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How does the scheduler work?

Scheduling is based upon AmigaOS(tm)'s round-robin scheme and provisions for multitasking. it is implemented using the following public Exec functions:

  • Dispatch(); // dispatch next available task.
  • Schedule(); // check if the current task should be rescheduled.
  • Reschedule(); // give up the current tasks remaining CPU time to other ready tasks, if there are any.
  • Switch(); // switch away from the current task.
  • Wait(); // make a task wait for incoming signals
  • Signal(); // signal a waiting task

These functions on their own provide a co-operative multi tasking system. The addition of an interrupt drives the full pre-emptive multitasking, forcing a task switch when the currently running task has used its time slice (quantum).

AROS implementation details

System startup

When the system is first brought up, there are no interrupts and the system is usually in a supervisor state. The kernel will first initialize the platform base hardware and processor, as well as interrupt controller - jumping into a user state and starting up exec.

During execs initialization it will prepare a "bootstrap" task structure and add a vblank interrupt handler to drive pre-emption, which it does by setting the quantum-expired and switch scheduling flags.

Exec will then initialize all the SINGLETASK resident modules. One of the first residents to initialize is execs late initialization - which is run after platform specific modules that may probe and initialize addition processor cores and timesources, etc., for the system to utilize. On SMP platforms the scheduler is started on additional cores and set to run an idle task.

After SINGLETASK residents are initialized, DOS is brought up which starts real boot process. DOS will launch a new process to run the disk boot sequence, and let the bootstrap task die, triggering the scheduler to begin.

Scheduling

Periodically the system will trigger an interrupt that results in Schedule being called to see if another task should run. If there is a readytask with the same or higher priority, and the current task has no pending exception and has used its time quantum schedule will signify the task should be switched out.

Switch is next called to put the currently running task into a ready state. Finally Dispatch is called to select the next suitable task and launch it.

Tasks waiting for input from other events, call Wait() and are put into a waiting task list. When a triggering event for them occurs, they have the appropriate signal bits set via Signal() and are moved into the ready list, so that the schedular can allow them to process the events when it is next time to run.

Caveats

Under AmigaOS the Exec fields ThisTask, Quantum, Elapsed, IDNestCnt and TDNestCnt are used to store the scheduling information used by Exec. AROS also supports SMP based systems, which have per-processor core versions of these fields so the exec versions are deprecated [1] and unavailable on those architectures. It is advised to use FindTask(NULL); to determine the current task in all cases to ensure code compiles for all AROS targets and gets the correct value. Other scheduling values should not be accessed.

[1]ThisTask does exist on binary compatible AROS targets and can be accessed as normal, however such code is not portable to all AROS targets.

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