Real Time Operating Systems - 4 threads managed with semaphores (Priority Ceiling Policy)

Author: Simone Contorno

Introduction

An overview of this program function.
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How it works

A rapid description of how the program works.
Go to How it works

Installation and Execution

How install and run this program in Linux.
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Conclusion

Conclusion.
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Introduction

This program manage 4 threads, using semaphores and Priority Ceiling Policy, in order to:

• Task 1_1 write something into the variable T1T2, Task 2_1 read from this one.
• Task 1_2 write something into the variable T1T4, Task 4 read from this one.
• Task 2_2 write something into the variable T2T3, Task 3 read from this one.

How it works

To do this, the following steps have been followed:

• Set the thread with the lowest period as the first one and the thread with the highest period as the last one.
• Compute the WCET (Worst Case Execution Time) for each task.
• Compute the Blocking Time for each task.
• Check if the schedulability if feasible or not computing the Utilization factor (U) and the Utilization factor lower upper bound (Ulub).
• Set attributes (including scheduling policy and priority) of each task.
• Set attributes (including Priority Ceiling) of each semaphore.
• Compute the next arrival time of each task.
• Create threads and wait for all of them.
• When these terminates, print missed deadlines of each task and terminate the program.

Now, let's notice what each Thread does:

• Tread 1:
  • Lock the first semaphore, execute Task 1_1 and unlock the semaphore;
  • Lock the second semaphore, execute Task 1_2 and unlock the semaphore;
  • Sleep until the next arrival time and compute the next one.
• Tread 2:
  • Lock the third semaphore, execute Task 2_2 and unlock the semaphore;
  • Lock the first semaphore, execute Task 2_1 and unlock the semaphore;
  • Sleep until the next arrival time and compute the next one.
• Tread 3:
  • Lock the third semaphore, execute Task 3 and unlock the semaphore;
  • Sleep until the next arrival time and compute the next one.
• Tread 4:
  • Lock the second semaphore, execute Task 4 and unlock the semaphore;
  • Sleep until the next arrival time and compute the next one.

Installation and Execution

Download the repository:

git clone https://github.com/simone-contorno/rtos-priority-ceiling

Now, to run 'rtos_priority_ceiling' type:

./exec.sh

Conclusion

Thanks to have read this file, i hope it was clear and interesting.