Pros: Provides flexibility in the applicability of scheduling policies across process types.
- Disadvantages: Can be inflexible if a process’s characteristics change over time. Also, there’s the potential for starvation of processes in lower-priority queues.
Multilevel Feedback Queue Scheduling
Multilevel Feedback Queue (MLFQ) is a variation of MLQ where processes are switched from one queue to another. The goal is to separate processes based on their CPU burst characteristics. For example, if a process runs out of its time quantum in one queue, it can be migrated to a low-priority queue with a greater time quantum. Processes that wait for a long time in lower-priority queues might be moved to higher-priority queues (ageing).
- Mechanism: Multiple queues with different priority levels and time quanta. Processes can move between these queues based on their CPU usage patterns.
- Advantages: More adaptable than MLQ, as it tries to dynamically adjust priorities to give preference to interactive, short-burst processes while preventing starvation of longer ones.
- Disadvantages: The most complex to implement, as it requires defining the parameters for each queue, the rules for moving processes between queues, and the initial queue assignment.
Comparative Analysis of CPU Scheduling Algorithms
| Algorithm | Allocation Strategy | Complexity | Typical Average Waiting Time | Preemption? | Starvation Potential? | Use Cases |
|---|---|---|---|---|---|---|
| FCFS | First to arrive gets the CPU. | Simple | Can be high | No | No | Simple batch systems. |
| SJF | Shortest next CPU burst. | More complex | Generally low | No | Yes | Batch systems where burst times are predictable. |
| SRTF | Shortest remaining CPU burst. | More complex | Often the lowest | Yes | Yes | Systems need to minimise average wait time, at the cost of overhead. |
| RR | Time slices are allocated to each process. | Medium | Moderate | Yes | No | Time-sharing systems, interactive systems. |
| Priority | Based on the assigned priority. | Simple/Medium | Varies based on priorities | Yes/No | Yes | Systems with processes of varying importance. |
| MLQ | Processes are assigned to fixed priority queues. | Medium/Complex | Can vary significantly | Often No | Yes | Systems with different classes of processes. |
| MFLQ | Processes move between priority queues based on CPU usage. | Very Complex | Generally good | Yes | Low | Modern general-purpose operating systems. |
Selecting the Optimal Scheduling Algorithm
The choice of the "best" CPU scheduling algorithms is highly context-dependent, hinging on the specific objectives and workload of the system. There is no universally superior algorithm. Factors such as the nature of the processes (I/O-bound vs. CPU-bound), the need for real-time guarantees, and the importance of fairness all influence the selection process. A deep understanding of the trade-offs inherent in each of these scheduling algorithms in os is crucial for designing efficient and responsive computing environments.