Scheduling (computing)

In computing, scheduling is the method by which work specified by some means is assigned to resources that complete the work. The work may be virtual computation elements such as threads, processes or data flows, which are in turn scheduled onto hardware resources such as processors, network links or expansion cards.

A scheduler is what carries out the scheduling activity. Schedulers are often implemented so they keep all computer resources busy (as in load balancing), allow multiple users to share system resources effectively, or to achieve a target quality of service. Scheduling is fundamental to computation itself, and an intrinsic part of the execution model of a computer system; the concept of scheduling makes it possible to have computer multitasking with a single central processing unit (CPU).

A scheduler may aim at one of many goals, for example, maximizing throughput (the total amount of work completed per time unit), minimizing response time (time from work becoming enabled until the first point it begins execution on resources), or minimizing latency (the time between work becoming enabled and its subsequent completion), maximizing fairness (equal CPU time to each process, or more generally appropriate times according to the priority and workload of each process). In practice, these goals often conflict (e.g. throughput versus latency), thus a scheduler will implement a suitable compromise. Preference is given to any one of the concerns mentioned above, depending upon the user's needs and objectives.

Windows Task Scheduler

Task Scheduler is a component of Microsoft Windows that provides the ability to schedule the launch of programs or scripts at pre-defined times or after specified time intervals: job scheduling (task scheduling). It was first introduced in the Microsoft Plus! for Windows 95 as System Agent but was renamed to Task Scheduler in Windows 98. The Windows Event Log service must be running before the Task Scheduler starts up.

This service should not be confused with the scheduler that allocates CPU resources to processes already in memory.

Versions

Task Scheduler 1.0

Task Scheduler 1.0 is included with Windows 2000, Windows XP and Windows Server 2003. It runs as a Windows Service, and the task definitions and schedules are stored in binary .job files. Tasks are manipulated directly by manipulating the .job files. Each task corresponds to single action. On Windows 95, Windows 98 and Windows Me, the Task Scheduler runs as an ordinary program, mstask.exe. It also displays a status icon in the notification area on Windows 95 and Windows 98 and runs as a hidden service on Windows Me, but can be made to show a tray icon. On Windows 2000, Windows XP and Windows Server 2003, it is implemented as a Windows service. Computer programs and scripts can access the service through six COM interfaces. Microsoft provides a scheduling agent DLL, a sample VBScript and a configuration file to automate Task Scheduler.