35.2. Overview of Realtime Process Scheduling

The standard kernel scheduling algorithm usually provides adequate performance and responsiveness for the mixture of interactive and background processes typically run on a system. However, realtime applications have more stringent requirements of a scheduler, including the following:

• A realtime application must provide a guaranteed maximum response time for external inputs. In many cases, these guaranteed maximum response times must be quite small (e.g., of the order of a fraction of a second). For example, a slow response by a vehicle navigation system could be catastrophic. To satisfy this requirement, the kernel must provide the facility for a high-priority process to obtain control of the CPU in a timely fashion, preempting any process that may currently be running.

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  Note:

  A time-critical application may need to take other steps to avoid unacceptable delays. For example, to avoid being delayed by a page fault, an application can lock all of its virtual memory into RAM using mlock() or mlockall() (described in Section 50.2).

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• A high-priority process should be able to maintain exclusive access to the CPU until it completes or voluntarily relinquishes the CPU.

• A realtime application should be able to control the precise order in which its component processes are scheduled.