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An On-line Approach to Reduce Delay Variations on Real-Time Operating Systems. Shengyan Hong. Delay Variation. What is delay variation? Difference between the max delay and the min delay of a real-time task. WCRT i (BCRT i ): the worst (best) case response time of the task i.

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delay variation
Delay Variation
  • What is delay variation?
    • Difference between the max delay and the min delay of a real-time task
  • WCRTi (BCRTi): the worst (best) case response time of the task i
delay variation1
Delay Variation
  • Why interested in reducing delay variation?
    • It degrades control system performance
  • Sources of delay variations
    • Task preemptions
    • Variations in task workload
    • Perturbations in physical environment
  • A 3DOF helicopter containing 4 periodic tasks
    • Position tasks: elevation, pitch and travel
    • Speed task
task model
Task Model
  • Original Task Model (Ci, Di, Pi)
    • Ci: task execution time, Di: task deadline, Pi: task period
  • IMF Task Model
    • Initial, Mandatory and Final subtasks
  • ADVR Task Model
    • Assume Cif=Cii
    • Final subtask (Cif, Difnew, Pifnew=Pi/Mi)
    • Mandatory subtask (Cimnew, Dimnew, Pi)
    • Maximize Mi as much as possible
problem formulation
Problem Formulation
  • Minimize delay variations of final subtasks
    • Dif: the deadline of the final subtask of task i, Cif: the execution time of the final subtask of task i
    • (Dif-Cif): the upper bound of delay variation
    • N: task number of the task set
    • Wi: a weighting factor that reflects the criticality of task i
  • Such that
    • task set is schedulable under EDF, and
    • subtask execution dependencies are observed
advr heuristic
ADVR Heuristic
  • Search an initial solution with optimal model transformation
    • Split original task period
  • Repeatedly search for a better solution based on the previous solution
    • Find smaller deadlines of final subtasks
  • Update the deadlines of mandatory subtasks
    • If ADVR has not converged, return to the previous step
  • Achieve fast convergence
    • Time complexity is pseudo polynomial
what is s ha r k
What is S.Ha.R.K.?
  • S.Ha.R.K. is an Open Source Real-time Kernel
  • It Supports:
    • Provide primitives to create, activate and run real-time tasks using scheduling algorithms.
    • Share data among tasks using resource reservation algorithms
    • Device drivers for most common hardware
  • ADVR implemented in S.Ha.R.K.
    • Task model modification
    • Scheduler internal data structure and algorithm modification
    • Application modification
solutions found
Solutions Found
  • Experimental Evaluation:
    • Compare ADVR with TBB, DRB
    • Part 1: tested on 9000 randomly generated task sets
    • Part 2: a case study of adaptive delay variation reduction
  • 9000 randomly generated task sets each consisting of 5 tasks
    • Utilization: fraction of time that the processor is working
  • ADVR performs best
    • ADVR founds the solutions of all the task sets
total average delay variation
Total Average Delay Variation
  • ADVR performs best
    • Difference between ADVR and the other methods increases with the increasing of utilization levels
average execution time
Average Execution Time
  • ADVR and DRB have comparable execution speed
    • DRV is a greedy algorithm, whose convergence is fast
iteration number of advr
Iteration Number of ADVR
  • ADVR is suitable for on-line use
    • ADVR finds an optimal solution for most of task sets within 50 iterations
case study 3dof helicopter
Case Study: 3DOF Helicopter
  • Implemented in S.Ha.R.K.
  • Compare results by ADVR with original Delay Variations, and those by TBB and DRB
    • The smaller DV, the better the method is.
  • When workload changes, see which method can adapt to it.