Helper Loop timing

What’s a Helper Loop?

A Helper Loop, in the DQMH framework context, is the name given to a loop executing code periodically through the Timeout case of an event structure.

You can implement a Helper Loop even if you are not using the DQMH framework

A Helper Loop is composed of the following elements:

A while loop.
An event structure with the Timeout case.
A non-negative value set to the Timeout input of the event structure.

The code you want to execute periodically is put into the Timeout case.

Figure 1. A basic Helper Loop

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How the the Timeout case execution is triggered

The Timeout case is executed when no events are detected during the time set to the Timeout input of the event structure. The Timeout counting starts when the event structure is called. The time spent executing code in the Timeout case and after the event structure execution is delaying the next Timeout count. Any event occurring reset the Timeout count. The Timeout case can be rarely (or never) executed when the event structure handles other events.

That’s why, setting a value to the Timeout input of the event structure is not enough to get a steady execution period of the Timeout case.

How the Helper Loop timing API ensure a steady period

Using the API to time your event structure allows you to:

get a steady execution period in any case.
know you are late executing the Timeout case.

The example provided demonstrates the different issues that can occur.

Figure 2. Example code

The Helper Loop are set to execute the Timeout case every 250 ms

Timeout reset

1	Hitting the Reset Timeout button triggers an event in the event structure.
2	In the incorrectly timed Helper Loop, the execution time is impacted by the reset.
3	In the correctly timed Helper Loop, the `Timeout` value is modified to take account of the reset.
4	The execution time is not impacted.

Figure 3. Timeout reset issue

You can see that the incorrectly timed Helper Loop executes fewer times than the correctly timed one.

Timeout case time execution

1	We simulate a code execution taking 100 ms.
2	In the incorrectly timed Helper Loop, the execution time is impacted by this code execution time.
3	In the correctly timed Helper Loop, the `Timeout` value is modified to take account of the time spent since the beginning of the `Timeout` case execution.
4	The execution time is not impacted.

Figure 4. Timeout case time execution

Late Timeout case execution

Even if you time your Helper Loop correctly, you can get late execution of the Timeout case. Indeed, if the code that executes after the beginning of the Timeout case execution takes more time than the desired period, there’s nothing to do.

The API provides a simple way to monitor if your Helper Loop runs at the expected period.

1	We simulate a code execution taking more than the desired period.
2	The period execution cannot be reached.

Figure 5. Late Timeout case execution