1. Latency and deadlines as performance metric in scheduling models for periodic, sporadic and aperiodic tasks:

An RTOS should quickly and predictably respond to the event. It should have minimum interrupt latency and fast context switching latency. Different models have been proposed for measuring performances. They are-

• Ratio of sum of interrupt latencies with respect to the sum of execution times.
• CPU loads.
• Worst case execution time with respect to the mean execution time.

Interrupt latencies in various task models can be used for evaluating performance metrics.

2. CPU load as performance metric-

i) Each task gives a load to the CPU that equals the task execution time divided by the task. The task execution time when a character is received must be less than 172us. Maximum load of the CPU is 1.

ii) The CPU load estimation in case of multitasking is as follows-

iii) Suppose there are M tasks. For multiple tasks, the sum of CPU loads for all the tasks and ISRs should be less than 1. The time outs and fixed time limit definitions for all task and ISRs should be less than 1.

iv) The time outs and fixed time limit definitions for tasks reduce the CPU load for higher priority tasks so that even the lower priority tasks can run before the deadlines. When task needs to run only once, then it is aperiodic in an application.

v) Scheduling of the tasks that need to run periodically with the fixed periods can be periodic and can be done with a CPU load very close to 1.

vi) When a task cannot be scheduled at fixed periods, it's schedule is called sporadic.

vii) A preemptive scheduler must take into account three Types of tasks,

• An aperiodic task needs to be preempted only once.
• A periodic task needs to be preempted after the fixed periods and it must be executed before its next preemption is needed.
• A sporadic task needs to be checked for preemption after a minimum time period of its occurrence.

3. Sporadic task model performance metric:

Consider following parameters