Simulation Stepping Options

Enable stepping back and configure number of time steps to move when stepping through simulation

Description

Use the Simulation Stepping Options dialog box to configure how many major time steps the simulation moves when you click Step Forward or Step Back and to enable stepping back. You can step backward in time in a simulation only after you enable stepping back using the Simulation Stepping Options.

You can adjust the simulation stepping options during simulation to balance simulation performance against the granularity of steps based on the runtime behavior of the system. For more information, see How Stepping Through Simulations Works.

Rapid accelerator simulations do not support stepping forward or backward. To step through a simulation, use normal or accelerator mode. When you want to use low-level debugging capabilities, such as stepping block by block, use normal mode.

For an example that shows all the options available for stepping through simulations, see Step Through Simulations Using the Simulink Editor. The table summarizes when and how you can use buttons in the Simulink^® Toolstrip to control the simulation execution.

Stepping Option	Description	Keyboard Shortcut	Availability
Continue	Continue simulation.	Ctrl+T or F5 On macOS, press command (⌘) instead of Ctrl.	Always available.
Step Forward	Advance the simulation by one or more time steps.	—
Step Back	Step simulation back by one or more time steps. To use this option, you must enable stepping back. For more information, see Simulation Stepping Options.	—	Available only when you enable stepping back using the Simulation Stepping Options dialog box.
Step Over	Advance simulation to just before the next block executes.	F10 (since R2023b) On macOS, press Shift+Command+O.	Available before starting a simulation and when a simulation is paused if Pause within time step is enabled in the Breakpoints List. (since R2024b) In R2024a: Available while simulation is paused if Pause within time step is enabled and the model has at least one active breakpoint. Before R2024a: Available only while paused within a time step.
Step In	Step into Model block, atomic subsystem, MATLAB Function block, or Stateflow® chart. When the next block to execute is not a Model block, atomic subsystem, or MATLAB Function block and the simulation is not paused on a Stateflow chart, the Step In button takes the same action as the Step Over button.	F11 (since R2023b) On macOS, press Shift+Command+I.	Available only when paused within a time step.
Step Out	Steps out of Model block, atomic subsystem, or Stateflow chart. When the next block to execute is not a Model block or atomic subsystem and the simulation is not paused on a Stateflow chart, the Step In button takes the same action as the Step Over button.	Shift+F11 (since R2023b) On macOS, press Shift+Command+U.

×

Open the Simulation Stepping Options

• Simulink Toolstrip: On the Simulation tab or the Debug tab, click the Step Back button.

• Simulink Toolstrip: On the Simulation tab or the Debug tab, click the Step Back button arrow and select Configure Simulation Stepping.

Examples

• Step Through Simulations Using the Simulink Editor
• How Stepping Through Simulations Works
• Debug Simulation Using Signal Breakpoints
• View Signal Values Using Port Value Labels

Parameters

Enable stepping back — Option to step backward in simulation

off (default) | on

Select this option to enable stepping backward as well as forward in time during simulation. When you select this option, you can configure how the software saves data during simulation to support stepping back.

Maximum number of saved back steps — Size of buffer that stores snapshots for stepping back

10 (default) | positive integer

Specify how many simulation snapshots to save to support stepping back. A simulation snapshot contains all the information required to restore the state of the simulation.

Simulation snapshots are stored in memory and are not accessible in the workspace during or after simulation. Saving more snapshots increases the amount of memory the simulation uses and can slow the speed of the simulation.

Dependencies

To enable this parameter, select Enable stepping back.

Interval between stored back steps — Number of major time steps between saved snapshots

10 (default) | positive integer

Specify the number of major time steps between each simulation snapshot the software saves to support stepping back.

Dependencies

To enable this parameter, select Enable stepping back.

Move back/forward by — Number of major time steps to step

1 (default) | positive integer

Specify the number of major time steps to step through each time you press the Step Forward or Step Back button.

