Embedded System Design by Peter Marwedel

Author:Peter Marwedel
Language: eng
Format: epub, pdf
Publisher: Springer International Publishing, Cham

Bold upward pointing arrows indicate the times at which jobs are released, or “ready.” At time , job enters a critical section after requesting exclusive access to some resource via an operation P. At time , job becomes ready and preempts . At time , fails getting exclusive access to the resource in use by and becomes blocked. Job resumes and after some time releases the resource. The release operation checks for pending jobs of higher priority and preempts . During the time has been blocked, a lower priority job has effectively blocked a higher priority job. The necessity of providing exclusive access to some resources is the main reason for this effect. Fortunately, in the particular case of Fig. 4.5, the duration of the blocking cannot exceed the length of the critical section of . This situation is problematic, but difficult to avoid.

In more general cases, the situation can be even worse. This can be seen, for example, from Fig. 4.6.

Fig. 4.6Priority inversion with potentially large delay

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