Confirmed: Objective-C ARC is slow. Don’t use it! (sarcasm off)

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Or so goes the argument. Still.

I wish Apple would just pull the plug and completely remove MRC support from LLVM. I’m getting tired, annoyed and sometimes angry when I browse and frequently find MRC code samples containing one or more blatant memory management issues.

Before I rant any further, this article is about testing the performance difference of ARC vs MRC code. I provide some examples, and the updated performance measurement project I’ve used before for cocos2d performance analysis, and the results of the full run at the bottom. I also split it into both synthetic low-level tests and closer to real-world algorithms to prove not one but two points:

ARC is generally faster, and ARC can indeed be slower – but that’s no reason to dismiss it altogether.

Measuring & Comparing Objective-C ARC vs MRC performance

Without further ado, here are the results of the low-level MRC vs ARC performance tests, obtained from an iPod touch 5th generation with compiler optimizations enabled (release build):

Times are in nanoseconds. A nanosecond (ns) is one billionth of a second (0.000 000 001 second). One Gigahertz (GHz) equals 1,000,000,000 Hz. One cycle of a 1 GHz CPU takes 1 nanosecond. A game that renders every frame in less than 16,700,000 nanoseconds runs at 60 frames per second. In other words, you can perform close to half a million Objective-C message sends every frame before your framerate drops below 60.

ARC vs MRC: Messaging Tests
Name Each (ns)
ARC: Message Send to Object 49
MRC: Message Send to Object 86
ARC: Assign (nonatomic, copy) property 329
MRC: Assign (nonatomic, copy) property 331
ARC vs MRC: Alloc/Init Tests
Name Each (ns)
ARC: Create Alloc/Init Object 4.844
MRC: Create Alloc/Init Object 4.881
ARC vs MRC: Autorelease Tests
Name Each (ns)
ARC: Create & Return Autorelease Object 5.769
ARC: Create Autorelease Object 5.840
MRC: Create & Return Autorelease Object 5.843
MRC: Create Autorelease Object 6.625

So the low-level tests are about equal and ARC usually has an advantage. Mainly due to faster autorelease pools and other low-level optimizations. This is why Apple says that ARC is generally faster than MRC.

Where ARC can actually be slower

That ARC is faster is not true in every case. Specifically where existing MRC code has already been fine-tuned to provide the best possible performance in regards to memory usage.

This is because in some situations ARC adds additional retain/release messages that are sort of optional under MRC. This happens for example when you send a message to a selector with one or more object (id) parameters. It also happens when you receive a temporary variable from a method – it gets retained and then released even though its lifetime is short and contained. For example:

// this is typical MRC code:

idobject=[array objectAtIndex:0];

[object doSomething];

[object doAnotherThing];

// this is what ARC does (and what is considered best practice under MRC):

idobject=[array objectAtIndex:0];

[object retain];// inserted by ARC

[object doSomething];

[object doAnotherThing];

[object release];// inserted by ARC

And again when sending messages to a selector which takes one or more objects as parameters:

// this is typical MRC code:

idobject=[array objectAtIndex:0];

[object doSomething];

[object doAnotherThing];

// this is what ARC does (and what is considered best practice under MRC):

idobject=[array objectAtIndex:0];

[object retain];// inserted by ARC

[object doSomething];

[object doAnotherThing];

[object release];// inserted by ARC

The reason why ARC adds these additional retain/release is because above all else ARC ensures correctness. And that code is correct under MRC as well. So why don’t we usually add these additional retain/release in MRC and it still works?

Because most of us aren’t writing multi-threaded applications, and even those who do will probably not understand why they’re treading dangerously close to the edge. In multithreaded applications said object could be released between the doSomething and doAnotherThing calls – thus crashing your app. Granted, it’s rare but when it rears its ugly head, you wish you wouldn’t have to debug it.

This explains why some algorithms run slower under ARC than the same code written in MRC. I took this Objective-C ARC genetic algorithm and converted it to MRC to make the comparison. You’ll see in the results that the ARC version is noticably slower than the MRC version mainly because of the additional retain/release:

ARC vs MRC: Algorithm Tests
Name Each (ns)
ARC: Unoptimized Contains String algorithm 1.696.808
MRC: Unoptimized Contains String algorithm 1.752.414
MRC: genetic algorithm 1.134.536.017
ARC: genetic algorithm 1.579.992.123

PS: don’t ask me why the “contains string” algorithm is faster under ARC. It just is.

Avoid using ARC for best performance, right?

No way!

Whoever thinks like that hasn’t understood what programming or ARC are all about. First and foremost, you want to write an app and release it without bugs, especially no crashes. ARC helps you tremendously with that.

While writing this very little MRC code for the project I had already introduced several leaks, one of which wasn’t even caught by the analyzer. It’s just too damn easy to write incorrect manual reference counting code. And it’s even more code to read and write, too. With ARC I have not had a single leak or other such issue in months! The static analyzer has changed from a tool that I used to run daily to one I run on a monthly basis only to confirm that there are few or no memory management issues.

For beginners MRC is even worse, because not understanding MRC they’ll randomly insert or remove release, retain and autorelease messages until it somehow works. Usually they just shift the problem around or change the nature of it: a crash becomes a leak, a leak becomes a dangling pointer. It makes me want to cry. It makes me want to not help anyone who is still using MRC code for no reason or simply because that’s the default in cocos2d.

That every day thousands of cute kittens have to die because developers are starting new cocos2d projects with MRC templates makes me really sad. I find solace only in knowing that Kobold2D and KoboldTouch help to preserve the imaginary kitten population.

Good programmers use ARC!

Spending as little time as possible on any extraneous tasks such as manual memory management and debugging issues because of it is a programmer’s primary concern. Maybe second only to writing readable and maintainable code. Performance is way at the bottom of the priority lists, even for game developers.

