Process Schedulers in Operating System

What is Process Scheduling in OS?

At its heart, process scheduling in an operating system is the complex task of deciding the order in which tasks run on the CPU. Since there are often more tasks than available CPU cores, the schedulers in OS play a crucial role in choosing which task gets CPU time and for how long. The main goal of scheduling is to make the best use of the CPU, reduce response times, and fairly share resources among different tasks.

Categories in Scheduling

Scheduling in operating systems involves efficient task management for optimal resource utilization. The main scheduling categories are:

1. Short-Term/CPU Scheduling:
   Determines which process runs on the CPU immediately, using algorithms like Round Robin and Priority Scheduling.
2. Long-Term/Job Scheduling:
   Controls process admission to balance responsiveness and resource use.
3. Medium-Term Scheduling:
   Manages process swapping to prevent memory congestion and ensure smooth operation.

Specialized schedulers in OS include:

1. I/O Scheduling:
   Optimizes input/output requests for devices.
2. Real-Time Scheduling:
   Prioritizes time-sensitive tasks.
3. Batch Scheduling:
   Executes non-interactive jobs in sequences.

These categories enable efficient multitasking and resource allocation within modern operating systems. For an in-depth understanding, explore this article by Scaler Topics on CPU Scheduling in Operating System.

Short-Term/CPU Schedulers in OS

The Short-Term/CPU Schedulers in OS play a pivotal role in managing the execution of processes on the central processing unit (CPU) within an operating system. This scheduler is responsible for selecting which process from the ready queue should be allocated the CPU for a limited time slice. Its primary objective is to maximize CPU utilization and ensure fair access to CPU time for all processes.

Key Characteristics

1. Time Slicing:
   The CPU scheduler assigns a fixed time slice, known as a quantum or time quantum, to each process in the ready queue. When the time slice expires, the CPU is preempted from the running process, and another process is selected to run.
2. Round Robin Scheduling:
   One of the commonly used algorithms in the Short-Term Scheduler is the Round Robin algorithm. In this approach, processes are serviced in a circular order, each receiving a time slice before moving to the next process in the queue. This ensures that all processes get an equal share of CPU time.
3. Priority Scheduling:
   Another approach is Priority Scheduling, where each process is assigned a priority value. The CPU scheduler selects the process with the highest priority to execute next. This can lead to the potential starvation of lower-priority processes if not managed carefully.

Importance

The Short-Term/CPU Scheduler significantly impacts system responsiveness and overall performance. Swiftly switching between processes creates an illusion of multitasking for users, allowing multiple applications to appear to be running simultaneously.

Challenges

While the Short-Term/CPU Scheduler enhances multitasking, it also introduces challenges like:

1. Context Switching Overhead:
   Frequent context switching between processes consumes time and resources.
2. Fairness:
   Ensuring that all processes receive fair CPU time can be complex, especially when dealing with processes of varying priorities.

Long-Term/Job Schedulers in OS

The Long-Term/Job Scheduler, also known as the Admission Scheduler, plays a fundamental role in managing the influx of processes within the operating system. Unlike the Short-Term Scheduler, which focuses on immediate execution, the Long-Term Scheduler decides which processes should be admitted into the system's queue for execution.

Key Functions

1. Process Admission:
   The Long-Term Scheduler decides which processes should be allowed into the system from a pool of waiting processes. This decision is based on available system resources, memory space, and CPU capacity.
2. Resource Utilization:
   By controlling the number of processes in the system, the Long-Term Scheduler helps optimize resource utilization. It ensures that the system is not overwhelmed with too many processes, which could lead to resource contention and performance degradation.
3. Balancing Throughput and Responsiveness:
   The Long-Term Scheduler aims to strike a balance between high throughput (the number of processes completed over time) and responsiveness (how quickly a process can start execution). Admitting too many processes could lead to lower responsiveness while admitting too few could reduce throughput.

