Real-Time Systems. Real-Time Communication. Hermann Härtig, Jork Löser (following Kopetz, Liu, Almeida, Jon Currey, Schönberg)

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1 Real-Time Systems Real-Time Communication Hermann Härtig, Jork Löser (following Kopetz, Liu, Almeida, Jon Currey, Schönberg)

2 Contents General Concepts IO Busses: PCI Networks as schedulable resources: Priority / Time-Driven / Weighted Round Robin Field Busses: CAN/Profibus/TTP Traffic Shaping on Switched LANs Switched WANs: ATM Resource Reservation, Group Communication: RSVP Middlware: RT-Corba 1.0 Real-Time Systems, 2006/7 RTCommunication, 2 2

3 General Scenario routers back bones real time bus / field bus real-time computer real-time computer real-time computer field bus IO bus sensors SPS controller actuator NC Real-Time Systems, 2006/7 RTCommunication, 3 di sk controller field bus 3

4 Purposes of a Real-Time Network Deliver communication services to the requesting nodes reliably, securely, efficiently and timely lower bound for bandwidth upper bound for latency and jitter Real-Time Node A Real-Time Network Real-Time Node B Real-Time Systems, 2006/7 RTCommunication, 4 4

5 Typical requirements Short data in control applications Large data in media applications Short periods (ms) monitoring, feedback control Fast aperiodic (ms) alarms Non real-time data configuration, logs Multicast Predictability in the presence of faults Real-Time Systems, 2006/7 RTCommunication, 5 5

6 Intra-Node Communication: IO Bus (PCI) All data transfers share the memory bus Memory bus conflicts PCI devices start transfers when data is available CPU-Memory-accesses are influenced by PCI devices Applications are slowed down by external load Memory Slowdown factor F: (ET: execution time) ET under external load/et without any external load application-specific depends on memory access pattern obtained by measurements attempts to statically predict CPU memory bus PCI Real-Time Systems, 2006/7 RTCommunication, 6 6

7 Some Measurements Source: Dissertation Sebastian Schönberg (see our Web-Pages) Real-Time Systems, 2006/7 RTCommunication, 7 7

8 Intra-Node Communication: PCI Bus PCI Bus specification does not prevent real-time Arbitration responsible for granting bus to devices Current implementations use round-robin arbitration Telecom Systems PCI bus for control data TDMA bus for real-time data Alternative: real-time capable PCI bus arbiter Assign individual share of the PCI bus to devices Enforce reservation Give unused bandwidth to time-sharing devices Research topic References: Schönberg, Philips Real-Time Systems, 2006/7 RTCommunication, 8 8

9 Networks as Schedulable Resources analogous to CPU scheduling: time driven priority driven (Weighted Round Robin) Analogy: WCET: Amount of traffic / message transmission time Periodic tasks: periodic messages messages are usually non-preemptive!! Deadline: max delay Admission: connection establishment Schedulers in switches or distributed on nodes Real-Time Systems, 2006/7 RTCommunication, 9 9

10 Examples Fieldbusses: CAN, Token Ring: priority based scheduler TTP/Flexray: time driven scheduling Switches: Delay EDD Weighted Round Robin (neither priority nor time driven) Real-Time Systems, 2006/7 RTCommunication, 10 10

11 Intra-Node Communication: Field Busses field bus real-time computer sensors real-time computer SPS Situation is moving Smart sensors, actors Wireless lans real-time computer actuator real-time computer NC Real-Time Systems, 2006/7 RTCommunication, 11 11

12 Fieldbusses Networks for process control, factory automation, cars, avionics, X-by-wire, embedded applications Networks are typically called fieldbuses: collapsed network stacks (application services access the data link directly) real-time transport short application messages (no need for fragmentation and reassembling) often a bus, hence a single broadcast domain, no routing Real-Time Systems, 2006/7 RTCommunication, 12 12

13 CAN - Controller Area Network Bosch GmbH, Version 2.0 released in 1991 ISO Standards (94) and (95) used in automotive industry, process control, manufacturing automation, embedded application domains cost-effective CSMA/BA: carrier sense multiple access/bitwise arbitration Real-Time Systems, 2006/7 RTCommunication, 13 13

14 Bus Arbitration with CAN (Priority Based) CSMA/BA (carrier sense multiple access/bitwise arbitration) Bit-wise collision resolution using message identifiers, sent from HSB to LSB Wired "and", implemented with bus drivers 0 - Dominant level 1 - Recessive level bus Node stops if a 0 is seen when sending a 1 Real-Time Systems, 2006/7 RTCommunication, 14 14

15 CAN Message ID: 11 Control: 6 Data up to: 64 CRC: 16 Various: 11 Bit stuffing to allow synchronization at bit level after 4 '0's or 4 '1's: dummy 1 or 0 Transmission time of a message in bit-times: C = databits+ (34+databits-1)/4 Real-Time Systems, 2006/7 RTCommunication, 15 15

