Advanced Memory Management on iOS and Android - Mark Probst and Rodrigo Kumpera
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Advanced Memory Management on iOS and Android - Mark Probst and Rodrigo Kumpera
- 1. Rodrigo Kumpera Runtime Engineer Xamarin kumpera@xamarin.com Advanced Memory Management on iOS and Android
- 2. Mark Probst Runtime Engineer Xamarin mark@xamarin.com Advanced Memory Management on iOS and Android
- 3. What a Garbage Collector Does • Gives you memory for your objects • Computes which objects are reachable • Gets rid of the rest (= garbage) h"p://www.flickr.com/photos/tweng/2235972313/
- 4. object reference object with reference one object ... … referencing ... … another Basic Garbage Collection
- 13. Two Generations • Generational hypothesis Most objects die young • Nursery Where (small) objects are born • Major Where they are copied to when they mature • Large Object Space Part of the major generation, objects > 8Kb Typically large arrays
- 14. Stop-the-World • Nursery collection pauses are short • Major collection pauses can take a long time • All threads registered with the runtime are stopped That includes the main run loop thread • iOS animations continue to run in a separate process
- 15. Common Issues • Let’s examine three common issues • Given a trivial piece code Find the issue Understand the problem Learn how to fix it
- 16. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 iOS Puzzle 1 public override void ViewDidLoad () { var imageView = new UIImageView (IMG_VIEW_POSITION); var image = UIImage.FromBundle ("grumpy-‐cat.jpg"); imageView.Image = image; View.Add (imageView); var button = UIButton.FromType (UIButtonType.RoundedRect); button.Frame = BUTTON_POSITION; View.Add (button); button.TouchUpInside += (sender, e) => { imageView.RemoveFromSuperview (); }; }
- 17. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 Something Is Missing public override void ViewDidLoad () { var imageView = new UIImageView (IMG_VIEW_POSITION); var image = UIImage.FromBundle ("grumpy-‐cat.jpg"); imageView.Image = image; View.Add (imageView); var button = UIButton.FromType (UIButtonType.RoundedRect); button.Frame = BUTTON_POSITION; View.Add (button); button.TouchUpInside += (sender, e) => { imageView.RemoveFromSuperview (); }; }
- 18. The garbage collector cannot see what’s behind an innocent object The Illusion 32 bytes 2 Mb C# UIImage ObjC UIImage
- 19. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 Dispose Your Resources public override void ViewDidLoad () { var imageView = new UIImageView (IMG_VIEW_POSITION); var image = UIImage.FromBundle ("grumpy-‐cat.jpg"); imageView.Image = image; View.Add (imageView); var button = UIButton.FromType (UIButtonType.RoundedRect); button.Frame = BUTTON_POSITION; View.Add (button); button.TouchUpInside += (sender, e) => { imageView.RemoveFromSuperview (); imageView.Dispose (); image.Dispose (); }; }
- 20. Using Dispose • Call Dispose() to release ownership of a resource • Use with large native resources, such as Images Sounds • Or scarce resources, such as Files Sockets • Remember, lifecycle must be manually defined
- 21. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 iOS Puzzle 2 public class CustomView : UIView { UIViewController parent; public CustomView (UIViewController parent) { this.parent = parent; } } public class Puzzle2Controller : UIViewController { public override void ViewDidLoad () { View.Add (new CustomView (this)); } }
- 22. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 Indirect Cycles public class CustomView : UIView { UIViewController parent; public CustomView (UIViewController parent) { this.parent = parent; } } public class Puzzle2Controller : UIViewController { public override void ViewDidLoad () { View.Add (new CustomView (this)); } }
- 24. How those cycles happen Indirect Cycles C# Puzzle2Controller CustomView View.Add (...) this.parent = ... Objec;ve-‐C 1 2
- 25. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 Using Weak References public class CustomView : UIButton { WeakReference<UIViewController> parent; public CustomView (UIViewController parent) { this.parent = new WeakReference<UIViewController> (parent); } } public class Puzzle2Controller : UIViewController { public override void ViewDidLoad () { View.Add (new CustomView (this)); } }
- 26. Indirect cycles • Inherited from Objective-C • How to detect When multiple objects point to each other • Breaking those cycles Use WeakReference By disposing the parent By explicitly nulling links • What can trigger it under the hood Non-wrapper subclasses of NSObject With reference count > 2
- 27. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 iOS Puzzle 2 (Bonus) public class CustomButton : UIButton { public CustomButton () {} } public class Puzzle2Controller : UIViewController { public override void ViewDidLoad () { var button = new CustomButton (); View.Add (button); button.TouchUpInside += (sender, e) => this.RemoveFromParentViewController (); } }
- 28. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 Watch Your Lambdas public class CustomButton : UIButton { public CustomButton () {} } public class Puzzle2Controller : UIViewController { public override void ViewDidLoad () { var button = new CustomButton (); View.Add (button); button.TouchUpInside += (sender, e) => this.RemoveFromParentViewController (); } }
- 29. Measure before assuming something is wrong Enabling GC logging on Android A Small Detour $adb shell setprop debug.mono.env "MONO_LOG_LEVEL=debug|MONO_LOG_MASK=gc" D/Mono ( 5862): GC_MAJOR: (user request) pause 2.82ms, total 3.06ms, bridge 20.96 major 608K/808K los 9K/20K
