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. The first simulation you run after enabling fast restart compiles the model. At the end of the simulation, the software skips the termination phase and reinitializes the simulation without compiling the model. While the model remains initialized for fast restart simulations, you can make these changes:

• Input data loaded using top-level input ports and From Workspace blocks

• The initial state of the model

• Values of run-to-run and runtime tunable parameters

Use fast restart to speed up simulation workflows when:

• You do not need to make structural changes to the model between simulations.

• You want to run multiple simulations of the same model while changing inputs or parameters in each simulation.

• Compiling the model takes several seconds or longer.

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. (since R2025a) 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.

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

• When a simulation is reinitialized in fast restart, you cannot:

  • Make structural changes to the model.

  • Change the values of nontunable parameters, such as sample times.

  • Save changes to the model. To save changes you make to the model while fast restart is enabled, first disable fast restart and then save 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 declare Pwork vectors

  • 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 enable 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.

• Parameters that affect the compilation of the model, such as sample times, dimensions, and data types, are not tunable while fast restart is enabled.

• The software issues a warning and ignores changes made to variables that define nontunable values while initialized in fast restart.

• Tuning the Initial Value parameter of a block is not supported while initialized in fast restart if the initial value is defined using a Simulink.Signal object.

• The Fixed-Point Tool provides limited support in fast restart simulations. To collect simulation and derived ranges and propose data types, you must disable fast restart.

• When the model contains multiple Model blocks that reference the same model, you cannot change which model reference instance has normal mode visibility while the top model is initialized in fast restart.

• Enabling and disabling the Simulink Profiler while initialized in fast restart is not supported. To use fast restart with the Simulink Profiler, enable fast restart, and then run profiling simulations using the Simulink Profiler. After you finish profiling simulation execution for the model, disable fast restart to terminate the profiling simulation. Disabling fast restart also disables the profiler. (since R2024a)

• Before R2024a: Fast restart is not supported in profiling simulations run using the Simulink Profiler.

• Running Model Advisor checks is not supported while initialized in fast restart.

• When you enable fast restart, the sim function always returns a single Simulink.SimulationOutput object, regardless of the syntax you use.

• Changes made in subsystem files for Subsystem Reference blocks do not propagate to the model while initialized in fast restart.