courseware Basics of Real-Time Scheduling Jan Madsen Informatics and Mathematical Modelling Technical University of Denmark Richard Petersens Plads, Building 321 DK2800 Lyngby, Denmark

SoC-MOBINET coursewareJan Madsen2 WAP application Need structure and to perform multiple processes - send requests to server - translate data received from server - render data on display - operating the user’s control panel - logically different operations - performed at different rates Tasks: Why?

SoC-MOBINET coursewareJan Madsen3 Why use multiple processes? Processes are a neat way to handle complex systems Processes introduce structure into the system design –understand each process independently –analyze communication between processes to determine system behavior Timing is easier to specify Communication between processes provide test points Suitable for team effort

SoC-MOBINET coursewareJan Madsen4 What is a process?  A process is a state and a procedure  State:  Running  Suspended  Deactivated  Procedure:  Sequence of separate steps  A process may interact with its surroundings

SoC-MOBINET coursewareJan Madsen5 Process execution deadline activation period

SoC-MOBINET coursewareJan Madsen6 Process execution Periodic –the period is the time between successive executions –the process rate is the inverse of its period Aperiodic –non-periodic process, typical an event handler Sporadic –hard real-time aperiodic process –usually, a minimum interarrival constraint is imposed

SoC-MOBINET coursewareJan Madsen7 Example: A software processes  A process includes:  code  memory which belongs to the process  connections to other processes  A process is a unique execution of a program  In most embedded systems, processes never die!

SoC-MOBINET coursewareJan Madsen8 CPU Memory Executing on a single CPU P 1 code & data P 2 code & data P 3 code & data P 1 state P 2 state P 3 state PC registers

SoC-MOBINET coursewareJan Madsen9 Execution of multiple processes On a single CPU –A Pentium processor On a distributed homogeneous system –A network of Pentium processors On a distributed heterogeneous system –A network of differing processors (CPUs and ASICs)

SoC-MOBINET coursewareJan Madsen10 About this lecture  Huge literature on scheduling theory  Aim is to give an overview of problems and possible solutions to the multi-task scheduling problem related to RTOS  Structure of lecture:  Some basic concepts and teminology  Uni-processor scheduling  Rate-monotonic scheduling  Earlies-deadlines-first scheduling  Multi-processor scheduling

SoC-MOBINET coursewareJan Madsen11 PART I Scheduling Terminology and Basic Concepts

SoC-MOBINET coursewareJan Madsen12 Design flow P1P1 P2P2 P3P3 Partitioning/clustering Break processes to increase parallelism Cluster processes to reduce communication Allocation PE 1 PE 2 Mapping Scheduling Communication P1P1 P3P3 P2P2 PE 1 PE 2 P1P1 P 2 & P 3

SoC-MOBINET coursewareJan Madsen13 Scheduling abc 1 2 os 3 4 scheduling abc

SoC-MOBINET coursewareJan Madsen14 Terminology Application Architecture constraints Scheduler Schedule Architecture Application Scheduling Constraints

SoC-MOBINET coursewareJan Madsen15 Architecture  Components  Processors  CPU, DSP, ASIC,...  Communication  Bus, network,...  Devices  sensor, actuator, display,...  Architecture may be fixed or (re-)configurable mem device abc os

SoC-MOBINET coursewareJan Madsen16 Application  Task  A module which can be envoked to perform a particular function  A schedulable entity  Characterized by its:  Timing constraints  Precedence contraints  Exclusion constraints  Resource requirements

SoC-MOBINET coursewareJan Madsen17 Timing constraints r1r1 r 1 = time at which task becomes released (or active) e1e1 e 1 = worst case execution time (WCET) d 1 = deadline, task should complete before this! d1d1 s1s1 s 1 = time at which task starts its execution T1T1 T 1 = period, minimum time between task releases 1

SoC-MOBINET coursewareJan Madsen18 Task execution  Periodic  the period is the time between successive executions  Aperiodic  non-periodic task, typical an event handler  Sporadic  hard real-time aperiodic task  typical, a minimum interarrival constraint is imposed

SoC-MOBINET coursewareJan Madsen19 Precedence constraints  p i precede p j if p i must finish before p j begins  process interrelate with each other to achieve:  synchronization  exchange data pipi pjpj pipi pjpj blocked

SoC-MOBINET coursewareJan Madsen20 Exclusion constraints  p i exclude p j if no execution of p j is allowed between the time that p i starts its computation and the time that p i complete its computation  Prevent simultaneous access to shared resources  data  memory  devices pipi pjpj pipi pjpj p j blocked

SoC-MOBINET coursewareJan Madsen21 Resource constraints  R = r 1, r 2, …, r m  Example:  R(p i ) = {r 1, r 3 } resource r 1 and r 3 is required by p i during its execution  may be used to  implement a critical section  enforce PE assignment of processes

SoC-MOBINET coursewareJan Madsen22 Scheduling approaches  Classical scheduling theory  Uniprocessor systems  RMS  EDF  Distributed real-time scheduling  Deterministic (static) scheduling  Dynamic scheduling

SoC-MOBINET coursewareJan Madsen23 Scheduling  Allocation  Determine number and type of processors/resources  Assignment  Binding tasks to processors  Scheduling  Determine execution order a b b a Task view 1 2 Architecture view a b

SoC-MOBINET coursewareJan Madsen24 Scheduling principles  Off-line  A scheduler performing its job before the scheduled system is put into operation  On-line  A scheduler performing its job at run-time, when the system is running  Typically list-based  When a task is released (ready) it is placed in a list  Scheduler select which task from the list to execute next  Selection based on some criteria – scheduling policy

SoC-MOBINET coursewareJan Madsen25 Scheduling areas  High-level Synthesis:  fine grained, operation/instruction level  schedule done off-line  single time constraint  Real-time operating systems (RTOS):  coarse grained, programs/sub-programs  scheduling done on-line  multiple time constraints

SoC-MOBINET coursewareJan Madsen26 SoC scheduling  Similarity to RTOS  Time constraints  Context switching  Task synchronization  Task communication  Difference to RTOS  Large design space  Variable granularity  Wide range of metrics  Time, power, cost,...  Flexibility, reliability,...  High degree of optimization  Dedicated hardware

SoC-MOBINET coursewareJan Madsen27  Non-preemptive scheduling  Preemptive scheduling Preemption vs. Non-preemption

SoC-MOBINET coursewareJan Madsen28 Metrics  Minimize:  c i  w c i i }{ max de L ii i   lateness, tardiness Schedule length p1p1 p2p2 p3p3 p4p4 p5p5 p1p1 p2p2 p3p3 p4p4 p5p5 d1d1 d2d2 d3d3 d4d4 d5d5