16 CAN - Performance Transmission rate from 5KBit/s to 1MBit/s Bit-synchronized bus access Bus length depends on transmission rate 40m with 1MBit/s, 1000 with 50KBit/s transmission rate increases with shorter busses... or with increased speed of light 2048 priorities (11 Bit) in version A, >500 Mio priorities (29 Bit) in version B Short messages (0-8 Byte) Efficiency depends on msg length: 0%..47% Real-Time Systems, 2006/7 RTCommunication, 16 16

17 Medium Access Control using a Token Ring Messages follow a logical ring Token required as permission to send (round robin scheduling) Node A (Token) sending Real-Time Network Node B - listening Node D - listening Node C - listening Real-Time Systems, 2006/7 RTCommunication, 17 17

18 Token Ring with Priorities (e.g. Profibus) 2 priority fields: Token Priority, Reservation Priority Token can be taken only, if priority field in token is lower than priority field in outgoing packet Reservation Priority in packet field is set to highest priority of waiting messages that are passed by packet Upon arrival at sender, the token priority is set to the packet priority reservation field Real-Time Systems, 2006/7 RTCommunication, 18 18

19 More on Tokens General medium access protocol Not necessarily a strict ring Master/Slave Table-based Functional Parameters Maximum token holding time at each node: THT Rings: resulting maximum token rotation time: TRT Problems: lost tokens duplicated tokens Real-Time Systems, 2006/7 RTCommunication, 19 19

20 TTP - Time Triggered Protocol 1990 at the Technical University of Vienna Targets at safety-critical applications Nodes: fault tolerant units (FTUs) Replicated bus Error detection, consistency checks Membership service Automatic clock synchronization References: Kopetz Real-Time Systems, 2006/7 RTCommunication, 20 20

21 TTP/C - Architecture Broadcast bus Transmission rate: 0.5, 1, 2 MBit/s up to 25 MBit/s with Ethernet Medium Medium Access Control: TDMA Time Division Multiple Access At a given time, one specific node accesses the bus API: shared memory state-only messages : no events via messages, IPC, interrupts data is transferred on a time-triggered basis Real-Time Systems, 2006/7 RTCommunication, 21 21

22 TTP/C - Bus Access Periodic Cluster Cycle Contains multiple TDMA rounds Contains multiple slots One node sends in each slot Up to 240 bytes Inter-Frame-GAP after each slot: 10µs - 100µs One slot per node and per round Bus access times stored in MEDL message descriptor list Distributed, static table within each node Real-Time Systems, 2006/7 RTCommunication, 22 22

23 Kopetz: Event vs. Time Triggered Alarm Monitor N N N N N N operator Bus: 100 Kbit/s N N N N 10 interface nodes Controlled object 40 sensors/node Each node supervises 40 alarm conditions and sends messages (as soon as possible) Deadline: 100 ms to notify operator Communication bandwidth: 100 kbits/second Real-Time Systems, 2006/7 RTCommunication, 23 23

24 Kopetz: Event Triggered (ex. CAN) Alarm Monitor N N N N N N operator Bus: 100 Kbit/s N N N N 10 interface nodes Controlled object 40 sensors/node Send message as soon as alarm condition is seen alarm name 8 Bit, CAN overhead 44+4 Bit 56 Bit allows 180 messages within deadline of 100ms worst case requirement: 400 messages... Real-Time Systems, 2006/7 RTCommunication, 24 24

25 Kopetz: Time Triggered (TDMA bus) Alarm Monitor N N N N N N operator Bus: 100 Kbit/s N N N N 10 interface nodes Controlled object 40 sensors/node Each node sends state message every 100ms Data field: 40 Bit + overhead 44 + gap 4 88 Bit allows 110 messages within 100ms period but, just 10 required! i.e. 10 % of bandwidth Real-Time Systems, 2006/7 RTCommunication, 25 25

26 Kopetz: Event vs. Time Triggered - Fault Detection Event Triggered: Client driven time out if no acknowledge Server does not learn about fault Time Triggered: Driven by Client and Server Server notices if message does not arrive in dedicated slot Real-Time Systems, 2006/7 RTCommunication, 26 26

27 The Replica Determinism Problem Sensors and Nodes may fail replicas (redundant, active nodes) may deliver different values, due to: different inputs (e.g. slightly differing measurements) preemptive scheduling race conditions nondeterminism (e.g. in languages) references to local time... Real-Time Systems, 2006/7 RTCommunication, 27 27

28 Kopetz: Rules How to Avoid use sparse global time base agreement on input static control structure deterministic algorithms Real-Time Systems, 2006/7 RTCommunication, 28 28

29 Real-Time Communication in WANs Performance objectives and constraints (bounds) rate (bandwidth) delay delay jitter (loss) Inherently dynamic nature Communication is connection oriented, to do: admission + connection establishment scheduling of packets (or cells) and to protect connections from each other shaping Often: group communication Real-Time Systems, 2006/7 RTCommunication, 29 29

