Game ProgrammingDebugging & Performance Optimization

Nick Prühs

Objectives

• To get an overview of techniques for preventing bugs beforehand

• To learn how to track down and properly remove bugs from your code

• To understand possible performance bottlenecks

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Why you should alwaysstart debugging immediately• Code entropy says your code will get worse, all the

time, unless you actively invest in preventing that

• Broken windows theory says the worse your code is, the worse it will become

• Tracking down bugs is harder in a larger code base

• Tracking down bugs is harder in a buggy code base

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Code Quality Tools

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Code Quality Tools

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Code Quality Tools

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/// <summary>

/// Attaches the passed component to the entity with the specified id.

/// Note that this manager does not check whether the specified id is valid.

/// </summary>

/// <exception cref="ArgumentNullException">

/// Passed component is null.

/// </exception>

/// <exception cref="InvalidOperationException">

/// There is already a component of the same type attached.

/// </exception>

public void AddComponent(int entityId, IEntityComponent component)

{

if (component == null)

{

throw new ArgumentNullException("component");

}

if (this.components.ContainsKey(entityId))

{

throw new InvalidOperationException(

"There is already a component of type " + component.GetType() + " attached to entity with id "

+ entityId + ".");

}

this.components.Add(entityId, component);

this.OnComponentAdded(entityId, component);

}

You need a repro. Period.

How can you be sure you’ve fixed it?

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Stack Traces

Object reference not set to an instance of an object

at LifeApplication.Initializer.CreateManagers () [0x00488] in Initializer.cs:481

at LifeApplication.Initializer.OnLoad () [0x00016] in Initializer.cs:235

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What’s null here?

// Initialize progress manager.

var progressConfig = newProgressConfig(this.unityLoader.Version.Code);

if (this.config.Progress.Encrypt)

{

progressConfig.SetEncryption(

this.config.Progress.Encryption.EncryptKey,

this.config.Progress.Encryption.EncryptIv);

}

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What’s null here?

// Initialize progress manager.

var progressConfig = newProgressConfig(this.unityLoader.Version.Code);

if (this.config.Progress.Encrypt)

{

progressConfig.SetEncryption(

this.config.Progress.Encryption.EncryptKey,

this.config.Progress.Encryption.EncryptIv);

}

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What’s null here?

// Initialize progress manager.

var progressConfig = newProgressConfig(this.unityLoader.Version.Code);

if (this.config.Progress.Encrypt)

{

progressConfig.SetEncryption(

this.config.Progress.Encryption.EncryptKey,

this.config.Progress.Encryption.EncryptIv);

}

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What’s null here?

// Initialize progress manager.

var progressConfig = newProgressConfig(this.unityLoader.Version.Code);

if (this.config.Progress.Encrypt)

{

progressConfig.SetEncryption(

this.config.Progress.Encryption.EncryptKey,

this.config.Progress.Encryption.EncryptIv);

}

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What’s null here?

// Initialize progress manager.

var progressConfig = newProgressConfig(this.unityLoader.Version.Code);

if (this.config.Progress.Encrypt)

{

progressConfig.SetEncryption(

this.config.Progress.Encryption.EncryptKey,

this.config.Progress.Encryption.EncryptIv);

}

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What’s null here?

// Initialize progress manager.

var progressConfig = newProgressConfig(this.unityLoader.Version.Code);

if (this.config.Progress.Encrypt)

{

progressConfig.SetEncryption(

this.config.Progress.Encryption.EncryptKey,

this.config.Progress.Encryption.EncryptIv);

}

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Divide-and-conquer

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Divide-and-conquer

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Divide-and-conquer

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Divide-and-conquer

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Divide-and-conquer

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Divide-and-conquer

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Conditional Breakpoints

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Logging

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On-Screen

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Crash Dump Analaysis

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C:\Program Files\Procdump>procdump.exe -ma -i D:\Temp\Dumps

