Get Started with Fast Restart
When you need to simulate a model iteratively to tune parameters, achieve a desired response, or automate testing, use fast restart to avoid compiling again. Fast restart allows you to perform iterative simulations without compiling a model or terminating the simulation each time. A generic workflow using fast restart involves the following steps.
Turn on fast restart using the Fast Restart button on the Simulink® Toolstrip or from the command line.
Simulate the model. The first simulation requires the model to compile, initialize and save a
ModelOperatingPointobject. Once the simulation is complete, it does not terminate. Instead, the model is initialized again in fast restart.
Perform any of these actions:
Change tunable parameters.
Switch between variable-step solvers.
Tune root-level inputs.
Modify base workspace, model workspace variables and data dictionary entries that are referenced by tunable parameters.
Change inputs to From File and From Workspace blocks.
Change the Initial state parameter for the next simulation.
Using the Signal Builder block, change data, rename signals and signal groups, and add new groups.
Change the signal logging override values for the model programmatically by using the
set_paramcommand and the
DataLoggingOverrideparameter. See Override Signal Logging Settings from MATLAB.
Once you have initialized a model in fast restart, you cannot
Change the dimension, type, or complexity of a signal or variable.
Make changes to a nontunable parameter such as sample time.
Make structural changes such as adding or deleting blocks or connections.
These changes require you to compile the model again. To make changes like these, turn off fast restart, make your changes, and repeat this procedure.
You do not need to recompile the model if you change the solver. You can pick appropriate solvers during runtime without having to go through an expensive recompilation process.
Simulate the model. The model uses the new values of parameters and inputs that you provided but does not compile again.
Once you have achieved the desired response, turn off fast restart.
When you turn off fast restart, Simulink does not store any compile information for the model. The model compiles when you next simulate the model.
Fast restart is not supported when
Use local solver when referencing modelis enabled in the Solver Pane.
Prepare a Model to Use Fast Restart
Before you simulate a model in fast restart, ensure that the model meets these requirements:
If you have enabled callbacks in the model, make sure they do not attempt to make structural changes when the model is reinitialized. For example, callbacks such as mask initialization commands get called at the beginning of each simulation. Therefore, avoid using mask initialization code that makes structural changes to the model.
All blocks in the model must support
The simulation mode is Normal or Accelerator mode.
When fast restart is on, you cannot save changes to the model after it compiles. Saving changes requires Simulink to discard information about the compiled state. To save any changes to the model, turn off fast restart first.
Enable Fast Restart
Use one of these methods to enable fast restart:
Click the Fast restart button on the Simulink Editor toolbar.
At the MATLAB® Command prompt, use
set_paramto enable fast restart. Type
Simulate a Model using Fast Restart
After you load your model and turn on fast restart, simulate the model.
Simulate the model by calling
simor clicking the Run button in the Simulink Editor toolstrip. The first simulation in fast restart requires the model to compile and save a
Once the simulation is complete, the status bar shows that the model is compiled in fast restart.
While fast restart is in progress, the simulation status becomes
Restarting, and the Simulink Editor becomes frozen like it is during model compilation and initialization.
Adjust tunable parameters in the model, such as the gain value of a Gain block, or tune root-level input values. You can also make changes to base workspace variables. You cannot adjust nontunable parameters such as sample time, because doing so requires the model to compile once more.
Simulate the model again. This time, the model does not compile. When you click the Play button or step forward, Simulink updates blocks that have new values as well as blocks that reference workspace variables.
When you are satisfied with your results, turn off fast restart by clicking the Fast restart button off.
To keep your changes, save the model.
After a model is initialized in fast restart, Simulink displays a warning if you attempt to make a structural change to the model. To make such changes, you must turn off fast restart.
Stop a Simulation
When you click Stop in the middle of a fast restart simulation:
Simulation does not terminate.
The model is in the initialized state.
You can now change tunable parameters in the model
You can terminate the simulation and exit fast restart by clicking the Fast restart button off.
Exit Fast Restart
You can exit fast restart only when the model is in the initialized state. After simulating, click the Fast restart button off. To do this programmatically, type:
Simulink terminates simulation.
Simulink discards any compiled information about the model.
The model must compile again the next time you simulate.
Fast Restart Methodology
Tuning Parameters Between Simulations
When a model is initialized in fast restart, in addition to block values and base workspace variables, you can tune parameters in the Data Import/Export and Solver panes in the Configuration Parameters dialog box (on the Simulation tab, under Prepare, click Model Settings).
Certain parameters are tunable between simulations only when the model is initialized in fast restart. They include:
Initial Value parameter of the IC block
Initial Output parameter of the Merge block
Data parameter of the From Workspace block
Signal parameter and signal groups of the Signal Builder block.
Switching Solvers Between Simulations
When fast restart is on, you can switch solvers between simulations. Only
valid solvers that can be switched to are listed in the
Solver list in the Configuration Parameters
dialog box. If you switch solvers using
between simulations and set to an invalid solver, you will see a warning and the
invalid solver will be ignored.
Model Methods and Callbacks in Fast Restart
When fast restart is on, Simulink calls model and block methods and callbacks as follows:
Use the ssIsFirstInitCond flag to guard code that should run only during the initialization phase of any simulation, including the first and subsequent initializations in fast restart.
Call model and block
Steps 1–5 apply to all simulations in Simulink (with or without fast restart).
For the first simulation in fast restart, capture a simulation snapshot. A simulation snapshot contains simulation state (
ModelOperatingPoint) and information related to logged data and visualization blocks. As part of the snapshot capture, call
This is a standard execution phase of any simulation, with or without fast restart.
Repeat these steps in a loop until stop time or a stop is requested.
After simulation ends, call model and block
StopFcncallbacks. This is a standard phase of any simulation, with or without fast restart.
Restore the simulation snapshot taken for fast restart. As part of the restore, call
Wait in a reinitialized state until one of these actions:
To run another simulation (programmatically or using the Simulink Editor) in fast restart, return to step 3.
To end Fast Restart mode and uncompile the model:
Call the model method
Do not call
StopFcncallbacks again at this point.
In some cases. the
methods are only called once and not for each successive Fast Restart
simulation. In these cases, these method calls are combined with calls to
CleanupRuntimeResources, respectively. These cases are as follows:
When an S-function contains custom
ModelOperatingPointsave and restore methods.
When an S-function sets the flag
When an S-function is placed inside the accelerated mode of a referenced model.
For more information on model callbacks, see Callbacks for Customized Model Behavior.
Operating Point and Initial State Values
You can change initial state values, including
ModelOperatingPoint, in between fast restart simulations.
ModelOperatingPoint object for initial state is used
in fast restart, every new simulation resets to the start time of the model and
not the snapshot time of each
Thereafter, on the first step forward, Simulink checks to see if a
ModelOperatingPoint has been
specified. If yes, Simulink restores it before computing the next step. Thus, the first
simulation step effectively fast forwards to the snapshot time of the specified
Analyze Data Using the Simulation Data Inspector
Fast restart supports data logging using the Simulation Data Inspector. Every
simulation in fast restart creates an SDI object with the name
<modelname> fast restart run <number>. The
number increments for each simulation.
Custom Code in the Initialize Function
When you place custom code in the Configuration Parameters > Simulation Target > Custom Code > Initialize function pane in the Model Configuration Parameters dialog box, this gets called only during the first simulation in fast restart.