DEEP DUMPSTER DIVING

A close look at .Net garbage collection

Ronn Black

October 2009

Why should I care?

Demo 1 (Word Count)using System;using System.Collections.Generic;using System.Diagnostics;using System.IO;

public class MyClass{

public static void RunSnippet(){

while(true){

Stopwatch watch = new Stopwatch();watch.Start();StreamReader sr = new StreamReader(@"C:\Users\Ronn\Documents\My

Code Snippets\Garbage Collection\catcher.txt");string text = sr.ReadToEnd();int wordCount = text.Split().Length;Console.WriteLine("{0} Words", wordCount);watch.Stop();Console.WriteLine(watch.ElapsedMilliseconds + " Milliseconds");

}

}

using System;using System.Collections.Generic;using System.Threading;using System.Runtime.CompilerServices;

public class MyClass{

static Byte[] bytes;public static void RunSnippet(){

Timer tmr = new Timer(M, null, 0, 1000);Thread.Sleep(20);for(int i = 0; i < 1000; i++)

bytes = new Byte[2000];

Console.ReadLine();}

static void M(object state){

Console.WriteLine("M - " + DateTime.Now);}

Demo 2

Unmanaged Memory Management

Unused AreaHi Address

Low Address

ReservedReserved

Args & VariablesArgs & Variables

StackStack

HeapHeap

Stack PointerUnused Area

NextObjPtr

NextObjPtr

RootsRoots

NextObjPtr

RootsRoots

NextObjPtr

~MyClass(){

//Do work here…}

MyClass.Finalize(){

//Do work here…}

=

Finalizers

RootsRoots

F

G

A

B

C

E

H

I

DFreachable Queue

C

E

F

I

Finalization Queue

F

G (x)

A

RootsRoots

B (x)

C

E (x)

H (x)

I (x)

DFreachable Queue

C

E (x)

F

I (x)

Finalization Queue

F

A

RootsRoots

C

E (x)

I (x)

D

E (x)

I (x)

Freachable Queue

C

F

Finalization Queue

Optimizations Generations

Newly created objects tend to have short lives. The older an object is, the longer it will survive. Groups objects by age and collects younger objects more

frequently than older objects. All objects added to heap are in generation 0. When an object survives the first garbage collection it is promoted

to generation 1. When garbage collection is triggered survivors from generation 1

are promoted to generation 2 and generation 0 survivors are promoted to gen 1.

As objects "mature", they are moved to the next older generation until they reach gen 2.

using System;using System.Collections.Generic;using System.Threading;using System.Runtime.CompilerServices;

public class MyClass{

static Byte[] bytes;public static void RunSnippet(){

Timer tmr = new Timer(M, null, 0, 1000);Thread.Sleep(20);for(int i = 0; i < 1000; i++)

bytes = new Byte[2000];

Console.ReadLine();}

static void M(object state){

Console.WriteLine("M - " + DateTime.Now);}

Demo 3 – WTF??

Demo 4 (CLR Profile Word Count)

using System;using System.Collections.Generic;using System.Diagnostics;using System.IO;

public class MyClass{

public static void RunSnippet(){

while(true){

Stopwatch watch = new Stopwatch();watch.Start();StreamReader sr = new StreamReader(@"C:\Users\Ronn\Documents\My

Code Snippets\Garbage Collection\catcher.txt");string text = sr.ReadToEnd();int wordCount = text.Split().Length;Console.WriteLine("{0} Words", wordCount);watch.Stop();Console.WriteLine(watch.ElapsedMilliseconds + " Milliseconds");

}

}

IDisposable public class MyClass : IDisposable

{

public void Dispose()

{

Dispose(true);

GC.SuppressFinalize(this);

}

protected virtual void Dispose(bool disposing)

{

if (!disposed)

{

if (disposing)

{

// Dispose managed

resources. Ex: Components.Dispose();

}

// Release ONLY unmanaged

resources. Ex: CloseHandle(handle);

}

disposed = true;

}

protected volatile bool disposed = false;

~MyClass()

{

Dispose(false);

}

}

[ComVisible(true)]

public interface IDisposable

{

void Dispose();

}

Usingusing System;

using System.Collections.Generic;

using System.Diagnostics;

Usingusing System;

using System.Collections.Generic;

using System.Diagnostics;

using (MyClass c = new MyClass())

{

//Do Some Work

}

Demo 5 - optimizeusing System;using System.Collections.Generic;using System.Diagnostics;using System.IO;

public class MyClass{

public static void RunSnippet(){

while(true){

Stopwatch watch = new Stopwatch();watch.Start();using(StreamReader sr = new StreamReader(@"C:…Garbage Collection\

catcher.txt")){

string line = "";int wordCount = 0;while((line = sr.ReadLine()) != null){

wordCount += line.Split().Length;}Console.WriteLine("{0} Words", wordCount);

}watch.Stop();Console.WriteLine(watch.ElapsedMilliseconds + " Milliseconds");

}}

Total Relocated Final Gen 0 Gen 1 Large Object Heap6,578,038 96,608 5,057,272 1,400,580 12 3,535,4645,473,972 101,501 1,441,201 2,097,172 103,992 9,328======== ======== ======== ========= ======== ===========-1,104,066 +4,893 -3,617,071 +696,592 +103,980 -3,526,136

Types of Memory LeaksManaged leaks – persistent object holds

a reference to an object expected to be garbage collected.

Unmanaged leaks - using unmanaged resources, dll’s or COM objects that leak memory.

Demo 7 - Leaky program?

public partial class ChildForm : Form { public ChildForm() { InitializeComponent(); }

private void ChildForm_Load(object sender, EventArgs e) { Program.Skin.PropertyChanged += new PropertyChangedEventHandler(Skin_PropertyChanged); }

void Skin_PropertyChanged(object sender, PropertyChangedEventArgs e) { this.BackColor = Program.Skin.BackgroundColor; this.label1.ForeColor = Program.Skin.ForegroundColor; } }

public partial class ChildForm : Form { public ChildForm() { InitializeComponent(); }

private void ChildForm_Load(object sender, EventArgs e) { Program.Skin.PropertyChanged += new PropertyChangedEventHandler(Skin_PropertyChanged); }

void Skin_PropertyChanged(object sender, PropertyChangedEventArgs e) { this.BackColor = Program.Skin.BackgroundColor; this.label1.ForeColor = Program.Skin.ForegroundColor; }

protected override void OnClosing(CancelEventArgs e) { base.OnClosing(e); Program.Skin.PropertyChanged -= Skin_PropertyChanged; } }

Contact & Reference Material http://msdn.microsoft.com/en-us/library/ms973837.aspx (Garbage

Collector Basics and Performance Hints) http://www.microsoft.com/downloads/details.aspx?FamilyID=a362781c

-3870-43be-8926-862b40aa0cd0&DisplayLang=en CLR Profiler for .Net 2.0 http://www.openasthra.com/multithreading/heap-overview/ Heap

Overview http://74.125.155.132/search?q=cache:44hDjSztDf4J:doc.bughunter.n

et/buffer-overflow/advanced-malloc-exploits.html+malloc+overview&cd=21&hl=en&ct=clnk&gl=us Advanced Malloc exploits

http://msdn.microsoft.com/en-us/magazine/cc534993.aspx http://www.microsoft.com/downloads/details.aspx?FamilyId=A362781

C-3870-43BE-8926-862B40AA0CD0&displaylang=en CLR Profiler

Ronn Black rblack@btsoft.org