Fix Stuttering in Unity: Optimize Garbage Collection

Posted by Gemma Ellison
./
August 2, 2025

Stutter-Free Unity: Taming the Garbage Collector

Stuttering in your Unity game is infuriating. One moment, everything’s smooth, the next, a jarring hitch throws off the player experience. Often, the culprit is the Garbage Collector (GC) working overtime. Let’s diagnose and fix that.

Understanding the Unity Profiler

The Unity Profiler is your best friend. Open it (Window > Analysis > Profiler) and run your game. Focus on the CPU Usage Timeline. Spikes labeled “GC Alloc” or “GC.Collect” indicate garbage collection. Dive deeper by selecting a spike and examining the “Hierarchy” or “Timeline” view to pinpoint the code responsible. Record your profiling notes in your game dev journal for later comparison.

Common Garbage Generation Culprits and Solutions

Several common coding practices lead to excessive garbage generation. Let’s tackle them one by one.

String Concatenation

Using the + operator to repeatedly concatenate strings creates new string objects each time, leaving the old ones for the GC.

Bad:

string message = "Hello";
message += ", ";
message += "World!";

Good:

using System.Text;

StringBuilder messageBuilder = new StringBuilder();
messageBuilder.Append("Hello");
messageBuilder.Append(", ");
messageBuilder.Append("World!");
string message = messageBuilder.ToString();

StringBuilder efficiently modifies strings in place, minimizing garbage.

Boxing/Unboxing

Boxing occurs when a value type (e.g., int, bool, struct) is implicitly converted to a reference type (e.g., object). Unboxing is the reverse. These conversions create temporary objects that the GC must clean up.

Bad:

ArrayList numbers = new ArrayList();
numbers.Add(5); // Boxing: int 5 is boxed into an object
int number = (int)numbers[0]; // Unboxing: object is unboxed into an int

Good:

List<int> numbers = new List<int>();
numbers.Add(5);
int number = numbers[0];

Using generic collections like List<int> avoids boxing/unboxing altogether.

Instantiating and Destroying Objects Frequently

Creating and destroying objects rapidly puts a strain on the GC. Object pooling reuses objects instead.

Bad:

GameObject newObject = Instantiate(prefab);
Destroy(newObject);

Good:

public class ObjectPool : MonoBehaviour
{
    public GameObject prefab;
    private Queue<GameObject> pool = new Queue<GameObject>();

    public GameObject GetObject()
    {
        if (pool.Count > 0)
        {
            GameObject obj = pool.Dequeue();
            obj.SetActive(true);
            return obj;
        }
        else
        {
            return Instantiate(prefab);
        }
    }

    public void ReturnObject(GameObject obj)
    {
        obj.SetActive(false);
        pool.Enqueue(obj);
    }
}

Use the GetObject() and ReturnObject() methods to manage your objects. Record the efficiency gains in your game development log.

Unnecessary Memory Allocation in Loops

Avoid allocating memory inside loops.

Bad:

for (int i = 0; i < 100; i++)
{
    Vector3 tempVector = new Vector3(i, i, i); // Allocation inside the loop
    // ... use tempVector
}

Good:

Vector3 tempVector = Vector3.zero; // Allocation outside the loop
for (int i = 0; i < 100; i++)
{
    tempVector.Set(i, i, i);
    // ... use tempVector
}

Camera.main

Accessing Camera.main repeatedly is slow because Unity uses FindGameObjectsWithTag under the hood. Cache the reference instead.

Bad:

void Update()
{
    Vector3 cameraPosition = Camera.main.transform.position; //called every frame
}

Good:

private Camera mainCamera;

void Start()
{
    mainCamera = Camera.main; //cache the camera
}

void Update()
{
    Vector3 cameraPosition = mainCamera.transform.position; //faster access
}

Structs vs. Classes

Classes are reference types, allocated on the heap and garbage collected. Structs are value types, often allocated on the stack. For small, frequently used data structures, structs can reduce garbage.

public struct Point
{
    public int x;
    public int y;
}

Use structs for simple data containers. Profile to confirm the performance benefit.

Leveraging System.GC

System.GC.Collect() forces a garbage collection. Avoid calling it every frame. Use it sparingly, perhaps after loading a new level or during a loading screen.

Track Your Progress

Remember to track all these optimizations in your game dev journal. Noting before-and-after performance using the Unity Profiler will give you concrete evidence of your improvements and guide your future optimization efforts. Experiment, analyze, and record what works best for your project.

Conclusion

Garbage collection stutter is a common problem, but with profiling and these techniques, you can significantly improve your game’s performance. Remember to profile frequently, identify garbage-generating hotspots, and apply appropriate optimization strategies. Consistent profiling and diligent record-keeping in your game dev journal are key to a smooth, stutter-free experience.