fixed-priority servers typical real-time systems are hybrids characterized by: periodic tasks that...
Post on 20-Dec-2015
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TRANSCRIPT
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Fixed-Priority Servers
Typical Real-Time systems are hybrids characterized by:
• periodic tasks that execute critical control activities
• aperiodic tasks that are event-driven
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Assumptions for Fixed Priority Servers:
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Scheduling Strategies we will explore:
• Background Service
First Come First Served (FCFS) service as availability permits
• Polling Server (PS)
Pseudo periodic task(s) provides slot for serving aperiodic tasks
• Deferrable Server (DS)
Unused capacity is saved for future aperiodic arrivals
• Priority Exchange Server (PE)
“Loans” unusable capacity to ready periodic tasks
• Sporadic Server
Loans unusable capacity to ready periodic tasks and delays recoup
• Slack Stealing
Passive task steals unnecessary slack time in scheduling
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Background Service
Background Service simply queues up aperiodic tasks and services them perhaps on a First Come First Served basis. There are no guarantees.
Advantages: Simple, Can use alternate scheduling algorithm for aperiodic tasks
Disadvantage: No inherent guarantees
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Background Service Example
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Polling Server
Advantages:
Advantages: Relatively easy to implement
Disadvantages: Not a big advantage in performance
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Polling Server Example
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Deferrable Server
Advantages: Much better response time for aperiodic tasks compared to polling
Disadvantages: More complex and DS violates the RM principle that the highest priority task runs when it is ready.
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Example of Deferrable Server
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Deferrable Server ExampleShows that Deferrable Server violates the RM task running guarantee
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Another Example of Deferrable ServerShows that the deferring can cause a task deadline to be missed
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Priority Exchange Server
Advantages: Provides a better Schedulability bound for periodic requests
Disadvantages: Provides worse response for aperiodic tasks compared to Deferrable Server
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Example of Priority Exchange
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Another Example of Priority Exchange
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Sporadic Server
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Example of Medium-Priority Sporadic Server
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Example of High-Priority Sporadic Server
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Slack Stealing
Advantages: Substantial improvement in aperiodic task response time
Disadvantages: Complexity
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Example of Slack Stealing
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Slack Stealing Example Continued: s=0
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Slack Stealing Example Continued: s=0
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Performance Comparison of PS, DS, PE, & SS
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Performance Comparison of Slack Stealer wrt background, PS, & SS
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Summary of Fixed-Priority Strategies
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Homework #5 Lecture 5/15
1) Schedule the following tasks with a Polling Server with intermediate priority.
2) Solve the same scheduling problem with a Sporadic Server with intermediate priority.
3) Solve the same scheduling problem with a Deferrable Server and highest priority.
4) Solve the same scheduling problem utilizing Slack Stealing.
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Slack Stealing Does not Lead to an Optimal Schedule
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Example to Show Non-existence of Optimal Server