Fault-Tolerant Real-time Scheduling using MicroC:OS-II

Project Report for EE8205

Fault-Tolerant Real-time Scheduling using MicroC:OS-II

Huy Pham, Moya Dai

April 7th, 2005

Ryerson UniversityRyerson University

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EE8205 ProjectEE8205 Project

ContentsContents1. Realtime System Description

2. Basic Algorithms Used in the Project

3. Our Implementation

4. Demonstration

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RealtimeRealtime System DescriptionSystem Description

Hard realHard real--time system:time system:

Deadline must not be missed.

Periodic Tasks: Periodic Tasks:

Deadline is the end of each period.

Fixed PriorityFixed Priority--Driven Scheme:Driven Scheme:

Task priority is determined at the beginning.

Fault may occur:Fault may occur:

System fault tolerance required.

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Basic AlgorithmsBasic Algorithms

Used in the ProjectUsed in the Project

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Two versions proposedTwo versions proposed

FaultFault--tolerance by Redundancy.tolerance by Redundancy.Each task has two versions: Primary and Alternate.Each task has two versions: Primary and Alternate.

Primary:Primary:Contains full function, produces good quality results. Contains full function, produces good quality results. Prone to failure for higher level of complexity and resource usaProne to failure for higher level of complexity and resource usagege.

Alternate:Alternate:Only the minimum required functions included, less precise but Only the minimum required functions included, less precise but acceptable results. acceptable results. Simpler, requiring less resource, correctness is fully tested.Simpler, requiring less resource, correctness is fully tested.

Objective:Objective:1.1. Guarantee either the primary or the alternate version of each taGuarantee either the primary or the alternate version of each task to be sk to be

correctly completed before the corresponding deadline.correctly completed before the corresponding deadline.2.2. Try to complete as many primaries as possible.Try to complete as many primaries as possible.

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Last Chance StrategyLast Chance Strategy• Each alternate is chosen to start at the latest possible time.

• The primaries are dynamically scheduled during the remaining intervals before their alternates.

• The alternates can preempt a primary when a time interval reserved for the alternates is reached.

• Whenever a primary is completed successfully, the scheduler remove its corresponding alternate and dynamically allocate the time intervals to other jobs.

• For those alternates affected, the reserved time intervals are reconstructed.

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The shorter the period the higher the priority Optimal for fixed priority-driven scheduling schemes.Primary tasks are assigned priorities according to the RM rule.An offline scheduler reserves time intervals for the alternates according to back-RM algorithm.

The Rate Monotonic AlgorithmThe Rate Monotonic Algorithm

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The Checking Available Time AlgorithmThe Checking Available Time AlgorithmProblem:Problem:

Subsequent jobs may be affected by the early job failures.

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Solution:Solution:Before selecting a primary to execute, check if there are enoughunallocated units of time for its completion. If not, the primary will not be activated, thus eliminate the wasteful execution for primaries as much as possible.

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The Eliminating Idle Time AlgorithmThe Eliminating Idle Time Algorithm

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CPU Idle

CPU Idle

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Solution:Solution:Two modes for each alternate:Prenotification and Postnotification

Postnotification mode: Starts at its notification time, and has higher priority than any primary.

Prenotification mode:An alternate is chosen to execute before its notification time only when the processor is about to be idle. It has lower priority than all primaries.

Problem:Problem: The processor will become idle, wasting cycles.

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Improved Algorithm (3 versions)Improved Algorithm (3 versions)

For frequently or permanent failure (continuous failures more than N times), replace the task with a backup version.

The backup version is never used when the primary is in its good performance or the failure time is less than N times.

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Our ImplementationOur Implementation

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MicroCMicroC OSOS--IIII•Open source

•Well documented and widely supported

•Small

•Simple interface

•Preemptive:

Each task must have a unique priority

Total of 64 priority levels, 0 is the highest, 63 is the lowest

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Scheduling with MicroC OS-II

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Scheduling with MicroC OS-II

Our Proposal:

Scheduler as a Task

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An Example Scheduling ProblemAn Example Scheduling Problem

Priority Assignment

Control 0A1 1A2 2P1 3P2 4

Two Tasks:T1(5, 2, 1)T2(6, 2, 2)

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Alternate Alternate // Alternate1// // Poll the clock and print out an ID string// every half a second, then move ourselves out of the// READY queue when time is up//static void Alternate1 (void *pdata){

while(running_time < duration){

Poll the clock and print out an ID stringevery half a second

}

MoveToDormant()

}

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PrimaryPrimarystatic void Primary1 (void *pdata){

x = random()if ( x < FAILRATE ){

// Run overtimewhile(1){


}}else{

// Run for our durationwhile(running_time < duration){


}

// Set a global var to indicate success, and quitP1Succeeded = 1MoveToDormant()

}}

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ControlControl// Control// // Fault-tolerant Real-time Scheduling by moving tasks // in and out of the READY queue appropriately.//static void Control (void *pdata){

// Build and initialize data structuresInitData()

// Schedulingwhile (1){

SheduleTasks()

}

}

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DemonstrationDemonstration

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EnhancementsEnhancements

Three Enhancements

Eliminating Idle Time

Taking into account variable task execution time

Keep track of the number of times a primary has failed, and stop scheduling it after a certain threshold.

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The The EEnd.nd.

Thank you!Thank you!

Fault-Tolerant Real-time Scheduling using MicroC:OS-II

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