Importance

Effective Long-Term Scheduling is crucial for maintaining a stable and efficient operating environment. It prevents resource exhaustion and ensures that the system can continue to operate smoothly even under heavy loads.

Challenges

While the Long-Term Scheduler offers benefits, it also faces challenges such as:

1. Determining Process Priority:
   Selecting which processes to admit based on priorities and available resources can be complex.
2. Resource Estimation:
   Estimating the resource requirements of processes accurately is essential for effective scheduling.

Medium-Term Schedulers in OS

The Medium-Term Scheduler, a crucial component within an operating system, is important in managing the number of processes residing in memory at any given time. This type of scheduling is essential for maintaining efficient memory utilization and overall system performance.

Key Functions

1. Process Swapping:
   The Medium-Term Scheduler is responsible for swapping processes between main memory and secondary storage (usually disk). When the system encounters a memory congestion situation, this scheduler transfers less frequently used or idle processes out of main memory and onto disk, freeing up memory space for active processes.
2. Memory Management:
   By controlling the number of processes in memory, the Medium-Term Scheduler helps prevent memory overload, which can lead to slow performance and system crashes. It ensures that active processes have sufficient memory to operate effectively.
3. Performance Optimization:
   Effective medium-term scheduling reduces memory contention, minimizes the need for excessive paging (data transfer between memory and disk), and enhances overall system responsiveness.

Importance

The Medium-Term Scheduler directly contributes to a stable and balanced operating environment. It prevents memory exhaustion and helps avoid situations where an excessive number of processes compete for limited memory resources.

Challenges

The Medium-Term Scheduler faces certain challenges, including:

1. Determining Swapping Criteria:
   Deciding when to swap a process out of memory involves evaluating factors like process priority, resource usage, and the likelihood of further CPU utilization.
2. Performance Impact:
   Frequent process swapping can introduce overhead due to disk I/O operations, potentially affecting system responsiveness.

Other Important Schedulers in OS

In addition to the primary schedulers, other specialized schedulers play pivotal roles in upholding system efficiency. These include:

1. I/O Scheduler:
   Tasked with determining the sequence in which I/O requests are serviced, the I/O scheduler aims to optimize the use of I/O devices, thereby limiting the time processes spend waiting for I/O operations to finalize.
2. Memory Scheduler:
   This scheduler manages and assigns memory to processes. Its job is to make sure processes get the memory they need to run, while also preventing memory issues.
3. Network Scheduler:
   In environments with multiple network connections, the network scheduler orchestrates the allocation of network bandwidth to different processes. This prevents any single process from overwhelming the network and obstructing the communication channels.

Short-Term Vs Medium-Term Vs Long-Term Scheduling

To understand their roles better, let's compare Short-Term, Medium-Term, and Long-Term Scheduling.

Two State Process Model Short Term

The Two-State Process Model is a conceptual framework used in operating systems to describe the various stages a process goes through during its lifecycle. The short-term state of this model refers to the active phase of a process, where it interacts directly with the CPU for execution.

States

1. Running State:
   In this state, the process is actively using the CPU to execute its instructions. It is the only state where a process is truly in control of the CPU and performing computations.
2. Ready State:
   A process enters the ready state when it's prepared for execution but is awaiting its turn to be allocated CPU time. Processes in this state are in the queue of ready-to-run processes, waiting for the CPU scheduler to select them.

Transitions

The transition from the ready state to the running state occurs when the CPU scheduler selects the process from the ready queue and allocates CPU time to it. The process then moves from the ready state to the running state. Similarly, a process transitions from the running state to the ready state when its time quantum expires or when it voluntarily yields the CPU.

What is Context Switching?

Context switching is a crucial task done by the operating system. It happens when the CPU switches from one task to another. The system carefully saves the current task's status, like register values and where it was in the program. Then, it loads either the status of the new task or the previously stored status of the task being switched to, letting it continue without interruption. While context switching is important for multitasking and managing tasks, it does add some extra work. This is because saving and restoring the task's status takes time and resources.