İzmir Institute of Technology Embedded Systems Lab. Real-Time Systems. Asst. Prof. Dr. Tolga Ayav Department of Computer Engineering

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1 İzmir Institute of Technology Embedded Systems Lab Real-Time Systems Asst. Prof. Dr. Tolga Ayav Department of Computer Engineering

2 Agenda Real-Time Systems RT Scheduling RT Kernels RT-Linux RT Executives DACOS Study of Embedded Systems in COMPENG

3 Real-Time Real-Time system is defined as a system where the correctness of the system depends not only the result of computations but also on the time at which it is produced. Therefore the time is the most important item to be managed.

4 Various Definitions Hard, Soft, Firm Real-Time Embedded System Deterministic System Failed System Fault, Error, Failure Safety Critical System Fail-Safe System

5 Real-Time Application A real-time application is composed of a set of coordinated tasks: Periodic Tasks: started at regular intervals and has to be completed before some deadline A-periodic Tasks: appeared irregularly, but within a bounded frequency Sporadic Tasks: their parameters are completely unknown

6 Defining a RT Task We can use following quintuple to express a task: b: begin time c: execution time f: frequency of a periodic task d: deadline

7 Schedulability For schedulability, at least the following conditions must be met:

8 Task Categories According to their time criticality: Hard Real-Time Tasks Soft Real-Time Tasks Non Real-Time Tasks (Background Task)

9 Hard Deadline Task

10 Safety Critical Task

11 Soft Deadline Task

12 Task Scheduling Scheduling Dynamic Scheduling (Online) Static Scheduling (Offline) Rate-Monotonic (Static Priority Driven Preemptive) Algorithm Architecture Adequation Earliest-Deadline-First (Dynamic Priority Driven Preemptive)

13 Rate-Monotonic Scheduling (1) Assumptions: Simple task model: No interprocess communication and all tasks are periodic Tasks have priorities which are inversly proportional to their periods. Tasks deadlines are equal to their periods. A high priority task may preempt lower priority tasks. Liu and Layland (1973) proved that for a set of n periodic tasks with unique periods, a feasible schedule that will always meet deadlines exists if the CPU utilization is:

14 Rate-Monotonic Scheduling (2) When number of tasks approaches to infinity, this utilization bound will converge to: Example: Task Execution Time τ1 1 8 τ2 2 5 τ Perio d Thus, the system is schedulable

15 Earliest-Deadline-First Alg. Priorities are changed dynamically Task with the earliest deadline gets the highest priority Unless RM, utilization may go up to 100%

16 Example Scheduling Three periodic tasks: A, B, C T is period, D is deadline and C is execution time uniprocessor

17 RTOS Operating system (OS) is a program that acts as an intermediary between a user and the computer hardware. The primary goal of an OS is to make the computer system convenient to use and the secondary goal is to use the hardware in efficient manner

18 RTOS Requirements Interrupt handling, guaranteed interrupt response Task (process) management (Support for scheduling of realtime tasks and preemptive scheduling) Interprocess communication and synchronization. Time management. Memory management I/O support (Support for communication with peripheral devices via drivers) High speed data acquisition Resource management (User control of system resources) Error and exception handling

19 RTOS Vendors OS name Vendor OS name Vendor Chorus Chorus VxWorks WindRiver ECOS Cygnus Windows CE Microsoft EPOC32 Symbian MicroC/OS White Horse Design OSE Enea AMX Kadak JavaOS Sun Microsystems Ariel Micoware LynxOS Lynx ARTOS Locamation NucleusPlus Acc. Tech ASP6x DAN enterprise OS/9 Microware Chimera Robotics PalmOS 3com CMX CMX company PSOS+ ISI Diamond 3L QNX QNX DOS6-XL General Software

20 Linux Architecture The standard LINUX kernel isn't a real-time kernel Some patches should be applied to fulfill the realtime requirements User s task Drivers Carte Pulse System libraries LINUX kernel I/O Hardware interrupts HARDWARE

