↑ Real Time Systems: Notes

Sporadic Server Example and Review for Exam

 

Topics to be Covered on Midterm Examination

(in priority order)

  1. Fixed-priority aperiodic servers: polling, deferrable, sporadic
    1. Can you apply them to an example?
    2. Can you find the formula for the worst-case interference caused by a server?
  2. Fixed-priority and EDF schedulability analysis results
    1. Do you know the utilization bounds for RM and EDF?
    2. Can you apply them?
    3. Can you derive the EDF utilization bound?
    4. Can you derive the fixed-priority (RM) response-time equation?
  3. Execution times
    1. Do you know the sources of execution time variation?

Topics to be Covered on Midterm Examination

The exam questions on Sporadic Server will be designed in a way that should permit answers based on any of the variants discussed in the text.

However, if in doubt, please try to preserve the fundamental property that the servers should not be able to cause any greater interference for lower priority tasks than would a periodic task with periodi ps and execution time es.

See the newly updated notes on sporadic servers, and the example below, for more information.

Old Exams, Homework, and Quizzes

  1. No excuse for missing anything on the prior midterm examination and homeeork for this year, or posted exams and quizzes from prior years!
  2. Homework from 2008, with solutions:
  3. Prior midterms exams:
  4. Quizzes from prior years:
  5. Do not assume the exam will be the same, or even similar to previous ones!
  6. Trust the outline of topics to be covered, above.

Sporadic Server Example from Text

The following are diagrams showing solutions to various parts of exercise 7.1 in Jane Liu's text. The sporadic server examples are for my own sporadic server variant given in the web notes, under "How to Correct the Definition?".

Sporadic Server with ps = 2 and es = 0.5

fig

Note that whenever the system is idle we can replenish all sporadic server budgets. This is safe because in our analysis of schedulabililty we can focus on just one busy interval at a time.

Deferrable Server with ps = 2 and es = 0.5

fig

In this examle I assume that the sporadic server stats at time zero, and so its replenishments occur at times 2.5, 5, 7.5, 10, etc.

Sporadic Server with ps = 1 and es = 0.25

fig

In the case indicated by "*" the normal replenishment coincides with an idle point, so that replenishment could be classified either way.

Deferrable Server with ps = 1 and es = 0.25

fig

Sporadic Server with ps = 3 and es = 0.75

fig

Note that because we are using RM priorities, the priority of the server drops below that of the task with period 2.5, now that the server has period 3.

Deferrable Server with ps = 3 and es = 0.75

fig
© 2006 T. P. Baker ($Id: wk10t.html,v 1.1 2008/08/25 11:18:48 baker Exp $)