The ppt contains detail about issues and scheduling technique of real-time systems. It includes scheduling both online and offline for uniprocessor system. The applications of real-time system is also there

1. Real Time System
Akansh Singhal
1402931009

2. Definition
• A real-time system is a type of hardware or software that operates with a time
constraint.
• The correctness of real-time system depends not only on logical results but also
on time.
• If real time system exceeds the time bonds it results in performance degradation
and/or malfunction of system.
• There are two types of real time system:
o Hard Real Time System
o Soft Real Time System
• Most of the embedded system are real time system.
• Example: Aircraft Traffic control, Boilers, Digital Controller, Telecommunication.

3. A/D
A/D
Sensor
Control Flow
Computation
Plant
D/A
Actuator
R(k)
Y(k)
R(t)
Y(t)
Y(k)
Digital Controller
Block Diagram
R(t) Reference Input

4. Contd…
E(t)=R(t)-Y(t)
Where R(t) is desired state.
Y(t) is measured state.
E(t) is error variable.
double PID(double In)
{
Out=P_gain*In_Igain*Insum+D_gain*InDiff;
Insum+=In;
InDiff=In-Inold;
Inold=In;
return out;
}
Implementation of Digital controller in C
A Digital Controller:
• Obtain I/P values from sensors.
• Can store state variables.
• Dynamic Parameters are known.
Formula:

5. Hard Real time system vs Soft Real Time System
S.No Parameter Hard Real Time System Soft Real Time System
1 Safety Critical Non Critical
2 Load Performance Predictable Degraded
3 File Size Small/Medium Large
4 Data Integrity Short Term Long Term
5 Redundancy Time Hard Required Soft Required
6 Error Detection Autonomus User assisted
7 Example Air Traffic Control Telecom

6. Issues in Real Time Computing
• Architectural Issues:-
 Processor Issues
 Network Issues
 Architecture for clock synchronization
 Reliability Evaluation
• Operating System Issues:-
 Task Assignment and Scheduling
 Communication Protocol
 Failure Management and Recovery
 Clock Synchronization
• Other Issues:-
 Programming Language Issues
 Database
 Performance Measures
Applications of Real Time
System
• Metal Industry
• Aviation
• Water Plant
• Petro Chemical
Air Traffic Control

7. Terminologies Related
• Task: Unit of Computations a processor executes.
• Job: Any instance of task is called of job. Its functionality is defined in task.
• Jitter Time: The task release in a range of time from {r-,r+}
• Release Time: It is instant of time at which job become available for execution.
• Deadline: It is instant of time by which its execution is required to be complete.
• Sporadic Release time: The job is released at random time for execution.
• Phase: Shifted release time of first job in task Ѱ
• Feasibility Interval: It is interval between release time and deadline {r,D}.
TasJ1.1 J1.2 J1.3 J1.4 J1.5 ------------ J1.nTask1

8. Task States
Suspended
Ready
Blocked
Running
Suspend Called
Suspended called
Scheduled
Block
Suspend Called

9. Schedulers
• A scheduler schedules job in their order of job execution.
• The components of schedulers are Heap, Timer, Task Memory.
Heap
Task Memory Task Memory
Timer
• The timer expiries at scheduling point. Job from task T is then scheduled from task T at time t.
Scheduler
Proficient
Equally
proficient
Optimal

10. Scheduling
Tasks FIFO QUEUE Execute
U=
𝒆𝟏+𝒆𝟐−−−𝒆𝒏
(𝒑𝟏+𝒑𝟐−−−𝒑𝒏)
• Scheduling is the process to schedule the task which are ready for execution.
• Scheduling point is a point at which scheduler takes decision regarding
which task to be run next.
• The CPU Utilization can be calculated as:-

11. Types of Scheduling in RTOS
Scheduling
Static or Offline
Clock Driven Cyclic
Dynamic or Online
Earliest deadline First
Or
Dynamic Priority
Algorithm
Rate Monotonic
or
Static Priority
Algorithm

12. Clock Driven Scheduling
• The job list as well as which task to be run for how long may be stored in a table.
• For scheduling n periodic tasks the scheduler develops a permanent schedule for
a period LCM(p1,p2,p3…..pn).
• Each time we have to set timer.
Task Start Time Stop time
T1 0 100
T2 101 150
T3 151 225
Pro: Time efficient, Simple and flexible.
Con: Can not handle sporadic or aperiodic task.

13. Cyclic Scheduler
• This scheduler deals with the major and minor cycle (frame).
• Repeat a precomputed cycle.
• A major cycle is divided into minor cycles called frame
• Major cycle=LCM(p1,p2,p3----pn) can be greater than LCM if F do not divide it evenly.
Major Cycle Major Cycle
Frames
𝟐 ∗ 𝐟 − 𝐠𝐜𝐝 𝒑𝒊, 𝒇 ≤ 𝑫𝒊
• The frame size should be greater than each task size 𝑓 ≥ max 𝑒𝑖 . This is for minimum context switch
• To minimize table size the frame size should squarely divide major cycle.
• The time constraint for each task based on deadline and release time is given with formula:-
Condition to choose frame size:-

14. Event Driven Scheduling:
• Can handle both sporadic and aperiodic task.
• Each task have some sort of priority that may be fixed or assigned at run time.
Earliest Deadline First:
• The Task priorities are assigned dynamically while execution.
• This scheduling is optimal but not used commercially.
• A task with reaching deadline has higher priority than the other tasks.
• This scheduling is best in uniprocessor, independent and non shareable
processors.
• The utilization U is less than 1.
• The major disadvantages are
o Runtime efficiency
o Resource sharing create problem
o Transient Overload Handling Problem

15. Gantt Chart for EDF

16. Rate Monotonic Algorithm
• The priority of task is proportional to frequency
• This is widely used algorithm and one of the optimal one
• The priorities in this scheduling are fixed
• The scheduling is preemptive one
• Task with shortest period get higher priority
Assumptions:-
• All task are periodic and not synchronize with other
• All process must have a release time equal to 0
Frequency
Priority
• It consider optimal in the sense that if set of processes can not be scheduled by this algorithm, It
can not be scheduled by any other scheduling algorithm
• Despite being optimal, RMA has bounded CPU utilization. The worst case CPU Utilization for
scheduling N processes is 2(2
1
𝑛 − 1).
• As number of process approaches infinity, CPU utilization fall approximately 69%

17. Thank You