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Pushing the limits of CAN - Scheduling frames with offsets provides a major performance boost

Pushing the limits of CAN - Scheduling frames with offsets provides a major performance boost. Nicolas NAVET INRIA / RealTime-at-Work http://www.loria.fr/~nnavet http://www.realtimeatwork.com Nicolas.Navet@realtimeatwork.com ERTS – 30/01/2008.

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Pushing the limits of CAN - Scheduling frames with offsets provides a major performance boost

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  1. Pushing the limits of CAN -Scheduling frames with offsets provides a major performance boost Nicolas NAVET INRIA / RealTime-at-Work http://www.loria.fr/~nnavet http://www.realtimeatwork.com Nicolas.Navet@realtimeatwork.com ERTS – 30/01/2008 Joint work with Mathieu GRENIER and Lionel HAVET

  2. In-vehicle networking : will CAN be able to keep up the pace? • Typically max. bus load is set to 35% • Not enough wrt to short/medium term bandwidth needs … • Solution 1: multiple CAN networks … but gateways induce heavy overhead • Solution 2: switch to FlexRay … expensive for bandwidth alone • Solution 3: optimize the scheduling of CAN frame .. Offsets provide a solution to make CAN predictable at higher network load (≥60%)

  3. Periods 20 ms 15 ms 10 ms Periods 20 ms 15 ms 10 ms 0 0 0 10 5 0 15 5 5 0 10 20 30 40 50 60 70 80 90 100 110 5 2,5 0 5 2,5 0 0 10 20 30 40 50 60 70 80 90 100 110 Scheduling frames with offsets ?! Principle: desynchronize transmissions to avoid load peaks Algorithms to decide offsets are based on arithmetical properties of the periods and size of the frame

  4. Frame response time Higher prio. frames frame CAN • Performance metric: worst-case response time System model (1/2) task ECU Frame Transmission request

  5. System model (2/2) • The offset of a message stream is the time at which the transmission request of the first frame is issued • Complexity: best choosing the offsets is exponential in the task periods → approximate solutions • Middleware task imposes a certain granularity • Without ECU synchronisation, offsets are local to ECUs

  6. Frame response time task Frame Transmission request Higher prio. frames frame CAN But task scheduling has to be adapted… ECU • In addition, avoiding consecutive frame constructions on an ECU allows to reduce latency

  7. Offsets Algorithm (1/3) • Ideas: • assign offsets in the order of the transmission frequencies • release of the first frame is as far as possible from adjacent frames • identify “least loaded interval” • Ex:f1=(T1=10), f2=(T2=20), f3(T3=20) f1,2 f1,1 f2,1 f3,1

  8. Offsets Algorithm applied on a typical body network 65 ms 21 ms

  9. Offsets Algorithm (3/3) • Low complexity andefficient as is but further improvements possible: • add frame(s) / ECU(s) to an existing design • user defined criteria : optimize last 10 frames, a specific frame, • take into account priorities • optimization algorithms: tabu search, hill climbing, genetic algorithms • …

  10. Efficiency of offsets : some insight (1/2) Work = time to transmit the CAN frames sent by the stations • Almost a straight line, suggests that our algorithm is near-optimal

  11. Efficiency of offsets : some insight (2/2) • A larger workload waiting for transmission implies larger response times for the low priority frames ..

  12. Computing worst-case response times with offsets

  13. AUTOSAR COM Frame-packingtask 5ms Computing frame worst-case response time with offsets • Waiting queue: • FIFO • Highest Priority First (HPF - Autosar) • Carmaker specific • Requirements : • handle 100+ frames • very fast execution times • ≠ waiting queue policy at the microcontroller level • limited number of transmission buffers 2 1 CAN Controller 9 6 8 bufferTx CAN Bus

  14. WCRT : State of the art • Scientific literature: • Complexity is exponential • No schedulability analysis with offsets in the distributed non-preemptive case • Offsets in the preemptive case : not suited for > 10-20 tasks • WCRT without offsets: infinite number of Tx buffers and no queue at the microcontroller level • Our software: NETCAR-Analyzer

  15. NETCAR-Analyzer : developed at INRIA, then RealTime-at-Work

  16. NETCAR-Analyzer : an overview • Worst-case response time on CAN with and without offsets • Proven near-optimal offsets assignments with user-defined performance criteria (e.g. WCRT of the 10 lowest prio. frames) • Exhibit the situations leading to the worst-case (results can be checked by simulations/testing) • Enable to dimension transmission/reception buffers (RAM) • Handle both FIFO and prioritized ECUs • Fast multi-core implementation (<1mn for 100 frames) • Industrial use since December 2006

  17. Performance evaluation : Experimental Setup WCRT of the frames wrt random offsets and lower bound WCRT reduction ratio for chassis and body networks Load increase : add new ECUs / add more traffic

  18. k # # d i d h i d N E C U M B F t t e w o r s e s s a g e s a n w r a m e p e r o s / d b B K i 1 5 2 0 7 0 1 2 5 5 0 2 t ¼ o y - s m s - s / h i b C K i 5 1 5 6 0 5 0 0 1 0 1 t ¼ a s s s - s m s - s Experimental Setup • Body and chassis networks With / without load concentration: one ECU generates 30% of the load • Set of frames generated with NETCARBENCH (GPL-licenced)

  19. 65 ms 32 21 17 Offsets in practice : large response time improvements (1/2)

  20. WCRT Reduction Ratio Chassis Networks Body Networks • Results are even better with loaded stations

  21. Offsets allow higher network loads • Typically: WCRT at 60% with offsets  WCRT at 30% without offsets

  22. Partial offset usage 65 ms 42 34 17

  23. ET CAN CAN with offsets TT-CAN + Complexity+ Determinism Conclusions • Offsets provide an cost-effective short-term solution to postpone multiple CANs and FlexRay • Tradeoff between Event and Time Triggered • Further large improvements are possible by synchronizing the ECUs …

  24. Questions, feedback? please contact me atnicolas.navet@realtimeatwork.com

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