Operating system scheduling prioritizes messages sent to a processor to maintain a constant workload and ensure timely completion of tasks. Long-term planning determines program space, mid-term scheduling frees up memory, and short-term scheduling supervises processor workload. Real-time systems require specific planning to ensure timely responses.
Operating system scheduling is the process of controlling and prioritizing messages sent to a processor. An internal operating system program, called a scheduler, performs this task. The goal is to maintain a constant amount of work for the processor, eliminating peaks and troughs in the workload and making sure that each process completes within a reasonable amount of time. While scheduling is important for all systems, it is especially important in a real-time system.
Since nearly every operation on a computer requires at least a small amount of processor time, the processor can be a major source of slowdowns and bottlenecks. In order to relieve stress on the processor and ensure that tasks are completed in a timely manner, most operating systems use some form of task scheduling. The OS planning process varies by system, but they tend to fall into familiar categories.
Planning is typically divided into three parts: long-, medium-, and short-term planning. Not all operating systems fully use every type – medium-term and long-term they are often combined – but they will use some combination of them. Each type of schedule gives the system a slightly different benefit.
Long-term planning revolves around getting programs into the planning process. When a new program is started, the long-term scheduler determines if there is enough room for the new entrant. Otherwise, the scheduler delays activating the program until there is enough space.
The mid-term scheduler decides which processes are idle and which are active. It leaves active processes alone and writes inactive ones to the hard drive. This frees up memory for other programs that enter through the long-term scheduler. When the medium- and long-term schedulers are combined, instead of delaying the activation of a new process, the scheduler simply swaps it in memory.
The short-term scheduler is the part that works directly with the processor. This part activates processes, sets priorities and supervises the processor workload. The short-term scheduler constantly tries to anticipate the computer’s needs to keep the processor running smoothly.
In most cases, operating system scheduling is a way to make a computer run more efficiently, but in a real-time operating system it is critical to its purpose. A real-time system has to run processes within a set time, and if these processes are late, their purpose is lost. These important programs require very specific system planning to make sure information and responses are where they need to be, when they need to be.
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