Scheduling algorithms are the core logic used by the short-term scheduler to decide which process from the ready queue should be given the CPU. Each algorithm has different characteristics and is optimized for different performance metrics.

● Concept: This is the simplest scheduling algorithm. Processes are dispatched to the CPU in the strict order of their arrival in the ready queue. It's akin to a single-line queue at a bank or a supermarket.
● Implementation: Typically implemented with a FIFO (First-In, First-Out) queue.
● Nature: Non-preemptive. Once a process gets the CPU, it holds it until it either completes its CPU burst (finishes execution or performs an I/O request) or voluntarily yields the CPU.
● Advantages:
○ Easy to understand and implement.
○ Fair in the sense that processes are served in the order they arrive.
● Disadvantages:
○ Convoy Effect: A major drawback. If a very long CPU-bound process arrives first, it will tie up the CPU, forcing all subsequent processes (even very short I/O-bound ones) to wait for a significant amount of time. This can lead to very poor average waiting times and low CPU utilization if many I/O devices are waiting.
○ Average waiting time can be quite high.
○ Not suitable for interactive systems where quick response times are crucial.

● Concept: This algorithm assigns the CPU to the process that has the smallest estimated next CPU burst time. It aims to minimize the average waiting time.
● Optimality: SJF is proven to be optimal in terms of minimizing the average waiting time for a given set of processes. This is because by always running the shortest job first, it clears the CPU quickly, allowing other processes to start sooner.
● Variations:
○ Non-Preemptive SJF: Once a process is given the CPU, it runs to completion of its CPU burst, even if a new process with a shorter burst arrives while it's executing.
○ Preemptive SJF (Shortest-Remaining-Time-First - SRTF): If a new process arrives in the ready queue with a CPU burst time shorter than the remaining time of the currently executing process, the currently executing process is preempted, and the new, shorter process is scheduled.
● Advantages:
○ Provides the minimum average waiting time among all scheduling algorithms.
○ Optimal for batch systems where burst times are known or can be reliably estimated.
● Disadvantages:
○ Practicality Issue: The biggest challenge is that it's impossible to know the length of the next CPU burst in advance. Operating systems can only estimate it (e.g., using exponential averaging of previous CPU burst times).
○ Starvation (Non-Preemptive): If there is a continuous stream of short jobs, a long job might never get the CPU, leading to indefinite postponement. Preemptive SJF (SRTF) reduces this risk but doesn't eliminate it entirely for extremely long jobs.
○ Increased overhead for estimating burst times.

● Concept: Each process is assigned a priority number (an integer). The CPU is always allocated to the process with the highest priority. If two processes have the same priority, FCFS is often used to break the tie.
● Priority Assignment:
○ Internal Factors: Can be based on process characteristics (e.g., time limits, memory requirements, I/O burst frequency).
○ External Factors: Set by users, system administrators, or based on the type of process (e.g., real-time processes, interactive processes, batch processes). Typically, a smaller integer value represents a higher priority.
● Variations:
○ Non-Preemptive Priority: The CPU is allocated to the highest-priority process, and it runs until it completes its CPU burst or blocks for I/O.
○ Preemptive Priority: If a new process arrives with a higher priority than the currently running process, the currently running process is preempted and put back into the ready queue, and the new, higher-priority process is given the CPU.
● Advantages:
○ Allows important or time-sensitive processes to be executed quickly.
○ Flexible to adapt to specific system requirements by tuning priorities.
● Disadvantages:
○ Starvation (Indefinite Blocking): The most significant problem. Low-priority processes may never get the CPU if there is a constant stream of high-priority processes. They may wait indefinitely.
○ Solution for Starvation: Aging: A technique where the priority of a process gradually increases the longer it waits in the ready queue. This ensures that even low-priority processes will eventually gain a high enough priority to be executed, preventing indefinite postponement.
○ Defining and assigning appropriate priorities can be complex and may require careful tuning.

