Scheduling Basic Concepts Ref: Hard Real-Time Computing Systems Giorgio Buttazzo Processes - Tasks - Threads

Nostalgia Automobile Computers: 2006: 1-2 million lines of code 2010: 10 million lines of code Apollo Guidance Computer First Microprocessor Microcontroller 99%+ of today’s computers are in embedded systems

The Osborne I (1981) Hardware: Dual 5¼-inch floppy disk drivesfloppy disk 4 MHz Z80 CPUMHzZ80CPU 65 kilobytes main memorykilobytesmemory Fold-down keyboard doubling as the computer case's lidkeyboard 5-inch, 52 character × 24 line monochrome CRT displaymonochromeCRT Parallel printer portParallelprinter Serial port for use with external modems or serial printersSerial portmodems Software: CPM Word Star SuperCalc Dbase II CBasic First Portable Computer

Orientation Schedule: An assignment of tasks to the processor, so that each task is executed until completion Scheduling Policy: Tasks assigned according to a predefined criterion Scheduling Algorithm: The set of rules that determines the ordering of tasks Active Task  Ready Task  Running Task (Ready Queue) (Dispatching)

Ready Queue

Definition of Schedule What does it say?

Preemptive Schedules

Categories of Schedules

Deadlines Deadline: Absolute – with respect to real-time (time zero) Relative – with respect to arrival time

Real-Time Task Parameters

Periodic vs Aperiodic Tasks

Precedence Constraints

Precedence Example

Critical Sections Binary Semaphore

Critical Section Example

Critical Section Model

Definition of Scheduling Problems

Classification of Scheduling Algorithms

Guarantee-Based Algorithms

Domino Effect Degradation