Classification of RTS. RTS Definitions. RTS Definitions

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1 Classification of RTS Dr. Hugh Melvin, Dept. of IT, NUI,G 1 Precise definition of RTS? Difficult due to the extent and scope of RTS System where a substantial fraction of the design effort goes into making sure that deadlines are met (Krishna/Shin) Response Time is an important parameter: The time between the presentation of a set of inputs to a system (stimulus) and the realisation of the required behaviour (response) including the availability of all associated outputs, is called the response time of the system (Laplante) Dr. Hugh Melvin, Dept. of IT, NUI,G 2 Alternative definitions System that must satisfy explicit (bounded) responsetime constraints or risk severe consequences, including failure (Laplante) System whose logical correctness is based on both the correctness of the outputs and their timeliness (Laplante) Failed System A system that cannot satisfy one or more of the requirements stipulated in the formal system specification (Laplante) Applies mostly to Hard RTS Dr. Hugh Melvin, Dept. of IT, NUI,G 3 1

2 Failed Systems RTS software rather than hardware usually at fault Emergence of Software Engineering Attempts by US DoD (SEI) to improve on the dismal record of software in military systems Has extended to general software industry All practical system are RTS? Time constraints are always bounded? Need for Classification of RTS Hard Firm Soft Differ by the consequences of missed deadlines Dr. Hugh Melvin, Dept. of IT, NUI,G 4 A Hard RTS is one in which failure to meet a single deadline may lead to complete and catastrophic system failure (Laplante) Eg. Power Plant Turbine Overspeed Protection Fuel Shutdown Flight Controller ABS (Antilock Braking System) Dr. Hugh Melvin, Dept. of IT, NUI,G 5 Dr. Hugh Melvin, Dept. of IT, NUI,G 6 2

3 12:45:01 23:01:97 Boiler Turbine Hard RTS VDU 1 BMS R30 Binary Field Inputs Binary Outputs Analogue Inputs Analogue Outputs AS101 AS112 Binary Field Inputs Binary Outputs Analogue Inputs Analogue Outputs AS013 AS017 Binary Field Inputs Binary Outputs Analogue Inputs Analogue Outputs AS131 AS133 VDU 2 VDU S\E s Office VDU Prog. Room Operators Keyboard 1 Operators Keyboard 2 S\E s Keyboard 1 Programmers Keyboard HW Analog Inputs HW Binary Inputs Annunciator Bus Coupler Tape Drive Storage Unit MEC-RT Data Storage Unit Bus 1 CS 275 Bus Bus 0 Printer 1 Printer 2 Printer Prog. Room Master Clock VDU 1 VDU 1 Local Printer VDU 1 VDU 2 5 1/4 FDD Storage Unit Alarms Printer Control Room VDU 2 VDU 3 Operator s Keyboard Fault Log Printer VDU 2 VDU 3 Operator s Keyboard Control Room LPA0 Local Terminals PG750 Maintenance Computer Operator s Annunciator Keyboard AS231 Unit Alarms System S\E s VDU & Keyboard MEC-RT Data Storage Unit Programming Room Printer MEC-RT Turbine Data Storage Unit Boiler OS254 Operations Programmers VDU & Keyboard Computer LPA1 VAX Maintenance Management System Dr. Hugh Teleperm Melvin, ME Dept. System of IT, Overview NUI,G 7 Unit 1 Unit 2 Unit 3 Bus 1 Bus 0 Bus 1 Bus 0 Bus 1 Bus 0 Bus A Bus Bus A Bus Bus A Bus Bus B Coupler Bus B Coupler Bus B Coupler Synogate Interface Synogate Interface Synogate Interface to to to Remote Operator OIS Server Station IBM NetFinity 5510 MP001 Gateway R30 Room Gateway Communications Room Station LAN MP002 Control Room Unit 1 Operator Station Control Room Unit 2 Control Room Unit 3 Operator Station Operator Station Dr. Hugh Melvin, Dept. of IT, NUI,G 8 Burner Management System Bus A Bus B N8 N Non-Redundant Control Circuits 2 out of 3 Double Redundant Safety Circuits Triple Redundant Safety Circuits Dr. Hugh Melvin, Dept. of IT, NUI,G 9 3

4 Hard RTS Emergence of Fly-by-wire control systems Military Aircraft Year Aircraft %Fns supported by S/W 1960 F F F (Source: W.S Humphreys Winning i with S/W, An Exec. Stgy, Add-Wes. 2002) Civilian aircraft similar Also Robotics Medical Devices ABS / Airbag Protection Many Hard RTS are embedded devices with limited and very specialised software designed for specific hardware Dr. Hugh Melvin, Dept. of IT, NUI,G 10 A Soft RTS is one in which performance is degraded but not destroyed by failure to meet response time constraints (Laplante) Eg. PC Designed for timesharing and multitasking Complex timesharing scheduler Good average performance acceptable Other consumer devices Camera, mobile phone IP-based Multimedia VoIP, IPTV, Gaming, Messaging Dr. Hugh Melvin, Dept. of IT, NUI,G 11 Soft RTS: Multimedia? Dr. Hugh Melvin, Dept. of IT, NUI,G 12 4

5 Soft RTS : VoIP Dr. Hugh Melvin, Dept. of IT, NUI,G 13 Soft RTS Private IP Network governed by SLA (Service Level Agreement) SLA specifies jitter/delay/loss/availability % Availability Lack of adherence results in : Irate customers loss of business Penalties imposed on provider Reservation Systems Dr. Hugh Melvin, Dept. of IT, NUI,G 14 A Firm RTS is one in which a few missed deadlines will not lead to total failure, but missing more than a few may lead to complete and catastrophic system failure failure (Laplante) Difficult to distinguish from Soft RTS Will stick with 2 levels Dr. Hugh Melvin, Dept. of IT, NUI,G 15 5

