Summary of Some Exercises

• Read the slides on periodic tasks. Download the example programs, try them, and try to modify them to understand how they work.
• Modify periodic-4.c to remove global variables. A possible solution is here.
• Now, implement a cyclic executive scheduler (see last three slides) for three tasks with periods 50ms, 100ms, and 150ms. The bodies of the jobs for the three tasks are in cyclic_test.c
  • Note: the last two slides are a copy of the slides from lesson II. The numbers in such slides are not referring to this exercise.
  • First hint: compute the minor cycle and the major cycle
  • Second hint: so, you need a periodic timer every ... ms
  • Third hint: write down a table showning which tasks should execute in every minor cycle
  • Question: how big must this table be?
  • Last hint: remember that in cyclic executive tasks are non-preemptable... They are not real tasks, but you can ``simulate'' them through function calls...
  • Solution
  • Solution using a scheduling table
• Try again with periods 60ms, 80ms and 120ms
• Modify the example to remove compilation warnings and to re-use the periodic timers code in a better way (instead of cut'n'pasting it in cyclic.c)
  • Possible solution
  • The proposed solution uses this code for periodic timers and this header file for describing its software interface
  • Also, here is the software interface for cyclic_test.c
• Rewrite the previous example (three periodic tasks, with periods 50ms, 100ms, and 150ms) using three real processes instead of a cyclic executive scheduler.
  • Use the same job bodies as in the previous example (download them from here). You can use this header file describing the prototypes of the three job bodies.
  • You now need to create three processes... You can reuse code from the fork() example
  • Each one of the three processes must be periodic... Reuse some code from one of the previous periodic-?.c examples
    • For example, here is the code for periodic tasks
    • And here is its software interface
  • A possible Solution
  • A More advanced solution that tries to set proper real-time priorities (according to RM)
• Rewrite the example again, this time using threads to implement the three tasks. You can look at thread.c to remember how to create a thread.
  • Can you see why global variables are evil, now?
  • A possible Solution (not setting real-time priorities)
  • A More advanced solution that tries to use proper real-time priorities
  • NOTE:These two solutions are using the factorised code for periodic timers and its software interface
• Read the slides on pthread Synchronisation. Download the example on mutexes (ex_mutex.c), read it, and try it.
  • See how mutex + condition variables can be used to implement conting semaphores
• Now, look at the message passing example, about condition variables. Here are some possible modifications at the previous program... WARNING: this program does not re-use existing code in an appropriate way; this is on purpose, and fixing the issue is left as an exercise. This is one possible solution.
• Look at another example on mutexes to see how to use Priority Inheritance.
• Finally, here is an example of real-time application