Dependable communication synthesis for distributed embedded systems Nagarajan Kandasamy, John P. Hayes, Brian T. Murray Presented by John David Eriksen and Jamie Unger-Fink

Overview Real-time distributed systems ▫Network ▫Sensors ▫Processors ▫Actuators Requirements ▫Strict performance targets ▫Fault-tolerance ▫Safety-critical operation

Related Work Network Protocols ▫Controller Area Network protocol (CAN)  Common protocol used in embedded systems networks.  Messages scheduled using variable priority levels ▫Time-Division Multiple Access (TDMA)  Time-multiplexed frame-based protocol Network topology considerations ▫Fixed network topology assumption ▫Synthesize topology given application requirements  Can be accomplished using a task graph model  Can be accomplished assuming a CAN or TDMA network protocol

TDMA Overview Time-Division Multiple Access (TDMA) communication protocol ▫Faster than CAN ▫Used in this paper TTP and FlexRay networking protocols

TDMA in FlexRay A round is a set of identical-sized slots determined by a given communication schedule. A processor Pj is allocated one or more sending slots during a round. Number of slots and size of slots determined by designer. A processor can only place messages in the slots allocated to it.

Topology Overview Topology restricted to multiple-bus systems Multiple-buses used for fault tolerance and to spread larger communication loads – network media typically low-bandwidth and inexpensive. Each processor connects to subset of communication buses Coprocessor attached to each processor handles message communication without interrupting process execution

Summary of Approach Target: a low-cost and reliable communication network Use a set of distributed applications modeled as task graphs {Gi}. Allocated messages to minimum number of buses {Bj} where each Bj has a fixed bandwidth.

Summary of Approach Target a generic TDMA protocol (FlexRay) Assume a multi-rate system where each graph Gi may have a different execution period period(Gi) Support dependable message communication by establishing redundant transmission paths between between processors. Maintain efficient network usage by reusing transmission slots allotted to a given processor between multiple messages sent by it.

Topology Development How is final number of necessary buses determined? ▫Initially, each message is assigned its own bus. ▫Iterative clustering heuristic used to minimize the number of buses used.  Clustering is NP-complete  Clustering heuristic SYNTH({mi}) used instead

Topology Development

Message Clustering SYNTH({mi}) ▫Cluster synthesis ▫Group message mi with existing cluster Cj if:  No two replicas of one k-FT message is allocated to Cj  All messages in Cj continue to meet deadlines.  Length of Cj communication schedule does not exceed application specifications.  Memory size of Cj communication schedule must fit in memory size of communication network co-processor.  This grouping process is considered to be a bin-packing problem (NP- Complete) so a heuristic used. ▫Results in efficient use of bus bandwidth through sharing and re-use of transmission slots between multiple messages whenever possible. ▫Each cluster assigned to separate bus in final topology.

Task Deadline Assignment Task scheduling range [ri, di] ▫ri – Release (start) time ▫di – Deadline Deadline computation ▫NP-Complete ▫Hueristic used instead (Natale and Stankovic)

Task Scheduling a)Initial task graph with execution times parenthesized and fixed execution period of 2000 microseconds. b)Path selection and slack allocation. c)Path {T1,T2,T4,T5} scheduled and removed. {T3} then scheduled. d)Final release/deadline scheduling range.

Task Scheduling Tasks assigned priorities according to the parameters of their schedule (release time and deadline) A processor executes the highest-priority task that has been released for execution Feasible only if: ▫All tasks can complete by their deadlines ▫All messages can be received by their deadlines

Case Study Applications modeled ▫Adaptive cruise control - Maintain safe following distance behind moving car ▫Electric Power Steering - Provide steering assistance ▫Traction Control - Maintain intended path on slippery terrain

Case Study Parameters Bus bandwidth of 250 KB/s Transmission slot width of 50 microsec. Max number of slots 16 Slot reuse on or off Round lengths fixed at three different sizes: 12, 16, and 10. Round lengths are harmonic multiples of three base periods: 3, 4, and 5.

Case Study Results No slot reuse yielded designs with less free slots that can be used for non-critical messages. Slot reuse yielded more free slots. Optimal solution used a period base 4 and yielded solution with three buses and plenty of free slots.

Conclusion Synthesis of low-cost TDMA communication network for distributed system showed to be feasible. ▫Proven via formal mathematical proofs. ▫Demonstrated via realistic case study. Future work: ▫Message scheduler on co-processors not part of this design. ▫Fault-tolerant allocation of tasks to processors not addressed.