6 Sample Rate and Response Time Sample rate refers to the rate at which a parameter is monitored Sample rate = (Sample interval time) Hz sample interval time = 10 msec Parameter must be monitored frequently enough so that changes to the system can be detected and acted upon within overall RTS reqds Once change is detected the response time must be sufficient to ensure correct/safe system operation Sample rate and Response time are both related to the underlying physical phenomena Tuning of system required Dr. Hugh Melvin, Dept. of IT, NUI,G 16 Sample Rate and Response Time Related to Underlying Physical Phenomenon Eg. Power System control / protection systems for Steam Pressure / Turbine Speed / Overvoltage 3 phenomena have very different natural characteristics Require very different Sample Times and Response Times to react safely to changing/dangerous conditions Eg. Flight Control / Car ABS Sample Rate and Response Times depend on Maximum velocity mph = relative velocity of 1200 mph» 1760 feet/sec (Min Vertical flight separation = 1000 ft = 0.6 sec) 2 cars at 120 kph = 66 m/sec.. Response time of msec reqd Dr. Hugh Melvin, Dept. of IT, NUI,G 17 Dr. Hugh Melvin, Dept. of IT, NUI,G 18 6

7 Sample Rate and Response Time Sample Rate ADC used for sampling No point in sampling more frequently than necessary Wasteful of /Memory Hard RTS Response time must be guaranteed When Aircraft Traffic Collision Avoidance Systems (TCAS) are engaged» Recall Tupolev TU154M / Boeing collision When a Power station at full output is suddenly disconnected from the National Grid When ABS is required to work Dr. Hugh Melvin, Dept. of IT, NUI,G 19 Analog Inputs via ADC Modules DAC output not shown 2 Wire Transmitter 4 20 ma 2 Wire Transmitter 0 10V Spare ASE 6DS1714-8AA Ch bar Ch rpm Ch 3 spare Ch 4 4 Wire Transmitter 0 20 ma Voltage 0 % 6600 Dr. Hugh Melvin, Dept. of IT, NUI,G 20 Other Event Any occurrence that causes the Program Counter (PC) to change nonsequentially i.e. a change of flow-of-control (Laplante) Synchronous v Asynchronous Events Synch: occur at predictable times in the flow on control eg. decision structure Asynch: occur at unpredictable times Periodic v Aperiodic Events Periodic: occur at regular intervals eg. timeslicing timer Aperiodic: occur at irregular events eg. interrupt (Sporadic = Very infrequent Aperiodic Event) Dr. Hugh Melvin, Dept. of IT, NUI,G 21 7

8 Synchronous + Periodic Cyclic Code eg. AS msec cycle Synchronous + Aperiodic Typical Branch Instruction Asynchronous + Periodic Clock Generated interrupt: Timeslicing QNX ticksize utility Asynchronous + Aperiodic Non-fixed period interrupt Dr. Hugh Melvin, Dept. of IT, NUI,G 22 Deterministic System System that for each possible state and set of inputs, a unique set of outputs and the next state of the system are known Importance of thorough system specification and testing 2OI 2 nd Order Ignorance- You cannot test for things you don t know you don t know Temporal Determinism Response Time is also known Critical and extra reqd for RTS Dr. Hugh Melvin, Dept. of IT, NUI,G 23 Utilisation (U) Measure of the percentage of non-idle processing 70% is useful rule of thumb based on scheduling theory (cf later) Consider task 1 n where task Ti has period p i,freq of 1/ p i and worst case execution time e i e i can be very difficult to quantify Utilisation factor (worst case) for task i ; u i = e i / p i n u i i 1 Overall U= Dr. Hugh Melvin, Dept. of IT, NUI,G 24 8

9 Task Characteristics T i p i d i e i r i Φi D i Dr. Hugh Melvin, Dept. of IT, NUI,G 25 RTS Components Hardware Specific to application Tend to be intensive Power Systems Fly-by-wire MM: VoIP Software Specific to Application Assembly language / C widely used Interface directly with hardware RT Programming Languages EG. Siemens OEM Programming Language Ada is most recognised Real Time Language Dr. Hugh Melvin, Dept. of IT, NUI,G 26 RTS Components RTOS Many Hard RTS do not have an OS as such Cyclic Executive approach used.. Software runs bare OS introduces too much complexity OS not needed if embedded device is simple enough or can be very well defined Soft-Firm RTS often utilise RTOS Facilitates multiple concurrent processes Requires Scheduler Process priorities ensure determinism Memory Management Eg. Mobile Devices / Network Switches Increasing use in Hard RTS : Mars Pathfinder Note: Conventional OS can be adequate for Soft RTS Dr. Hugh Melvin, Dept. of IT, NUI,G 27 9

10 Simple Control System Process Field Inputs / Outputs Conveyor Belt Taco-generator Drive Unit E Y Signal Conditioning Process Control Processor (incl ADC/DAC) Thyristor Drive Unit Speed Setpoint Belt Running Drive Temp High Operator Interface Dr. Hugh Melvin, Dept. of IT, NUI,G 28 Less simple control System : Boeing 777 Dr. Hugh Melvin, Dept. of IT, NUI,G 29 RTS: Closing Remarks RTS more concerned with predictability rather than absolute response times Providing faster processors will convert a PC (Soft RTS) to a faster PC (Soft RTS), not a Hard RTS. Many Hard RTS are embedded devices Specific hardware Customised and limited software Simplified OS/ RTOS..or No OS Guarantees are provided through simplicity, precise definition, good design and overprovisioning Dr. Hugh Melvin, Dept. of IT, NUI,G 30 10