In the last 15 years at the MathWorks I have been asked about once per month “how do I know what the execution order of my XXXX is”. If you come a textual language execution order is directly specified by the order in which you write the lines of code.

1. My Function
1. Do A
2. Do B
3. Do C

With multiple functions, a threaded OS or event driven interrupts this is more complex, but at its’ heart it is directly specified in the language. In contrast Simulink determines execution order based on Data Flow; so…

Which executes first, Chicken or Egg?

The principal of data flow based execution is that calculations are performed once data is present. We will start with the simplest example possible, one path direct feed through.

In this example we have a data flow from left to right (Inport #1 to Outport #1). The calculation is
Output = Input * Chicken * Egg

In this example we have introduce a Unit Delay block. This changes the execution order
Output = LP * Egg
LP = Input * Chicken

In this case the “Egg” calculation takes place first since we have existing information from the unit delay allowing calculation of the output value.

In our next example the execution order is arbitrary; since there is no data dependencies between “Chicken” and “Egg” output 1 or output 2 could be calculated first. (Note: Simulink provides a “Sorted Execution order”; these are the red numbers you see in the image. The lower numbers are executed first.

Controlling execution order (avoiding egg on your face)

In the last example we showed that with independent data flows execution order is resolved by Simulink; however, there are instances where the execution order will matter (1). The “obvious” and very wrong solution is to add a data dependency to the path.

By adding a “Zero value gain” and sum block to the “Egg path” I have forced the “Chicken path” to be executed first (4). For reasons that should be obvious this is a bad idea. Anyone who looked at the model would think “Why are they doing that” and they would be correct in asking that question. The recommended approach is to make the execution order explicite using a Function driven approach

In this case the “Egg” is executing second; this is known from the “B” number in the block, “ChickenPath” is BO, while “EggPath” is B1; the lower number executes first. For more complex execution orders a Stateflow chart can be used to define the execution order

In this “tongue in beak” example we see that “eggs” only execute during the “WeHaveEggs” and the “Dinosaur” states. Once we hit the “WeHaveBoth” state (after many eons) the Chicken executes first. In my next post I will give examples of best practices for controlling execution order with Stateflow charts.

Footnotes:

1. Within the subsystem the execution order does not matter. However there are several cases where it can matter.
1. Time limited execution: If the given function has a limited time for execution allocated and it is possible that all of the calculations may not be able to be performed in that time period. In that case you would want to control the execution order
2. Consumption by parallel process: If the data from one (or more) of the paths is used by a parallel process and that process needs data “first” then you want to control the execution process.
3. Determinism: For some analysis locking down execution order will simplify the execution task.
2. I debated even showing this image, however I have seen many “cleaver”(3) engineers come up with this solution.
3. Yeah, I realized I was autocorrected to “cleaver” as in something that cuts or chops . It was an error at first. Then I realized that I liked that more, they are not really “clever” rather they are just chopping apart a problem.
4. This could be considered a form of “playing chicken”