CS452 - Real-Time Programming - Spring 2017

Lecture 27 - Pathologies II.

Public Service Annoucements

  1. Train Control II demo on Tuesday, 18 July.
  2. The exam will start at 19.30, July 28, 2017 and finish at 22.00, 29 July 2017.


Pathologies

1. Deadlock

One or more tasks will never run again.

2. Livelock (Deadly Embrace)

Definition

All tasks continue running; no progress is made. This is something like an infinite loop.

3. Critical Races

Example

  1. Two tasks, A & B, at the same priority
  2. A is doing a lot of debugging IO
  3. B always reserves a section of track before A, and all is fine.
  4. Debugging IO is removed
  5. A reserves the section before B can get it, and execution collapses.
  6. Lower priority of A to the same level as C.
  7. Now C executes faster and gets a resource before D .
  8. You shuffle priorities forever, eventually reverting to put back in the debugging IO.
But, sometimes you can't return to the running with excessive I/O state because too much has changed.

Definition

The order in which computation is done is an important factor in determining whether or not it is successful. Without knowing it you have created a program the correctness of which is execution order dependent.

Critical races, like Livelock can be

Symptoms

  1. Small changes in priorities change execution unpredictably, and drastically.
  2. Debugging output changes execution drastically.
  3. Changes in train speeds change execution drastically. How do you tell this apart from a bad calibration? (Your application knows where the train is and it doesn't help.)

Solutions

  1. Explicit synchronization
  2. Gating is a technique of global synchronization

4. Performance

Changes in performance of one task with respect to another often give rise to critical races

The hardest problem to solve

In practice, how do you know you have performance problems? Problems I have seen:

Priority

The hardest thing to get right

Problems with priority

  1. Priority inversion
  2. One resource, many clients
  3. Tasks try to do too much

Congestion

  1. Too many tasks trying to run at once (long ready queues).

Output

  1. Too much terminal output interferes with train controller communication
  2. Requests to poll the sensors get backed up in the serial server, or whoever provides output buffering.

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