- 30. 01 02 03 04 05 06 07 08 09 10 11 Android Puzzle public class Tweet {} public class Puzzle1 : ListActivity { protected override void OnCreate (Bundle bundle) { base.OnCreate (bundle); var data = new Tweet[] { tweet0, tweet1, tweet2, tweet3 }; ListAdapter = new ArrayAdapter (this, Resource.Layout.TextViewItem, data); } }
- 31. 01 02 03 04 05 06 07 08 09 10 11 Over-Sharing public class Tweet {} public class Puzzle1 : ListActivity { protected override void OnCreate (Bundle bundle) { base.OnCreate (bundle); var data = new Tweet[] { tweet0, tweet1, tweet2, tweet3 }; ListAdapter = new ArrayAdapter (this, Resource.Layout.TextViewItem, data); } }
- 32. A tale of two heaps Cross-Heap References ArrayAdapter ArrayList Tweets C# Object Java Object
- 33. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 Do It All from C# Land public class Tweet {} public class TweetAdapter : BaseAdapter<Tweet> { List<Tweet> tweets; public override Tweet this[int position] { get { return tweets [position]; } } public override int Count { get { return tweets.Count; } } } public class Puzzle3 : ListActivity { protected override void OnCreate (Bundle bundle) { var data = new List<Tweet> () { tweet0, tweet1, tweet2, tweet3 }; ListAdapter = new TweetAdapter (this, data); } }
- 34. The a!er effect Cross-Heap References TweetsAdapter List<Tweet> Tweets C# Object Java Object
- 35. Avoid Cross-Heap References • It’s expensive for Java to see a C# object And vice-versa • Performance cost of language crossing • Higher Garbage Collector costs • Your objects are effectively being mirrored So using twice as much memory
- 36. Performance Tips • The less you allocate, the less o!en the GC runs • The less live data you have, the quicker the GC runs • Small, short-lived objects are cheap • Don’t allocate large (> 8Kb) objects that die young • Avoid writing to reference fields • Better: avoid having reference fields • Don’t use free lists
- 37. Q&A
- 38. Use SGen unless you have good reason not to SGen vs Boehm
- 39. Memory Management on iOS • Reference Counting • Each object has a reference count field • Incremented when something points to it • Decremented when something stops pointing to it • Object is released when count equals to zero
- 40. Xamarin.iOS Approach • Two classes of objects • Wrappers Comes from native frame UIButton, NSString, etc • User types User code that extend the above MyButton
- 41. Wrappers • Retain on construction • Release on finalization/dispose • We don’t care if the managed object goes away
- 42. User Types • Retain/Release like wrappers • Can have custom state • We make a gc handle (think of a static field) point to the object if reference count > 1 • Managed object is kept around if native is interested in it
- 43. 01 02 03 04 05 06 07 08 09 10 11 User Types class MyButton : UIButton {} ... public override void ViewDidLoad () { var button = new MyButton (); //ref count == 1, no gc handle this.View.Add (button); //ref count == 2, gc handle created ... button.RemoveFromSuperView (); // ref count == 1, gc handle removed }
- 44. First Rule of Finalizers: Don’t Use Finalizers! • They’re not guaranteed to run within any deadline • They don’t run in a specific sequence • They make objects live longer • The GC doesn’t know about unmanaged resources
- 45. Finalizers • Run in a dedicated finalizer thread • Run concurrently with other threads • Keep their objects and those referenced from there alive for another GC round
- 46. Root has Finalizer an object with a finalizer must be kept alive Finalizer Illustration
- 47. Root has Finalizer so must all objects reachable from it Finalizer Illustration
- 48. Root has Finalizer so must all objects reachable from it Finalizer Illustration
- 49. Root has Finalizer they die in the next collection Finalizer Illustration
- 50. Root they die in the next collection Finalizer Illustration
- 52. Root has Finalizer the finalizer resurrects them Finalizer Illustration
- 53. Root the finalizer resurrects them Finalizer Illustration
- 54. Root the finalizer resurrects them Finalizer Illustration
- 55. Stuff You Can Do with Finalizers (and Probably Shouldn’t) • Resurrect their objects • Re-register for finalization • Non-critical vs critical finalizers
- 56. Nursery Root 1 Root 2 Major Heap keeping track of major→nursery references The Write Barrier
- 57. Nursery Root 1 Root 2 Major Heap keeping track of major→nursery references The Write Barrier
- 58. What Are Roots? • static variables • Stack frames in managed threads • Finalizer queue • References registered in unmanaged code
- 59. 01 02 03 04 05 06 07 Android Puzzle 2 public class CommitLogAdapter : BaseAdapter { Dictionary<string, CommitObject> commitCache; CommitObject GetCommit (string hash) { return commitCache [hash]; } }
- 60. 01 02 03 04 05 06 07 Too Many Objects public class CommitLogItemAdapter : BaseAdapter { Dictionary<string, CommitObject> commitCache; CommitObject GetCommit (string hash) { return commitCache [hash]; } }
- 61. It must inspect special objects and everything they reference Android GC Interop CommitLogAdapter commitsCache
- 62. 01 02 03 04 05 06 07 Use Static Caches public class CommitLogItemAdapter : BaseAdapter { static Dictionary<string, CommitObject> commitCache; static CommitObject GetCommit (string hash) { return commitCache [hash]; } }
- 63. Taking care of special objects • If it extends the Android framework, it’s a special object • The Garbage Collector scans them in a different way That is slower It rescans all objects they reference Avoid pointing to large group of objects • Explicitly manage the lifecycle of long living objects Static caches and collections Helps measuring consumption