CS452 - Real-Time Programming - Spring 2011

Lecture 27 - Pathologies

Pubilic Service Announcement

  1. Train availability.
  2. Second train control demo. July 14.

Pathologies

As we go down this list both pathology detection and the time of an edit-compile-test cycle grow without bound.

1. Deadlock

2. Livelock (Deadly Embrace)

Definition

Two or more tasks are READY. For each task, the state of the other tasks prevents progress being made regardless of which task is ACTIVE.

A higher level of coordination is possible.

Two types of livelock exist

  1. Ones that are the result of bad coding
  2. Ones that are inherent in the application definition

Looking for solutions we prefer ones that avoid the central planner

Usually occurs in the context of resource contention

Livelock that's Really Deadlock

Solutions

  1. Make a single compound resourse, BUT
  2. Impose a global order on resource requests that all clients must follow.
  3. Create a mega-server that handles all resource requests

Real Livelock

Proprietor1 & proprietor2 fail the requests

Livelock that's Really a Critical Race

We could try to make the clients a little more considerate

while ( no resources ) {
   Send( prop1, get1es1, result );
   while ( result == "sorry" ) {
      Delay( ... );
      Send( prop1, getres1, result );
   }
   Send( prop2, getres2, result );
   if ( result == "sorry" ) Send( prop1, relres1, ... );
}

Inherent Livelock

Remember the example where two trains come face to face, each waiting for the other to move. They will wait facing one another until the demo is over, probably polling.

What's hard about solving this problem?

In real life,

What's most easy for you to do is to programme each engineer with

  1. detection, e.g.,
  2. work around, e.g.,

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.

Definition

The order in which computation is done is an important factor in determining whether or not it is successful.

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.

`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

Inherent Critical Races

Above we recommended solving livelock inherent to the train set by a

  1. detect
  2. back-off
  3. work around

strategy. With this strategy we don't know which train will back off, only that almost all the time only one train will back off. Thus, in the long run, there are three possible futures

  1. Train A backs off; train B follows through.
  2. Train B backs off; train A follows through.
  3. Both train A and train B back-off.

Which future occurs is the result of who wins a critical race. Yet this the existence of this critical race is not necessarily a bug. If all three futures accomplish the goal of your project, the critical race does not determine whether or not you achieve your goal, only the specific way in which the goal is achieved.

Watch the trains when such a critical race occurs. In my observation this is when they are most likely to look intelligent.

4. Time Lags

You can't send to the train controller a command like

You can only send a command like

When you send this command depends on your knowledge of the size of the delay times the velocity of the train. (There are ways you have probably already discovered where you don't need separate measurements of these two qualtities. Think about what stopping distance actually is.)

Hunting

Oscillation

5. 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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