ProcDump v7.0 - Writes process dump files

Copyright (C) 2009-2014 Mark Russinovich

Sysinternals - www.sysinternals.com

With contributions from Andrew Richards

Set to:

HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug

(REG_SZ) Auto = 1

(REG_SZ) Debugger = "C:\Program Files\Procdump\procdump.exe" -accepteula -ma

-j "D:\Temp\Dumps" %ld %ld %p

Set to:

HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows NT\CurrentVersion\AeDebug

(REG_SZ) Auto = 1

(REG_SZ) Debugger = "C:\Program Files\Procdump\procdump.exe" -accepteula -ma

-j "D:\Temp\Dumps" %ld %ld %p

ProcDump is now set as the Just-in-time (AeDebug) debugger.

Crash Dump Analaysis

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C:\Program Files\Procdump>procdump.exe -u

ProcDump v7.0 - Writes process dump files

Copyright (C) 2009-2014 Mark Russinovich

Sysinternals - www.sysinternals.com

With contributions from Andrew Richards

Reset to:

HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug

(REG_SZ) Auto = <deleted>

(REG_SZ) Debugger = "C:\WINDOWS\system32\vsjitdebugger.exe" -p %ld -e %ld

Reset to:

HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows NT\CurrentVersion\AeDebug

(REG_SZ) Auto = <deleted>

(REG_SZ) Debugger = "C:\WINDOWS\system32\vsjitdebugger.exe" -p %ld -e %ld

ProcDump is no longer the Just-in-time (AeDebug) debugger.

Crash Dump Analaysis

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Deferencing a nullptr that will cause a crash

Crash Dump Analaysis

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Minidump File Summary in Visual Studio

Crash Dump Analaysis

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Debugging a Minidump in Visual Studio

Pair Programming

29 / 12Source: University of Utah, UIT

And if nothings helps …

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And if nothings helps …

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TRY AGAIN TOMORROW!

Some are really nasty …

• Remote systems

• Race conditions

• Mobile development, embedded systems, drivers

• “Release Mode only” bugs

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Quoting My Tutor

“If the bug is not where you expect it to be,

you better start looking for it where you’re not expecting it to be.”

- Hagen Peters

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Why you should neverstart optimizing immediately• Your code base will change over time, a lot, most

likely removing some of the code you’ve spent time on optimizing

• Optimized code tends to be hard to read• … and thus, hard to debug.

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Performance Optimization

1. Profile first!

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Performance Optimization

1. Profile first!

2. Profile again!

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Performance Optimization

1. Profile first!

2. Profile again!

3. Identify the bottlenecks: CPU vs. GPU vs. Memory

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Fighting CPU Bottlenecks

Pooling

Trades memory for CPU performance.

Re-uses objects to prevent costly construction and destruction.

Requires proper (cheap) reset of pooled objects.

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Fighting CPU Bottlenecks

Caching

Trades memory for CPU performance.

Stores computed values for later use.

Requires proper cache invalidation whenever the input changes.

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Fighting CPU Bottlenecks

Bucketing

Trades accuracy for CPU performance.

Distributes computations across multiple frames by dividing operation into multiple input sets.

Can only be applied if player doesn’t notice difference immediately (e.g. updating AI just twice per second).

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Memory Bottleneck

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Memory Bottleneck

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Memory Bottleneck

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GPU Bottleneck

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Frame Debugger

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Frame Debugger

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Frame Debugger

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Frame Debugger

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Frame Debugger

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Gotcha!

Always turn off logging before profiling!

Otherwise, disk I/O will lead to false results.

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Hint

If no native profiling tools are available (or applicable), you can always fall back to utility classes

such as System.Diagnostics.Stopwatch.

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References

• McShaffry. Debugging Your Game … or “That’s not supposed to happen!”. Austin Game Conference, 2003.

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Thank you!

http://www.npruehs.de

https://github.com/npruehs

@npruehs

dev@npruehs.de