● Concept: Specifically designed for time-sharing systems to provide fair CPU allocation and good response times for interactive users. It is a preemptive, cyclic algorithm.
● Time Quantum (Time Slice): Each process is given a small unit of CPU time, called a time quantum (or time slice), typically 10 to 100 milliseconds.
● Mechanism:
○ Processes are placed in a circular ready queue.
○ The CPU scheduler picks the first process from the ready queue.
○ The process is given the CPU for one time quantum.
○ If the process completes its CPU burst within the quantum, it releases the CPU and terminates or blocks for I/O.
○ If the process does not complete its CPU burst within the quantum, it is preempted by the timer interrupt, and its context is saved. It is then moved to the end of the ready queue, and the CPU is allocated to the next process in the queue.
● Advantages:
○ Fairness: Each process gets a fair share of the CPU over time.
○ Good Response Time: Particularly for short interactive jobs, as they don't have to wait for long jobs to finish. This makes it suitable for interactive systems.
○ Prevents starvation (assuming a non-zero time quantum).
● Disadvantages:
○ Performance depends heavily on the time quantum size:
■ Large Quantum: If the quantum is very large (approaching infinity), RR effectively degenerates into FCFS.
■ Small Quantum: A very small quantum leads to excessive context switching. The CPU spends more time switching between processes than executing useful work, reducing overall throughput. This is pure overhead.
○ Average waiting time can be longer than SJF for some workloads.

● Concept: This algorithm partitions the ready queue into multiple separate queues, each with its own scheduling algorithm. Processes are permanently assigned to a specific queue based on their characteristics.
● Example Partitioning:
○ Foreground (Interactive) Queue: Processes that require quick response times (e.g., text editors, web browsers). Might use Round-Robin.
○ Background (Batch) Queue: Processes that are less sensitive to response time (e.g., large data computations, print jobs). Might use FCFS.
○ System Processes Queue: For operating system processes.
● Inter-Queue Scheduling: There needs to be a scheduler to determine which queue gets access to the CPU. This can be done using:
○ Fixed-Priority Preemptive Scheduling: Higher-priority queues are served first. If the foreground queue is not empty, background processes will not run. This can lead to starvation for lower-priority queues.
○ Time Slicing between Queues: Each queue receives a certain percentage of CPU time. For example, the foreground queue might get 80% of CPU time, and the background queue gets 20%. Within their allocated time, processes in each queue are scheduled according to their own algorithm.
● Advantages:
○ Allows different types of processes to be scheduled appropriately based on their requirements.
○ Reduces scheduling overhead by not needing to consider all processes in the system at once.
● Disadvantages:
○ Static Assignment: Processes are permanently assigned to a queue, which can be inflexible. If an interactive process suddenly becomes CPU-bound, it remains in the interactive queue, potentially harming other interactive processes.
○ Potential for Starvation: If higher-priority queues always have processes ready, lower-priority queues might never get CPU time (in fixed-priority scheduling).

● Concept: This is the most complex but also the most flexible and general scheduling algorithm. It allows processes to move between different queues based on their CPU burst characteristics or behavior. This mechanism aims to separate processes with different CPU burst patterns.
● Dynamic Adaptation:
○ New Process Entry: A new process typically enters the highest-priority queue.
○ Demotion: If a process consumes its entire time quantum in a higher-priority queue, it is demoted to a lower-priority queue (e.g., it's likely a CPU-bound process). Lower-priority queues usually have larger time quanta or use FCFS.
○ Promotion (Aging): To prevent starvation, a mechanism called "aging" is typically implemented. Processes that wait for too long in a lower-priority queue are promoted to a higher-priority queue, ensuring they eventually get a chance to execute.
○ I/O Bound Preference: Processes that block for I/O before their quantum expires are typically kept in their current queue or moved to a higher-priority queue upon returning from I/O, as they are likely I/O-bound processes that should be prioritized for responsiveness.
● Configuration Parameters: An MLFQ scheduler is defined by:
○ The number of queues.
○ The scheduling algorithm for each queue.
○ The method used to determine when to upgrade a process to a higher-priority queue (aging).
○ The method used to determine when to demote a process to a lower-priority queue.
○ The method used to determine which queue a process will enter when it needs service.
● Advantages:
○ Highly Flexible and Adaptable: Can be tuned to achieve a good balance of response time, throughput, and fairness.
○ Approximates other algorithms: With proper configuration, it can behave like SJF (by prioritizing short CPU bursts), RR, or FCFS.
○ Prevents Starvation: Through the aging mechanism.
○ Favors I/O-bound processes: By keeping them in higher-priority queues or promoting them, it ensures good responsiveness for interactive applications.
● Disadvantages:
○ Complexity: Very difficult to implement and configure optimally due to the large number of parameters and the dynamic nature of process movement between queues.
○ Requires careful tuning to achieve desired performance characteristics.