CSC 7103 20091029

CSC 7103 20091029 - CSC 7103 Page 1 of 3 Task pre-emption...

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CSC 7103 10/29/2009 Page 1 of 3 Task pre-emption with semaphores. Figure 1 – Semaphore example Figure 2 – Nested semaphores Figure 3 illustrates priority inversion because when T 1 locks, it blocks, and even though T 2 has a lower priority it is still allowed to run first because of the semaphore blocking. Figure 3 – Semaphore example showing priority inversion Chain blocking is shown in Figure 4. Here T 3 runs because T 1 and T 2 share a semaphore. And T 1 is blocked by a lower priority process because of the semaphores. How can we avoid the wasted time for T 1 ? Figure 4 – Chain blocking example How to remove the problem of priority inversion? The simplest way is for tasks which are holding semaphores needed by a higher priority process to get the priority of the process that needs the semaphore. This is called the priority inheritance protocol. More details:
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CSC 7103 10/29/2009 Page 2 of 3 1. All tasks are assigned rate monotonic priorities 2. If T h is blocked by T l , then the priority of Pr(T h ) = Pr(T l ) 3.
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