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

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 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

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 2004, Santa Fe, New Mexico 3 General framework - 2 Communication across different subsystems takes place through gateways

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 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 –...

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 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 deadline allocation strategies: – Isometric – Maximum Schedulability Laxity and comparison of their relative performance

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 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

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 2004, Santa Fe, New Mexico 7 FTT brief overview Elementary Cycle structure Synchronous window  Conveys the time-triggered traffic  The TM contains the EC-Schedule Asynchronous window  Event triggered traffic, real and non-real-time

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 2004, Santa Fe, New Mexico 8 Isochronous vs anisochronous architectures Non synchronized FTT segments may lead to high end-to-end latency (synchronous traffic)

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 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

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 2004, Santa Fe, New Mexico 10 Isochronous vs anisochronous architectures Isochronous architecture – Requires clock synchronization CPU overhead Communication overhead – EC lengths constrained to be harmonic – Tight control on the inter- network traffic latency – Reduced end-to-end latency Anisochronous architecture – Lower CPU/Network overhead – Unconstrained EC length – Lower efficiency in inter- network traffic handling, leading to a higher end-to- end latency

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 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: SM i,j ={C i,j P i,j D i,j Pr i,j  i,j } – D i e2e : end–to-end deadline of message i

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 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

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 2004, Santa Fe, New Mexico 13 Deadline allocation scheme Isometric allocation scheme – Message deadline equally divided between all the involved networks – Simple computation – No need to know global system state but … – May lead to bottlenecks

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 2004, Santa Fe, New Mexico 14 Deadline allocation scheme Maximum schedulability laxity – Assign deadlines according to the relative network workload – Normalized utilization:

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 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

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 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 Asynch. window length (% of EC) Initial network util. (%) Schedulab. level (% of synch. bandw.) minmaxminmaxminmax

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 2004, Santa Fe, New Mexico 17 Simulation results Number of scheduled messages Average network utilization ratio

Message routing in multi-segment FTT networks: the isochronous approach Paulo Pedreiras, Luís Almeida, WPDRTS’ April, 2004WPDRTS 2004, Santa Fe, New Mexico 18 Conclusion There may be advantages from using segmented time-triggered networks Reduncing latency of inter-segment traffic requires global synchronization – Isochronous vs Anisochronous architectures – The Isochronous architecture provides a better control 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