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What is the Scheduler?

The "task scheduler" (or often "scheduler") is the part of the software that schedules which task to run next. The scheduler is the part of the software that chooses which task to run next.

The scheduler is arguably the most difficult component of an RTOS to implement. Schedulers maintain a table of the current state of each task on the system, as well as the current priority of each task. The scheduler needs to manage the timer too.

In general, there are 3 states that a task can be in:

   1. Active. There can be only 1 active thread on a given processor at a time.
   2. Ready. This task is ready to execute, but is not currently executing.
   3. Blocked. This task is currently waiting on a lock or a critical section to become free.

Some systems even allow for other states:

   1. Sleeping. The task has voluntarily given up control for a certain period of time.
   2. Low-Priority. This task only runs when all other tasks are blocked or sleeping.

There are 2 ways the scheduler is called:

    * the current task voluntarily yield()s to the scheduler, calling the scheduler directly, or
    * the current task has run "long enough", the timer hardware interrupts it, and the timer interrupt routine calls the scheduler.

The scheduler must save the current status of the current task (save the contents of all registers to a specified location), it must look through the list of tasks to find the highest priority task in the Ready state, and then must switch control back to that task (by restoring it's register values from memory).

The scheduler should first check to ensure that it is enabled. If the scheduler is disabled, it shouldn't preempt the current thread. This can be accomplished by checking a current global flag value. Some functions will want to disable the scheduler, so this flag should be accessable by some accessor method. An alternate method to maintaining a global flag is simply to say that any function that wants to disable the scheduler can simply disable the timer. This way the scheduler never gets called, and never has to check any flags.

The scheduler is generally disabled inside a critical section, where we do not want to OS to preempt the current thread. Otherwise, the scheduler should remain active.

Scheduling Policies - Video: