Real Time Operating Systems Lecture 10

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Real Time Operating SystemsLecture 10

• David Andrews
• dandrews_at_eecs.ukans.edu

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What We Will Cover Today

• Operating Systems Services
• Base Components
• Real Time Considerations
• Task Management
• Time Triggered
• Event Triggered
• Interprocess Communications
• Blocking
• Non-Blocking
• Time Management
• Services
• Maintenance

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Operating Systems Functionality

• Considerations
• Footprint An Issue
• Small Numbers of Programs
• Virtual Memory Will Not be Required For Our Study

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Task Taxonomy

• Simple Task (S-task)
• No synchronization within task
• Executes from beginning to end, subject only
  to pre-emption by interrupts
• Might be triggered by some other task
  initially
• Might communicate results to some other task
• But, no internal synchronization with other
  tasks
• Complex Task (C-task)
• Has some synchronization with other tasks
• May have to wait for another task to process
  a request
• May have to wait for I/O to complete
• May have to wait for resource to become
  available (memory, network)
• Timing depends on the rest of the system
• So, C-tasks are bad because they increase
  complexity
• And, C-tasks make it more difficult to
  analyze system timing properties

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C-Task Execution time

• S-Task execution time plus
• Waiting for other processes
• Not so bad if other tasks are S-Tasks and it
  is just a remote procedure call
• General case can be impractical to handle
• General solution
• Break C-tasks into S-tasks separated by
  interactions
• Then, make sure that S-tasks are scheduled far
  enough apart for interaction to complete
• Great when it works...

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Task Management

• Key component is scheduler
• Static Schedule Built up Prior To Running
• Scheduler Develops Time Triggered Schedule
• Periodic/Predictable
• Dynamic Schedule Built During Run Time
• Can Be Based on Worst Case Execution Time (WCET)
• Scheduler Invoked by events
• Timer, Interrupts, suspend, resume, etc.

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Worst Case Execution Time (simple version)...

• Assume a simple task (S-task) that just executes
  its own code without coordinating with other
  threads or I/O
• How hard could it be to determine timing?
• Timing is once through the code jitter due to
• Non-fixed loop iteration count
• Non-fixed number of recursive calls
• Garbage collection
• Conditional statement execution asymmetry
  (if/else path timings)
• Effects of hardware non-determinism
• Cache
• Data-dependent instruction execution time

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Plus S-Task Context Swapping

• Overhead from task switching OS Hardware
• multiply by number of pre-emptions
• Gives WCAO (Worst-Case Administrative
  Overhead) due to pre-emptions

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Plus Effects From Nested Interrupts

• What if you have multiple prioritized interrupts
• Worst case execution time for interrupt at
  Priority X is Execution time for interrupt at
  Priority 1 (perhaps multiple times)
• Execution time for interrupt at Priority 2
  (perhaps multiple times)
• Execution time for interrupt at Priority 3
  (perhaps multiple times)
• Execution time for interrupt at Priority X
  (perhaps multiple times)
• You can bound this as long as the period of each
  interrupt is bounded
• Same algorithm as for computing worst case CAN
  message latency(!)
• Scheduling on a CPU is a dual problem to
  scheduling on a network

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Task Management

• Time Triggered
• A priori build Task-Descriptor Table (TADL)

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Interprocess Communications

• IPC can support Messages, or Common Data Regions
• Messages
• We have already discussed synch/asynch
• Asynchronous can be implemented in S-Tasks
• Synchronous (suspend) can be implemented in
  C-Tasks
• Common Data Regions
• Implemented via shared memory
• Introduces critical regions
• Semaphore Operations needed
• Test and Set

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Time Management

• Time Management provides
• Clock synchronization
• Time stamping
• Watchdog Timers
• User Accessible
• Operating System

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Real Time Operating Systems

• Unix feel
• QNX
• LinxOS
• Smaller RTOSs
• OS9, Microware
• VxWorks (be sure to get the memory protection
  extension)
• pSOS
• RTX, VenturCom
• A few research/freeware RTOS systems
• KURT (KU real Time)
• RT-Mach, CMU
• others
• Windows WinCE embedded NT, Windows RT add-on
  products

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Questions To Ask Your RTOS Vendor

• Real Time Operating System
• Purports to ensure timeliness of task
  execution
• Uses task priorities and a scheduling
  algorithm
• Hard questions
• What is the tasking model (preemptive or
  non-preemptive)? Scheduler?
• What is the longest task switch latency?
• Function of interrupt masking time
• How big is the footprint for a real system?
  (Not a stripped-down toy system)
• Does it have inter-task memory protection?
• How much of POSIX does it really support?
• How long does the system take to reboot?
• What is the runtime license fee?
• How robust is it (and what is the exception
  handling model?)