# Understand physics performance issues

> Understand performance issues related to physics in your application.

The physics simulation runs on a separate fixed frequency update cycle from the main logic's update loop, and can only advance time via a [`Time.fixedDeltaTime`](/engine/6000.6/script-reference/unityengine/time/fixeddeltatime.md) per call. This is similar to the difference between [`Update`](/engine/6000.6/script-reference/unityengine/monobehaviour/update.md) and [`FixedUpdate`](/engine/6000.6/script-reference/unityengine/monobehaviour/fixedupdate.md). For more information, refer to the documentation on the [Time window](/engine/6000.6/manual/unity-editor/editor-settings-reference/comp-manager-group/class-time-manager.md).

When a heavy logic or graphics frame takes a long amount of time, the Profiler has to call the physics simulation multiple times per frame. This means that an already resource-intensive frame takes even more time and resources. This might cause the physics simulation to temporarily stop according to the **Maximum Allowed Timestep** value, which you can set in the Project Settings window (menu: **Edit > Project Settings > Time**)

To detect this in your project, select the [CPU Usage Profiler module](/engine/6000.6/manual/analysis/profiler/visualizing-data/cpu/cpu.md) and check the number of calls for **Physics.Processing** or **Physics.Simulate** in the Overview section in the **Hierarchy** view.


**CPU Usage Profiler with the value of 2 in the Calls column:**
![](/api/media?file=/engine/6000.6/media/images/profiler-physics-cpu-module.png)

In this example image, the value of 2 in the Calls column indicates that the physics simulation was called two times over the last logical frame.

A call count close to 10 might indicate an issue. As a first solution, reduce the frequency of the physics simulation, and if the issue continues, check what might have caused the heavy frame before the physics system had to use a lot of simulation calls to catch up with the game time. Sometimes, a heavy graphics frame might cause more physics simulation calls later on in a scene.

For detailed information about the physics simulation in your scene, select the search box at the top of the module details pane, search for **Physics.Processing**, and then select **Calls** from the dropdown at the top right of the pane. This displays the names of the physics system tasks that run to update your scene, such as the following:

* **Pxs:** PhysX solver, which are physics system tasks that joints require and resolving contacts for overlapping Rigidbody components.
* **ScScene:** Used for tasks that update the scene, run the broad phase and narrow phase, and integrate Rigidbody components (moving them in space due to forces and impulses).

## Additional resources

* [Physics Profiler module reference](/engine/6000.6/manual/physics-section/physics-overview/profiler-physics.md)
* [Physics Debug window reference](/engine/6000.6/manual/physics-section/physics-overview/physics-debug-visualization.md)