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 Schedulinga) 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.