30 Real-Time Communication in WANs find connection (reservations) determine local properties at each switch of connection depending on switch-local scheduler add up delays take care for jitter (provide buffer, traffic shapers) Real-Time Systems, 2006/7 RTCommunication, 30 30

31 Schedulers in Switches EDF: e.g., Delay-EDD Service Discipline static priorities... see text books weighted round robin Real-Time Systems, 2006/7 RTCommunication, 31 31

32 Scheduling Example: Weighted Round Robin Principle: Scheduler looks at input buffers in round robin fashion processes weighted number of units of messages per round Units of messages: cells, packets,... Messages: e i max length of message in units minimum interarrival time p i Real-Time Systems, 2006/7 RTCommunication, 32 32

33 Scheduling Example: Weighted Round Robin wt i each connection is given wt i slots per round. RL e i round length (switch design parameter) max length of message expressed in units processed per slot if wt i =1 wt i <= RL Roundlengths? Weights? Delay? Per Node, end to end? Real-Time Systems, 2006/7 RTCommunication, 33 33

34 Scheduling Example: Weighted Round Robin ff Delay per Switch: RL * ( e i / wt i ) number of hops (switches passed) RL is bound by delay guarantees of switch for example, to guarantee that messages are transmitted within a period: RL <= min p i wt i >= e i / p i / RL end to end delay: ( e i / wt i + -1) * RL Real-Time Systems, 2006/7 RTCommunication, 34 34

35 Reservation: ATM sender establishes connection to receiver a route is determined and routing information is stored on the switches along this route fixed size packets ( Bytes) are sent the route is determined by (VPI,VCI) pair in each cell (Virtual Path Id, Virtual Channel IP) change at each hop real time traffic classes: constant bit rate traffic variable bit rate traffic with bounded delay Real-Time Systems, 2006/7 RTCommunication, 35 35

36 RSVP - Ressource Reservation Protokoll Network reservation protocol for QoS Defined in 1997 as RFC 2205 Unidirectional data transfer Receiver: Multicast or Unicast address Sender: always Unicast address one-to-many group communication IP-Protocol, such as TCP or UDP transparent routing Data transfer uses UDP/TCP/... "Session": receiver-ip, protocol, port Real-Time Systems, 2006/7 RTCommunication, 36 36

37 RSVP - Connection Establishment Sender offers data stream: "Path message" Receiver replies with "Recv" message merging of recv-messages for multicast (Source: Eduad Siemens) Real-Time Systems, 2006/7 RTCommunication, 37 37

38 RT CORBA (1.0) Extensions, not modifications Fixed Priority Systems End to end predictability for fixed priority CORBA systems Real-Time Systems, 2006/7 RTCommunication, 38 38

39 RT CORBA Extend CORBA by: Thread priorities Bound duration of thread priority inversion Bound latency of operation invocation In each RT-CORBA system envolved: RT OS with scheduling mechanism RT object request broker RT communication Application Real-Time Systems, 2006/7 RTCommunication, 39 39

40 Priorities Assign platform independent RT-CORBA priority to invocations... Map to priorities of underlying OS consistently between nodes with different native priority schemes //IDL module RTCORBA { typedef short priority; const priority minpriority = 0; const priority maxpriority = 32767;} Real-Time Systems, 2006/7 RTCommunication, 40 40

41 Some Extensions to CORBA Thread interface: Scheduling Policy Two server priority models Client priority propagation Priorities assigned at server side Thread pools Preallocation (reduce priority inversion) Partitions threads to isolate applications Real-Time Systems, 2006/7 RTCommunication, 41 41

42 Priority Banded Communication Multiple Connections to reduce priority inversion Banding Each connection may represent a range of priorities Different range in each band (including 1) Real-Time Systems, 2006/7 RTCommunication, 42 42

43 Summary Field busses Simulate processors Scheduling analysis analog to processors Use time driven / static priority / table based scheduling Switched LANs traffic shaping within the nodes, no policing inside the switch still the area of research is moving into automation WANs Dynamic Use local scheduling disciplines in switches/routes to enforce system wide behaviour Admission for connections required Real-Time Systems, 2006/7 RTCommunication, 43 43

44 References J.-Y. Le Boudec and P. Thiran: Network Calculus. Springer Verlag, LNCS volume 2050, H. Kopetz: Real-Time Systems. Kluwer Academic Publishers, Othmar Kyas: ATM-Netzwerke. DATACOM-Buchverlag, J. Loeser, H. Härtig: Low-latency Hard Real-Time Communication over Switched Ethernet. In Proceedings of the 16th Euromicro Conference on Real-Time Systems (ECRTS 2004), June 2004, Catania, Italy. Malcolm, Zhao: Hard Real-Time Communication in Multiple-Access Networks. In Real Time Systems Journal, Kluwer Academic Publishers, S. Schönberg: Impact of PCI-Bus Load on Applications in a PC Architecture. In Proceedings of the 24th IEEE Real-Time Systems Symposium (RTSS 2003), December 2003, Cancun, Mexico. Real-Time Systems, 2006/7 RTCommunication, 44 44

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