Message routing in multi-segment FTT networks: the isochronous approach

1. Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida {pedreiras,lda}@det.ua.pt DET – IEETA Universidade de Aveiro Aveiro-Portugal Workshop on Parallel and Distributed Real-Time Systems 2004 (WPDRTS04). April 26 th and 27 th , 2004, Santa Fe, New Mexico

2. General framework • Industrial systems (in a broad sense) are more and more integrated • As the system size grows, so does its complexity • A possible approach to handle complexity is to build the system by composing subsystems • Breaking a large network in segments may: • Facilitate the system management • Increase the traffic schedulability level • Isolate independent traffic • Allow the physical extension of the network WPDRTS 2004, Santa Fe, New Mexico

3. General framework - 2 • Communication across different subsystems takes place through gateways WPDRTS 2004, Santa Fe, New Mexico

4. General framework - 3 • For real-time applications it is necessary to guarantee the schedulability of both: • Intra-network traffic • Inter-network traffic • Category of problems addressed in several contexts: • Voice and video on WANs • Multi-computer systems interconnected by mesh networks • Wireless networks • Switched Ethernet networks • ... WPDRTS 2004, Santa Fe, New Mexico

5. Contribution • Multi-segment support to the Flexible Time-Triggered communication paradigm (FTT): • Comparison, in terms of end-to-end latency, between Isochronous and Anisochronous architectures • For the isochronous architecture: • Two deadlineallocation strategies: • Isometric • MaximumSchedulability Laxity and comparison of their relative performance WPDRTS 2004, Santa Fe, New Mexico

6. FTT brief overview • The FTT paradigm main operational characteristics: • Centralized scheduling with operational flexibility • Master/Multi-slave cooperation model • Support for distinct traffic classes • Event /Time-Triggered traffic, with temporal isolation • Hard/Soft/Non real-time timeliness requirements • How it works? • Traffic is allocated in fixed duration time slots ( Elementary Cycle - EC ) • Bus time is organized in an infinite succession of ECs • ECs start with a trigger message (TM) sent by the Master WPDRTS 2004, Santa Fe, New Mexico

7. FTT brief overview • Elementary Cycle structure Asynchronous window • Event triggered traffic, real and non-real-time Synchronous window • Conveys the time-triggered traffic • The TM contains the EC-Schedule WPDRTS 2004, Santa Fe, New Mexico

8. Isochronous vs anisochronous architectures • Non synchronized FTT segments may lead to high end-to-end latency (synchronous traffic) WPDRTS 2004, Santa Fe, New Mexico

9. Isochronous vs anisochronous architectures • Synchronized FTT segments may lead to lower end-to-end latency End-to-end deadline equal to sum of intermediate deadlines WPDRTS 2004, Santa Fe, New Mexico

10. Isochronous vs anisochronous architectures Anisochronous architecture • Lower CPU/Network overhead • UnconstrainedEC length • Lowerefficiency in inter-network traffic handling, leading to a higher end-to-end latency Isochronous architecture • Requires clock synchronization • CPU overhead • Communication overhead • EC lengths constrained to be harmonic • Tightcontrol on the inter-network traffic latency • Reduced end-to-end latency WPDRTS 2004, Santa Fe, New Mexico

11. Deadline allocation scheme • The problem Given a message end-to-end deadline, how to compute the intermediate deadlines in each one of the involved networks? • System model • FTT isochronous networks • Interconnection via gateway nodes that fully comply with the FTT trasmission control policy • Synchronous message i of network j characterized by: • SMi,j={Ci,jPi,j Di,j Pri,ji,j} • Die2e : end–to-end deadline of message i WPDRTS 2004, Santa Fe, New Mexico

12. Deadline allocation scheme • For each message i having to cross networks 1..k: • Latency: • Goal: and feasible message sets in each one of the intermediate networks WPDRTS 2004, Santa Fe, New Mexico

13. Deadline allocation scheme • Isometric allocation scheme • Message deadline equallydivided between all the involved networks • Simple computation • No need to know global system state but … • May lead to bottlenecks WPDRTS 2004, Santa Fe, New Mexico

14. Deadline allocation scheme • Maximum schedulability laxity • Assign deadlines according to the relative network workload • Normalized utilization: WPDRTS 2004, Santa Fe, New Mexico

15. Deadline allocation scheme • Maximum schedulability laxity(cont) • Deadline computation: • Compared with the isometric strategy: • Requires global data (individual network utilization) • More complex ( O(k) instead of O(1) ) but … • Higher schedulability • Best suited for systems requiring on-line QoS management • Load balancing WPDRTS 2004, Santa Fe, New Mexico

16. Simulation results • FTT implementation on CAN • EC =10ms • Bit rate=125kbps • FTT overheads = 7% / EC • Messages between 1 and 8 data bytes • Periods between 10 and 60 ECs, Deadlines=Periods • Number of networks between 1 and 5 WPDRTS 2004, Santa Fe, New Mexico

17. Simulation results Number of scheduled messages Average network utilization ratio WPDRTS 2004, Santa Fe, New Mexico

18. Conclusion • There may be advantages from using segmented time-triggered networks • Reduncinglatency of inter-segment traffic requires global synchronization • Isochronous vs Anisochronous architectures • The Isochronous architecture provides a bettercontrol of inter-network traffic latency • Two methods to compute inter-network message deadlines: • A simple isometric allocation scheme • An allocation scheme that partitions the deadline according to the leeway of each intermediate network WPDRTS 2004, Santa Fe, New Mexico