Tips

• To inspect signal values while stepping through the simulation, display port value labels on signals. Select a signal. Then, on the Debug tab, under Tools, in the Output Values button group, click Show Output Value of selected signal .

• To start stepping through a normal or accelerator mode simulation that is already running, pause the simulation. Then, use the Step Forward and Step Backward buttons.

• To start stepping from a specific time within a simulation, specify a pause time. On the Debug tab, under Breakpoints, specify the Pause Time in seconds. When you start the simulation by clicking Run, the simulation runs until the specified time and pauses. From there, you can step through the simulation.

• To debug the behavior of a model around a point of interest, you can add a signal breakpoint and specify the condition such that the simulation pauses at the time of interest. Then, use the Step Forward and Step Backward buttons. For more information about setting breakpoints in your model, see Breakpoints List.

• The stepping and breakpoint options in the Simulink Editor do not have a direct programmatic equivalent available in the software. The Simulink debugging programmatic interface includes options for setting breakpoints and stepping through simulations. However, the behavior of these functions is not identical to the behavior of the stepping options in the Simulink Editor. For more information about programmatic debugging, see Simulink Debugging Programmatic Interface.

Version History

Introduced in R2012b

R2023a: Performance improvement for simulations with back stepping enabled

Enabling stepping back in simulation has a smaller effect on simulation performance. To support stepping back in simulation, the software captures simulation snapshots, which adds overhead and can slow simulation. The performance improvement for simulations with stepping back enabled varies between models, with smaller improvements for models that use certain blocks and features, including:

• Referenced models configured to simulate in accelerator mode

• Simscape™ blocks

• S-functions that implement custom methods for saving and restoring the S-function operating point

You can assess the performance improvement for a model by running equivalent simulations of the model in R2022b and R2023a with back stepping enabled and checking the execution time in the simulation metadata. For example, to assess the performance improvement for the model slexAircraftExample, follow these steps using R2022b and then R2023a:

1. Open the model.

   openExample(...
       "simulink_aerospace/AircraftLongitudinalFlightControlExample",...
       supportingFile="slexAircraftExample")

2. To capture timing information in the simulation metadata, including the time spent in the execution phase of the simulation, check that the Single simulation output parameter is enabled.

   In the Simulink Toolstrip, on the Modeling tab, click Model Settings. On the Data Import/Export pane, select Single simulation output.

3. Enable stepping back.

   On the Simulation tab, click Step Back. In the Simulation Stepping Options dialog box, select Enable stepping back.

4. In the Simulation Stepping Options dialog box, check that the Maximum number of saved back steps and Interval between stored back steps parameters both use the default value of 10. Then, click OK.

5. Use the same start and stop times for the simulation in R2022b and R2023a. For this simulation, use a stop time of 1000 seconds.

   On the Simulation tab, in the Stop time field, enter 1000.

6. To run the simulation, on the Simulation tab, click Run.

7. Check the execution time in the simulation metadata captured in the simulation output.

   execTime = out.SimulationMetadata.TimingInfo.ExecutionElapsedWallTime;

The table summarizes performance improvements for two example models by comparing the execution time between simulations run in R2022b and R2023a. The model asbhl20 shows a smaller performance improvement because the model uses features that involve custom operating point implementations, including a referenced model that simulates in accelerator mode. The simulations were timed on a Windows^® 10, AMD^® EPYC 74F3 @ 3.19 GHz test system.

Model Name	Simulation Duration	Approximate Execution Time in R2022b	Approximate Execution Time in R2023a	Approximate Execution Time Improvement
slexAircraftExample For more information about the model, see Aircraft Longitudinal Flight Control.	0 to 1000 seconds	2.3 s	1.0 s	56%
asbhl20 For more information about the model, see HL-20 Project with Optional FlightGear Interface (Aerospace Blockset).	0 to 61 seconds	760 s	650 s	14%

For more information about how stepping back and the simulation stepping options affect simulation performance, see How Stepping Through Simulations Works.