How Fast Restart Improves Iterative Simulations

In a typical simulation, when you simulate a model, the software:

1. Compiles the model

2. Simulates the model

3. Terminates the simulation

While developing a model, you typically simulate a model repeatedly as you iterate the design. For example, you might calibrate input values or block parameters for a particular response. Changing these values or parameters does not always require compiling the model before simulating again. However, in the classic workflow, each simulation compiles the model, even if the changes do not alter the model structurally. Each compile slows down the process and increases overall simulation time.

Fast restart allows you to perform iterative simulations without compiling a model or terminating the simulation each time. Using fast restart, you compile a model only once. You can then change values for root inports, values for model initial state, and From Workspace block data, tune parameters and root inputs and simulate the model again without spending time on recompiling. Fast restart associates multiple simulation phases to a single compile phase to make iterative simulations more efficient.

Use fast restart when your workflow does not require structural changes to the model. Also, fast restart is better suited if the workflow involves any of these factors:

• The model requires multiple simulations in which simulation inputs or parameters change in every iteration.

• The compile time of the model is several seconds or longer.

• You need to switch solvers during runtime to pick the appropriate solvers without having to go through an expensive recompilation process.

Fast restart is supported for all normal and accelerator simulation workflows and for programmatic and scripted rapid accelerator simulation workflows. For more information about using fast restart in rapid accelerator simulations, see Script Iterative or Batch Simulations Using Fast Restart.

Limitations

• Fast restart is supported for only programmatic and scripted rapid accelerator simulations. Fast restart is not supported for rapid accelerator simulations run from a user interface, such as the Simulink Editor, or for programmatic workflows that use the set_param function with the SimulationCommand name-value argument to start a simulation. (since R2025a)

• For model hierarchies that contain one or more referenced models configured to use a local solver, fast restart is supported by only normal mode simulations. For more information, see Use Local Solvers in Referenced Models. (since R2025a)

• When a model is in the reinitialized state, you cannot:

  • Make structural changes.

  • Make changes to nontunable parameters such as sample time.

  • Save changes to the model. You must turn off fast restart to save any changes to the model.

• Fast restart is not supported for models that contain blocks that do not support saving and loading model operating points. These blocks include:

  • SimEvents^® blocks from before R2016a

  • MATLAB^® function blocks that contain system objects

  • S-functions that do not implement the model operating point get and set methods but have Pwork vectors declared

  • From Multimedia File

  • To Multimedia File

  • Multipath Rician Fading Channel

  • Multipath Rayleigh Fading Channel

  • Derepeat

  • DC Blocker

  • Stack

  • Queue

  • Read Binary File

  • Write Binary File

  • Video Viewer

  • Frame Rate Display

  • Video From Workspace

  • Video To Workspace

• You cannot turn on fast restart in a model if it directly or indirectly references a model that simulates in accelerator mode and contains Simscape™ blocks.

• Between simulations, fast restart does not handle changes to design data, such as bus properties.

• Parameter tunability limitations apply. See Tunability Considerations and Limitations for Other Modeling Goals.

• The Fixed-Point Tool provides limited support when a model is simulated in fast restart. You must exit fast restart to collect simulation and derived ranges, and propose data types.

• When there are multiple model references to the same referenced model, you cannot change the model visibility when the model is in the reinitialized state.

• Fast restart is not compatible with the Simulink^® Profiler.

• When simulating a model in fast restart, you cannot run checks using Model Advisor.

• When you enable fast restart, the sim function supports only the single output Simulink.SimulationOutput form, regardless of the syntax you use.

• When you enable fast restart, you cannot pass non-tunable parameters as arguments to the sim function.

• When you simulate a model with a Subsystem Reference block in fast restart mode, any changes made in the subsystem file will not propagate to the model as they are structural changes.