Non Preemptive Scheduling of Periodic Mixed Criticality Real-Time - - PowerPoint PPT Presentation

Non Preemptive Scheduling

• f Periodic Mixed Criticality

Real-Time Systems

Jasdeep Singh, Luca Santinelli, Federico Reghenzani Konstantinos Bletsas, Zhishan Guo

Outline

• Real-time systems
• Probabilistic Real-time systems
• Mixed criticality systems
• Graph and Tree model
• Schedule
• Worst Case Evaluation

Real-Time applications

• System of tasks to be executed on processors with

resources

• Require real-world timing guarantees
• Done through scheduling: Arrange task execution in time
• Scheduling uses task WCET
• Real execution time is Rarely equal to WCET

Probabilistic R/T systems

• Practically, execution time is rarely equal to WCET
• Instead of WCET => pWCET (probabilistic worst case

execution time)

• pWCET: worst case probability distribution of various

possible execution times

• pWCET is assumed given

P r

• b

a b i l i s t i c q u a n t i fi c a t i

• n
• f

p e s s i m i s m i n W C E T !

Mixed Criticality Systems

• MC Systems: Tasks with different criticalities
• Each criticality corresponds to system criticality
• Task takes more time than ‘expected’=> system HI mode
• Lower and Upper WCET
• System HI mode => All LO tasks are dropped; schedulability of HI ensured
• Model represents clear line between processor demands

System:LO

Tasks:LO Tasks:LO Tasks:HI Tasks:HI

System:HI

Tasks:HI Tasks:HI Tasks:LO Tasks:LO More time to execute If one HI task takes longer than “usual”

D e t e r m i n i s t i c a d j u s t m e n t

• f

p e s s i m i s m i n W C E T !

MC from PWCET

Graph and Tree Model

• Mix of tasks with three level criticality
• Each has a probability

The Schedule

• Graph -> Tree
• Available paths: all jobs
• Valid paths: no job misses

deadline (evaluate response time)

• Dangerous path: no job misses

deadline in higher criticalities

• Schedule: Optimize (Tree minus

non-valid and dangerous paths)

• Optimize: Allowing maximum

jobs to execute, independent of job entering high criticality S c h e d u l e i s

• p

t i m i z e d i n r e s

• u

r c e u s a g e !

• No System-wide mode switch
• Ensured that no job misses deadline, in any criticality
• Schedule adjusts if job enters high criticality, drops only necessary
• nes, at lower criticalities, maximum utilization
• Schedule is a tree and knows which branch to take at which event
• Complexity is reduced by checking for deadline miss while tree

construction

• Quantify the probability of system entering higher criticalities (we

can only quantify)

The Schedule

Evaluation: Response Time

• Convolution has hidden assumption: task/jobs arrive at

same time

• Convolution safe but pessimistic
• We propose:

N

• w

R e s p

• n

s e t i m e g i v e s t h e p r

• b

a b i l i t y

• f

t a s k e n t e r i n g H I ! N O T W C E T

Propositions for Mixed Criticality

• Response time for MC: System HI mode is a run-time

information

• Probability and Schedule: Probability comes into play,

schedule can depend on the probability

• Schedule for optimal Probability: Probability from

response time now depends on schedule

• Be prepared for worst case: execution scenarios upper

bounded, schedule adapts for system entering higher mode; Previous model to upper bound

Thank you !