[RTOS Fundamentals]

The scheduler is the part of the kernel responsible for deciding which task should be executing at any particular time. The kernel can suspend and later resume a task many times during the task lifetime.

The scheduling policy is the algorithm used by the scheduler to decide which task to execute at any point in time. The policy of a (non real time) multi user system will most likely allow each task a “fair” proportion of processor time. The policy used in real time / embedded systems is described later.

In addition to being suspended involuntarily by the kernel a task can choose to suspend itself. It will do this if it either wants to delay (sleep) for a fixed period, or wait (block) for a resource to become available (eg a serial port) or an event to occur (eg a key press). A blocked or sleeping task is not able to execute, and will not be allocated any processing time.


Referring to the numbers in the diagram above:

  • At (1) task 1 is executing.
  • At (2) the kernel suspends (swaps out) task 1 …
  • … and at (3) resumes task 2.
  • While task 2 is executing (4), it locks a processor peripheral for its own exclusive access.
  • At (5) the kernel suspends task 2 …
  • … and at (6) resumes task 3.
  • Task 3 tries to access the same processor peripheral, finding it locked task 3 cannot continue so suspends itself at (7).
  • At (8) the kernel resumes task 1.
  • Etc.
  • The next time task 2 is executing (9) it finishes with the processor peripheral and unlocks it.
  • The next time task 3 is executing (10) it finds it can now access the processor peripheral and this time executes until suspended by the kernel.

Next: RTOS Fundamentals – Context Switching

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