Dispatch Latency

The most significant element in scheduling behavior for real-time applications is the provision of a real-time scheduling class. The standard time-sharing scheduling class is not suitable for real-time applications because this scheduling class treats every process equally. The standard time-sharing scheduling class has a limited notion of priority. Real-time applications require a scheduling class in which process priorities are taken as absolute. Real-time applications also require a scheduling class in which process priorities are changed only by explicit application operations.

The term dispatch latency describes the amount of time a system takes to respond to a request for a process to begin operation. With a scheduler that is written specifically to honor application priorities, real-time applications can be developed with a bounded dispatch latency.

The following figure illustrates the amount of time an application takes to respond to a request from an external event.

Application Response Time

The overall application response time consists of the interrupt response time, the dispatch latency, and the application's response time.

The interrupt response time for an application includes both the interrupt latency of the system and the device driver's own interrupt processing time. The interrupt latency is determined by the longest interval that the system must run with interrupts disabled. This time is minimized in Oracle Solaris by using synchronization primitives that do not commonly require a raised processor interrupt level.

During interrupt processing, the driver's interrupt routine wakes the high-priority process and returns when finished. The system detects that a process with higher priority than the interrupted process is now ready to dispatch and dispatches the process. The time to switch context from a lower-priority process to a higher-priority process is included in the dispatch latency time.

The figure titled Internal Dispatch Latency illustrates the internal dispatch latency and application response time of a system. The response time is defined in terms of the amount of time a system takes to respond to an internal event. The dispatch latency of an internal event represents the amount of time that a process needs to wake up a higher priority process. The dispatch latency also includes the time that the system takes to dispatch the higher priority process.

The application response time is the amount of time that a driver takes to: wake up a higher-priority process, release resources from a low-priority process, reschedule the higher-priority task, calculate the response, and dispatch the task.

Interrupts can arrive and be processed during the dispatch latency interval. This processing increases the application response time, but is not attributed to the dispatch latency measurement. Therefore, this processing is not bounded by the dispatch latency guarantee.

Internal Dispatch Latency

With the new scheduling techniques provided with real-time Oracle Solaris, the system dispatch latency time is within specified bounds.