FixedUpdate vs Update in Unity - Performance Comparison

Performance: FixedUpdate vs Update

MEDIUM IMPACT

This concept affects the smoothness and responsiveness of game frame rendering and physics calculations.

Updating physics calculations in a Unity game

Unity

void FixedUpdate() {
    // Physics movement code here
    rb.AddForce(force);
}

FixedUpdate runs at a fixed timestep, ensuring consistent physics calculations and smooth motion.

📈 Performance GainStable physics simulation with consistent frame pacing improves perceived smoothness.

Updating physics calculations in a Unity game

Unity

void Update() {
    // Physics movement code here
    rb.AddForce(force);
}

Physics updates in Update cause inconsistent physics steps because Update runs at variable frame rates.

📉 Performance CostCauses jittery physics and uneven frame pacing, leading to poor user experience.

Performance Comparison

Pattern	Frequency	Consistency	Input Responsiveness	Verdict
Physics in Update	Variable (per frame)	Inconsistent physics steps	Good	[X] Bad
Physics in FixedUpdate	Fixed timestep	Consistent physics steps	N/A	[OK] Good
Input in FixedUpdate	Fixed timestep	May miss inputs	Poor	[X] Bad
Input in Update	Variable (per frame)	Captures all inputs	Excellent	[OK] Good

Rendering Pipeline

Unity's rendering pipeline separates physics updates (FixedUpdate) from frame rendering (Update). FixedUpdate runs at fixed intervals for physics calculations, while Update runs every frame for rendering and input.

→Physics Simulation

→Game Logic Update

→Rendering

⚠️ BottleneckRunning physics in Update causes inconsistent physics steps, leading to jitter and uneven frame pacing.

Optimization Tips

1Use FixedUpdate for physics to keep simulation stable and consistent.

2Use Update for input and rendering to ensure responsiveness and smooth visuals.

3Avoid mixing physics code in Update to prevent jitter and uneven frame pacing.

Performance Quiz - 3 Questions

Why should physics updates be done in FixedUpdate instead of Update?

A Because FixedUpdate runs at a fixed timestep ensuring consistent physics simulation

B Because FixedUpdate runs every frame for smooth rendering

C Because Update cannot access physics components

D Because FixedUpdate is faster than Update

DevTools: Unity Profiler

How to check: Open Unity Profiler, record gameplay, and check CPU usage for FixedUpdate and Update calls. Look for spikes or inconsistent timings.

What to look for: Consistent FixedUpdate timings indicate stable physics; irregular Update timings may affect input responsiveness.

Practice

1. What is the main difference between Update and FixedUpdate in Unity?

easy

A. Both run at the same time but handle different components.

B. Update runs every frame; FixedUpdate runs at fixed time intervals.

C. FixedUpdate runs every frame; Update runs at fixed time intervals.

D. Update is only for physics; FixedUpdate is for animations.

Solution

1. Step 1: Understand the timing of Update

   Update is called once per frame, so its timing depends on the frame rate.

2. Step 2: Understand the timing of FixedUpdate

   FixedUpdate is called at fixed intervals, independent of frame rate, ideal for physics calculations.

3. Final Answer:

   Update runs every frame; FixedUpdate runs at fixed time intervals. -> Option B

4. Quick Check:

   Update = frame-based, FixedUpdate = fixed interval [OK]

Hint: Remember: Update = frame, FixedUpdate = fixed time [OK]

Common Mistakes:

• Confusing which method runs at fixed intervals
• Thinking FixedUpdate depends on frame rate
• Assuming Update is for physics only

2. Which of the following is the correct syntax to declare the FixedUpdate method in a Unity C# script?

easy

A. void FixedUpdate() { /* code here */ }

B. void fixedUpdate() { /* code here */ }

C. void Fixedupdate() { /* code here */ }

D. void Fixed_Update() { /* code here */ }

Solution

1. Step 1: Check method name capitalization

   Unity requires exact method names with correct capitalization; FixedUpdate starts with capital F and U.

2. Step 2: Verify method signature

   The method must be declared as void FixedUpdate() with no parameters.

3. Final Answer:

   void FixedUpdate() { /* code here */ } -> Option A

4. Quick Check:

   Correct capitalization and signature = void FixedUpdate() { /* code here */ } [OK]

Hint: Method names are case-sensitive; use exact spelling [OK]

Common Mistakes:

• Using wrong capitalization like fixedUpdate
• Adding underscores in method names
• Adding parameters to FixedUpdate

3. Consider this Unity C# script snippet:

void Update() {
    Debug.Log("Update called");
}

void FixedUpdate() {
    Debug.Log("FixedUpdate called");
}

If the game runs at 60 frames per second and the fixed timestep is 0.02 seconds, which output will appear more frequently in the console?

medium

A. Both will appear the same number of times.

B. "FixedUpdate called" will appear more frequently.

C. "Update called" will appear more frequently.

D. Neither will appear because Debug.Log is disabled.

Solution

1. Step 1: Calculate Update calls per second

   At 60 FPS, Update runs 60 times per second.

2. Step 2: Calculate FixedUpdate calls per second

   Fixed timestep is 0.02 seconds, so FixedUpdate runs 1/0.02 = 50 times per second.

3. Final Answer:

   "Update called" will appear more frequently. -> Option C

4. Quick Check:

   60 (Update) > 50 (FixedUpdate) calls per second [OK]

Hint: Update frequency = FPS; FixedUpdate frequency = 1/fixedDeltaTime [OK]

Common Mistakes:

• Assuming FixedUpdate runs more often than Update
• Ignoring fixed timestep value
• Confusing frame rate with fixed update rate

4. You wrote this code to move a Rigidbody smoothly:

void Update() {
    rb.AddForce(Vector3.forward * forceAmount);
}

But the movement is jittery. What is the best fix?

medium

A. Move the AddForce call into FixedUpdate.

B. Increase forceAmount in Update.

C. Call rb.MovePosition inside Update instead.

D. Disable physics in Rigidbody.

Solution

1. Step 1: Identify physics update timing

   Physics updates happen in fixed time steps, so applying forces in Update causes inconsistent timing and jitter.

2. Step 2: Move physics code to FixedUpdate

   Calling AddForce inside FixedUpdate matches physics timing and smooths movement.

3. Final Answer:

   Move the AddForce call into FixedUpdate. -> Option A

4. Quick Check:

   Physics code belongs in FixedUpdate [OK]

Hint: Put physics code in FixedUpdate to avoid jitter [OK]

Common Mistakes:

• Applying physics forces in Update
• Trying to fix jitter by increasing force
• Disabling physics instead of fixing timing

5. You want to create a smooth player movement that responds to input and uses physics. Which is the best way to organize your code?

hard

A. Apply forces in Update, ignore input.

B. Read input and apply forces both in Update.

C. Read input and apply forces both in FixedUpdate.

D. Read input in Update, apply movement forces in FixedUpdate.

Solution

1. Step 1: Understand input timing

   Input should be read every frame in Update to avoid missing user actions.

2. Step 2: Understand physics timing

   Physics forces should be applied in FixedUpdate for consistent physics simulation.

3. Step 3: Combine input and physics correctly

   Store input values in Update, then use them in FixedUpdate to apply forces smoothly.

4. Final Answer:

   Read input in Update, apply movement forces in FixedUpdate. -> Option D

5. Quick Check:

   Input in Update + physics in FixedUpdate = smooth movement [OK]

Hint: Input in Update, physics in FixedUpdate for smooth control [OK]

Common Mistakes:

• Reading input in FixedUpdate causing missed inputs
• Applying physics in Update causing jitter
• Ignoring input or physics timing differences