Tasks

Tasks

PeriodicThe period is the amount of time between each iteration of a regularly repeated task

Non PeriodicRespond to randomly arriving events .

Time drivenThe task is automatically activated by the kernel

at regular intervals.

Event drivenThe task is activated upon the arrival of an event

or through an explicit activation.

Tasks

• Non periodic tasks can be converted to periodic tasks– Converting an interrupt handler to a

polling task. – Instead of reacting to an external event

the moment it occurs, the system polls the external input regularly.

– If the awaited event is detected, the appropriated action is performed.

Tasks

Tasks

HardMissing a deadline may cause catastrophic

effects on the controlled system.

SoftMissing a deadline only causes a performance

degradation.

• Sensory acquisition• low-level control• sensory-motor planning

• Reading data from the keyboard• User command interpretation• Message displaying• Graphical activities

Schedule

• A schedule is a particular assignment of tasks to the processor.

τ3τ2τ1Idle Idle

t1 t2 t3 t4

t

Schedule algorithm

Schedule algorithm

PreemptiveThe running task can be

temporarily suspended in the ready queue toexecute a more important task.

Non PreemptiveThe running task cannot besuspended until completion.

Schedule Preemptive

t1 t2 t3 t4t

τ1

τ2

τ3

t5

Schedule Algorithms Constraints

• Timing constraints– Activation, completion, jitter.

• Precedence constraints– They impose an ordering in the

execution.

• Resource constraints– They enforce a synchronization in the

access of mutually exclusive resources.

Some Schedule Algorithms

• First Come First Served• Shortest Job First• Priority Scheduling• Round Robin Not suitable for Real Time!!!!!

First Come First Served

• It assigns the CPU to tasks based on their arrival times.– Very unpredictable– Response times strongly depend on task

arrivals.

Shortest Job First

• It selects the task with the shortest computation time.

Shortest Job First

• Sometimes…is not feasible

Priority Scheduling

• Each task is assigned a priority: e.g. [0, 255]- The task with the highest priority is selected for execution.

• - Tasks with the same priority are served FCFS.

• Problem: starvation – low priority tasks may experience long

delays due to the preemption of high priority tasks.

Round Robin

• The ready queue is served as FCFS• Each task cannot execute more than

Q time units (Q = time quantum).• When Q expires, the task is put back

in the queue.

CPUτ3τ2τ1

Activation

Q expired

Round Robin

• Each task runs as it was executing alone on a virtual processor n times slower than the real one!!!

Q

nQ

Real Time Algorithms

– Tasks can be scheduled by • Relative deadlines Di (static)• Absolute deadlines di (dynamic)

t

di

τi

Di

Earliest Due Date (EDD)

– It selects the task with the earliest relative deadline. • All tasks arrive simultaneously• Fixed priority (Di is known in advance)• Preemption is not an issue• It minimizes the maximum lateness (Lmax)

Earliest Deadline First (EDF)

– It selects the task with the earliest absolute deadline • Tasks may arrive at any time• Dynamic priority (di depends on arrival)• Full preemptive tasks• It minimizes the maximum lateness (Lmax)

Earliest Deadline First Example

Periodic Tasks Scheduling

– Defining the problem• Each group of tasks has to start after the

period defined. • Each group of tasks has to finish before the

next period come.

Cyclic Scheduling

It has been used for 30 years in military systems, navigation, and monitoring systems.

Examples:– Air traffic control– Space Shuttle– Boeing 777

Cyclic Scheduling Method

– The time axis is divided in intervals of equal length (time slots).

– Each task is statically allocated in a slot in order to meet the desired request rate.

– The execution in each slot is activated by a timer.

Cyclic Scheduling Example

• Specification :– Task A f=40 Hz, T=25 ms– Task B f=20 Hz, T=50 ms– Task C f=10 Hz, T=100 ms

0 ms 25 ms 75 ms50 ms 100 ms

125 ms

175 ms

150 ms

225 ms

200 ms

Λ

Λ Minor Cycle

T

T Major Cycle

Assumptions guaranteed!!!:

CA + CB ≤ Λ

CA + Cc ≤ Λ