Post on 26-Mar-2015
Processing Order: Cooperative vs. Preemptive
Process B2Process B2
Process AProcess A
Process CProcess C
Process B1Process B1
Process DProcess D
Process A (low priority, cyclic)Process A (low priority, cyclic)
Process C (high priority, cyclic)Process C (high priority, cyclic)
Process B (low priority)Process B (low priority)
Process D (high priority)Process D (high priority)
OS
Preemptive MultitaskingCooperative Multitasking
Processes (tasks) are run in an infinite loop
If task takes too much time (B), need to divide it
No overhead occurs
Scheduling is performed by the OS Long task is suspended if needed Overhead: caused by OS internals
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Task States (ITRON Case)
Ready: could run but the CPU is busy
Executing: running on the CPU
Blocked: waits for something (I/O, signal, resource, etc.)
Dormant (hibernation): created task that is not started
NON-EXISTENT: deleted or not created task
The RTOS implements a Finite State Machine for each task and manages its transitions.
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Task State Transitions
Ready Executing
Dormant state
Task scheduled
Preemption
Wait state
Wait for the event
Task can be executed at any time
unlock waiting
resuming
waiting
suspend
Execution is suspended
Only one task running
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Preemption
１ ２ ３ Ａ
Ready to run Executing
Ａ １ ３ ２
１２
３ ＡTime
High Priority
ready-to-run state Executing
dormant state
dispatch
pre-emption
waiting state
A1 2 3
waiting
unlock waiting
suspend
resuming
Task is in the ready state Task is scheduled to run – running state
Priority of some task is changed to high
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Close
Example: the task runs until finishing the function
Execution of tasks to complete
TASK(…) { ………… ………… …………} Hibernation
Ready to run Executing
２ ３
Ａ
１
ready-to-run state Executing
dormant state
dispatch
pre-emption
waiting state
A1 2 3
waiting
unlock waiting
suspend
resuming
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Starting
Start task executing from the beginning.
Task was executing, then it was suspended
TASK(…) { ………… ………… …………}
Hibernation
Ready to run Executing
２ ３
Ａ１
ready-to-run state Executing
dormant state
Dispatch
pre-emption
waiting state
A1 2 3
waiting
unlock waiting
suspend
resuming
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Wait
Example: Wait for an event for 40 msec.
Running task is waiting for an event. When AN event rises, task gets ready for execution.
TASK(…) { ………… 40msec Wait …………}
Wait state
Ready to run Executing
２ ３
Ａ
１
ready-to-run state Executing
dormant state
Dispatch
pre-emption
waiting state
A1 2 3
waiting
unlock waiting
suspend
resuming
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Wait Release
Start execution of the task from the last point.
Waiting task becomes ready to run.
TASK(…) { ………… 40msec Wait …………}
Wait state
Ready to run Executing
２ ３
Ａ１
ready-to-run state Executing
dormant state
Dispatch
pre-emption
waiting state
A1 2 3
waiting
unlock waiting
suspend
resuming
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