5 Echtzeitsysteme

Question 1

Name the two requirements needed for real time.

Answer 1

• Computation is correct

- Computation finishes in time (before deadline)

Question 2

Was ist die Hard real time?

Answer 2

after a specific deadline, the results are not nuseful anymore

Question 3

Was ist die Soft real time?

Answer 3

the usefulness of the results decrease after a specific deadline

OR:
Statistically suffienciently high percentage is availabel before the deadline

Question 4

How many processes and resources are needed for Deadlock?

Answer 4

• 2 Processes
• 2 Resources

Example: Task A, B and Resources R1, R2
A occupies R1
B occupies R2
B requires R1
A requires R2
--> Deadlock

Question 5

How many processes and resources are needed for Priority Inversion?

Answer 5

• 3 Processes
• 1 Recource

Example: Tasks A, B, C and Recource R1
Prio(A) > Prio(B) > Prio(C)
C occupies R1
A starts, (pre-empting C)
A requires R1
B starts
B terminates (problem)
C continues, terminates
A occupies R1, terminates

Question 6

How to fulfill Real-Time requirements?

Answer 6

► Polling
► Main loop and interrupts
► Real-time operating system

Question 7

Definition Schedule:

Answer 7

Mapping of an execution sequence to a task system

Question 8

Definition Feasibility:

Answer 8

A schedule is feasible, if no deadline is violated.

Question 9

Definition Schedulability:

Answer 9

A task system is schedulable, if a feasible schedule exists.

Question 10

What is behind the “earliest deadline first scheduling”?

Answer 10

Execute the task with smallest D first.

Question 11

How can priority inversion be prevented?

Answer 11

• Priority Inheritance Protocol

- Priority Ceiling Protocol

Question 12

Priority Inheritance Protocol:

Answer 12

► Task A (high priority) needs a resource R
► Task B (low priority) currently holds R
► As soon as A starts waiting for R
▪ B inherits A’s priority
▪ B cannot be preempted by an intermediate priority
▪ No priority inversion possible
► Complex:
▪ Current holder of a resource must be determined
▪ Holder’s priority must be changeable while running

Question 13

Priority Ceiling Protocol

Answer 13

► Ceiling priorities are assigned to the resources
► A resources priority is the highest priority of all tasks that may use it
► A task is upgraded to the resource’s priority as soon
as the task holds it.
► Can be calculated during design time
► Not optimal: T2 is upgraded to R1’s priority even if T4 is inactive