CS452 - Real-Time Programming - Fall 2010

Lecture 25 - Pathologies

Public Service Announcements

  1. Due date for Tracking 1
  2. Comment on halting the CPU

Pathologies

1. Deadlock

One or more tasks will never run again. For example

  1. Task sends to itself (local: rest of system keeps running, task itself will never run)
  2. Every task does Receive( ) (global: nothing is running)
  3. Cycle of tasks sending around the cycle (local: other tasks keep running)

Kernel can detect such things

Potential deadlock can be detected at compile time

Solutions

2. Livelock (Deadly Embrace)

Usually occurs in the context of resource contention. For example

Kernel(s) cannot easily detect livelock

Possible solutions

  1. package resource combinations together in one proprietor
  2. global order on resource requests
  3. ethernet algorithm

Could consider this a form of critical race.

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 more slowly, and D gets a different resource before C .
  8. You shuffle priorities forever, eventually reverting to leave in the debugging IO.

Theory of relativity and the event horizon.

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.

`Drastically' means chaos in both senses of the term

  1. Sense one: a small change in the initial conditions produces an exponentially growing change in the system
  2. Sense two: exercise for the reader.

Solutions

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

4. Performance

The hardest problem to solve

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

Layered abstraction are costly

e.g. Notifier -> SerialServer -> InputAccumulater -> Parser -> TrackServer

Hardware

  1. Turn on optimization, but be careful
  2. Turn on caches

Size & align calibration tables by size & alignment of cache lines


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