Scheduling (computing)

Computers have many jobs to do.

A helper decides which job goes first. This helper is called a scheduler. It gives the computer parts what they need.

One goal is to keep the computer busy. It also tries to make sure no job waits too long. This helps the computer feel fast.

Sometimes, the helper must make hard choices. It might pick one job over another. This helps the computer do many things at once.

In some machines, like robots, timing is key. The helper must finish jobs by a certain time. This keeps the machine working well.

This smart helper makes sure everything runs smoothly.

Computers have many jobs to do at once.

To manage these jobs, they use a scheduler. A scheduler is a part of the computer system that assigns resources to tasks. Resources can be parts like the CPU, which is the brain of the computer. The scheduler makes sure these parts stay busy. It also helps many users share the same computer fairly.

There are different kinds of schedulers. A long-term scheduler decides which jobs are allowed to enter the system. It picks a mix of jobs that use the CPU and jobs that use other parts. This keeps the system balanced. A medium-term scheduler moves jobs in and out of the main memory. This is called swapping.

A short-term scheduler makes very fast decisions. It picks which job runs right now.

Some schedulers are preemptive. This means they can stop a running job to start a new one. Others are cooperative. These can only stop a job if the job lets them. A special part called a dispatcher then gives control to the new job. Schedulers use different rules to work. One rule is called first-come, first-served. This means jobs are handled in the order they arrive.

Computers are amazing at doing many things at once. To do this, they use a system called scheduling. Scheduling is the way a computer assigns resources to different tasks. These resources might be the processor, network links, or expansion cards. The tasks being managed can be threads, processes, or data flows.

A special part of the system called a scheduler handles all this work. It makes sure the computer stays busy and works fairly for everyone.

A scheduler has many important goals to reach. One goal is to maximize throughput, which is the total amount of work finished in a certain time. Another goal is to minimize wait time or latency. Latency is the time from when a task is ready until it is actually finished. Schedulers also try to ensure fairness by giving equal time to different processes. Sometimes these goals conflict with each other. Because of this, the scheduler must find a good compromise.

Operating systems use different types of schedulers to stay organized. The long-term scheduler, or admission scheduler, decides which jobs enter the system. It controls the mix of CPU-bound and I/O-bound processes. A CPU-bound process spends most of its time doing math and logic. An I/O-bound process spends more time doing input and output tasks. The medium-term scheduler helps by moving processes in and out of main memory. This process of moving things to secondary memory is called swapping.

The short-term scheduler, also known as the CPU scheduler, makes very fast decisions. It chooses which process in the memory gets to use the CPU next. This happens after things like clock interrupts or system calls. This scheduler can be preemptive, meaning it can forcibly stop a process. If it cannot stop a process, it is called a cooperative scheduler. A module called the dispatcher then carries out the actual switch. The dispatcher saves the state of the old process and loads the new one. This quick switch is known as dispatch latency.

Schedulers use different rules, or algorithms, to manage their work. One simple rule is First-In, First-Out, or FIFO. This means tasks are handled in the exact order they arrive. This is very easy to do but can sometimes cause a convoy effect. This happens when one long task makes many short tasks wait. Other rules include priority scheduling, like Earliest Deadline First. This is used in real-time systems to make sure tasks meet their deadlines. These rules help everything from printers to large computer clusters run smoothly.

In computing, scheduling is the essential process of assigning resources to perform specific tasks. These resources can include processors, network links, or expansion cards. The tasks being managed are often threads, processes, or data flows. This activity is managed by a specialized mechanism known as a scheduler. Scheduling is fundamental to computation and is an intrinsic part of a computer's execution model. It is the concept that makes multitasking possible even on a system with a single central processing unit (CPU).

A scheduler typically aims to achieve several specific goals. One primary goal is maximizing throughput, which is the total amount of work completed per time unit. Another goal is minimizing wait time, which is the duration from when work becomes ready until it begins execution. Schedulers also work to minimize latency or response time. In interactive systems, response time is the interval between a task becoming ready and the system providing the first output to the user. Finally, schedulers aim for fairness, which means providing equal or appropriate CPU time to each process based on its priority and workload. In practice, these goals often conflict, such as the tension between throughput and latency. Therefore, a scheduler must implement a suitable compromise based on user needs.

Operating systems utilize up to three distinct types of schedulers to manage these tasks. The first is the long-term scheduler, also called the admission scheduler. This module decides which jobs or processes are admitted to the ready queue in main memory. It dictates the degree of concurrency and manages the split between different types of processes. Specifically, it balances I/O-bound processes, which spend more time on input/output, and CPU-bound processes, which spend more time on computations. A good mix is vital for performance. If all processes are I/O-bound, the short-term scheduler will have little to do. If all are CPU-bound, hardware devices may go unused.

The medium-term scheduler provides another layer of management by handling memory. It temporarily removes processes from main memory and places them in secondary memory, such as a hard disk drive. This process is commonly referred to as swapping out or swapping in. The scheduler may swap out a process if it has low priority, is not active, or uses too much memory. This frees up main memory for other processes. In some modern systems, the medium-term scheduler also performs long-term scheduling tasks. It can use a method called demand paging, where segments of a binary are swapped in only when they are required.

The short-term scheduler, or CPU scheduler, makes decisions much more frequently than the other types. It selects which in-memory process will be allocated the CPU after a clock interrupt, an I/O interrupt, or a system call. This scheduler can be preemptive, meaning it can forcibly remove a process from the CPU to start a new one. If it cannot force a process off the CPU, it is known as a cooperative scheduler. To execute these decisions, the system uses a module called the dispatcher. The dispatcher performs context switches, which involve saving the state of the old process and loading the state of the new one. The time required for this switch is known as dispatch latency.

To distribute resources, schedulers follow specific rules called scheduling disciplines or algorithms. One of the simplest methods is First-In, First-Out (FIFO), also called First-Come, First-Served (FCFS). This algorithm queues processes in the exact order they arrive. While it has minimal overhead, it can suffer from the convoy effect, where long processes cause short processes to wait a long time. Other more complex algorithms include priority scheduling. For example, Earliest Deadline First (EDF) is a dynamic algorithm used in real-time operating systems. It searches for the process closest to its deadline to determine the next task for execution.

Scheduling is critical in various specialized environments. In real-time environments, such as industrial robotics, schedulers must ensure processes meet strict deadlines to keep the system stable. In computer networks, scheduling algorithms manage packet traffic in routers. Advanced wireless networks, like LTE, use channel-dependent scheduling to improve efficiency. These systems can assign resources to users who can best utilize them based on current conditions. Whether managing a single laptop or a massive supercomputer, scheduling ensures that computing resources are used effectively and fairly.