21 RT-Linux Projects RT-Linux is an operating system, in which a small real-time kernel co-exists with standard Linux kernel The real-time kernel sits between standard Linux kernel and the h/w. The standard Linux kernel sees this real-time layer as actual h/w The real-time kernel intercepts all hardware interrupts. Only for those RTLinux-related interrupts, the appropriate ISR is run. All other interrupts are held and passed to the standard Linux kernel as software interrupts when the standard Linux kernel runs. The real-time kernel assigns the lowest priority to the standard Linux kernel. Thus the realtime tasks will be executed in real-time Realtime tasks are privileged (that is, they have direct access to hardware), and they do NOT use virtual memory.

22 Real-Time Linux Architecture Hardware abstraction layer real-time interrupt dispatcher, real-time scheduler Inter processes communication services The patches available are: RTLinux, xenomai and RTAI (Real-Time Application Interface) User s task Real Time task Drivers System libraries Linux kernel I/O Real-time Scheduler I/O Software interrupts Real-time plugin Hardware interrupts HARDWARE

23 RTAI Block Description The software architecture of RTAI is made of: 1 I/F to Linux HW Management (HAL): basically a data structure. 3 basic components (dispatcher, scheduler, fifo's). 1 I/F (set of functions) used in user tasks to initialize and start the components. From a Linux point of view these entities populate modules.

24 Control flow in a RTAI/Linux system Virtual Interrupt Control using sti() cli()

25 RT-Linux Tasks (1) gcc c simple.c insmode simple.o rmmod simple.o

26 RT-Linux Tasks (2)

27 RTAI Project For further information, installation details and various documents, please refer to:

28 Real-Time Executives Simplified RTOS Not a real-time operating system itself Usually has task scheduling and time management functions May rely on other OSs, like DOS.

29 DACOS (Data Acquisition and Control Operating System) It s a real-time executive in fact. Runs on MS-DOS operating system. Most functions require a special Hardware: DACES Supports: Data Acquisition. Pipes, variables, processes. Periodic tasks.

30 DACOS Components custom.h : The definitions of the max numbers for tasks, pipes, variables etc. dacos.h : All prototypes for functions and data structures used by the real-time system. calls.h : The definitions for the system calls. defs.asm : Some definitions for the assembly functions. dacos.c : Almost all of the function implementations of the real-time executive. kernel.asm : The assembly language routines providing task switching which cannot be done well within C. main.c : Includes main function which does the initialization of the executive and shell functions for interacting with user. qcl -c -AL -Zi -Ox -Aw -Gs dacos.c qcl -c -AL -Zi -Ox -Aw -Gs main.c qcl -c -AL -Zi -Ox -Aw -Gs kernel.asm link /co /stack:16384 dacos.obj kernel.obj main.obj, dac.exe

31 DACOS Features Scheduling: Preemptive Round-Robin. Each task is given a fixed 500µs time slice. Interprocess communication: Pipes and variables. DAC.EXE runs on DOS. It reads load.dac system configuration file. The tasks are stored as procs/*.bin files. Use DACES hardware: 16 channel analog inputs, 16 digital inputs and 16 digital outputs. The executive DAC.EXE is only 127KB.

32 System Configuration File pipe.name[string].size[unsigned integer]. var.name[string].initial_value[integer] sample.channel_number[integer0-15].sampling_period [unsigned integer].pipe_name[string]. pro.name[string].parameter1.parameter2.parameter3 timer.period[unsigned int].name. parameter1. parameter2. parameter3

33 Use of Pipes for IPC

34 Sample LOAD.DAC

35 Sample Task: average.c

Chapter 19: Real-Time Systems. Operating System Concepts 8 th Edition,

Chapter 19: Real-Time Systems. Operating System Concepts 8 th Edition, Chapter 19: Real-Time Systems, Silberschatz, Galvin and Gagne 2009 Chapter 19: Real-Time Systems System Characteristics Features of Real-Time Systems Implementing Real-Time Operating Systems Real-Time