If you’re more concerned about performance than code correctness, you’re not doing a good job because you set the wrong priorities. ARC ensures that you get the most out of your time and your app’s runtime.

And then any performance difference you can measure between ARC and MRC code in synthetic tests is practically null and void in real world applications. You’re usually doing a lot more elsewhere, like rendering, and often you don’t even have much control over those parts (game engine, foundation, etc). And you always have room for optimization just by writing better code.

The genetic algorithm I picked is such an example. The code repeats a terribly wasteful memory management pattern regardless of whether you’re using ARC or MRC. Refactor or rewrite the essential parts of the code in C, or just optimized Objective-C, and you’ll likely see gains far greater than the difference between ARC and MRC in the test result above.

Finally, typically less than 5% of your code contributes to 95% of its runtime performance! It would be downright stupid not to use ARC for the remaining 95%, and hand-tune only the 5% where it’ll make an actual difference in responsiveness, speed or framerate.

Final words

Arguing against using ARC because it may be slower in some well-defined situations really makes me wonder one thing: if you’re that concerned about performance, why are you even using Objective-C to begin with?

Objective-C has been proven to be slower than C countless times already. Your argument is invalid!

You can certainly make significantly greater performance improvements by rewriting certain performance-critical code to C than you could ever hope to achieve by avoiding ARC and sticking to MRC. Fortunately only very few developers will ever have to consider doing so.

And why are we using Objective-C?

It’s because Objective-C is easier to use and safer than plain C! In that sense, using ARC is a logical next step for any Objective-C programmer because it makes your work even easier and your app more stable. And when you do find an actually noticable performance difference, compared to one that’s only measurable, you can always -fno-objc-arc the hell out of that code.

Complete Test Results (iPod touch 5G)

This is from a different run, so the values you’ve already seen earlier may differ. Comparing the results gives you an indication of how much of an error range there is in these values.

The results involving cocos2d are not directly comparable to the previous performance tests I did because the updated performance test project uses cocos2d v2.1 (rc0a), the previous one used cocos2d v1.0.1.

ARC vs MRC: Messaging Tests

Name Each (ns)
ARC: Message Send to Object 61
MRC: Message Send to Object 61
ARC: Assign (nonatomic, copy) property 329
MRC: Assign (nonatomic, copy) property 330

ARC vs MRC: Alloc/Init Tests

Name Each (ns)
ARC: Create Alloc/Init Object 4.747
MRC: Create Alloc/Init Object 4.811

ARC vs MRC: Autorelease Tests

Name Each (ns)
ARC: Create Autorelease Object 5.503
ARC: Create & Return Autorelease Object 5.608
MRC: Create & Return Autorelease Object 5.612
MRC: Create Autorelease Object 6.342

ARC vs MRC: Algorithm Tests

Name Each (ns)
ARC: Unoptimized Contains String algorithm 1.691.563
MRC: Unoptimized Contains String algorithm 1.750.430
MRC: genetic algorithm 1.144.589.482
ARC: genetic algorithm 1.559.268.544

Loading Textures

Time it takes to load and unload the same 1024×1024 texture using a variety of different image file formats, compression and color bit depths.

Name Each (ns)
PVRTC2.pvr.ccz 300.497
PVRTC4.pvr.ccz 306.025
PVRTC2.pvr 3.752.015
PVRTC4.pvr 7.255.280
PVRTC2.pvr.gz 10.308.318
PVRTC4.pvr.gz 19.612.099
RGBA8888.pvr 57.411.790
RGBA8888.pvr.ccz 85.919.379
RGBA8888.png 98.239.494
RGBA8888.pvr.gz 100.842.780

Node Hierarchy (children)

The performance of functions that act on the node hierarchy (children list) depends heavily on the number of children.

Name Each (ns)
reorderChild w/ 100 Nodes 258
reorderChild w/ 2,500 Nodes 258
reorderChild w/ 10 Nodes 259
reorderChild w/ 500 Nodes 261
getChildByTag w/ 10 Nodes 1.188
getChildByTag w/ 100 Nodes 5.998
addChild with tag 6.758
removeChildByTag 16.493
getChildByTag w/ 500 Nodes 29.022
getChildByTag w/ 2,500 Nodes 178.572

Array Tests

Testing Cocos2D’s CCArray performance against regular NSMutableArray.

Name Each (ns)
CCArray objectAtIndex: 248
CCArray exchangeObjectAtIndex 267
NSMutableArray objectAtIndex: 285
CFArray GetValueAtIndex 319
CCArray withCapacity addObject: 385
CCArray addObject: 385
NSMutableArray insertObject: atIndex:0 468
NSMutableArray withCapacity addObject: 539
NSMutableArray addObject: 540
CCArray removeLastObject 734
NSMutableArray removeLastObject 1.039
CCArray removeObjectAtIndex 1.546
NSMutableArray removeObjectAtIndex 1.776
NSMutableArray exchangeObjectAtIndex 2.069
NSMutableArray insertObject: atIndex:random 2.709
CCArray insertObject: atIndex:random 4.479
CCArray insertObject: atIndex:0 8.430
NSMutableArray fast enumeration 9.353
CCArray indexOfObject 17.732
CCArray containsObject 17.799
NSMutableArray indexOfObject 46.535
NSMutableArray containsObject 47.374
NSMutableArray makeObjectsPerformSelector 125.084
NSMutableArray makeObjectsPerformSelector withObject 137.865
CCArray makeObjectsPerformSelector 138.679
CCArray makeObjectsPerformSelector withObject 152.657
CCArray fast enumeration 333.421
CCArray enumeration 408.957
NSMutableArray enumeration 450.680
NSMutableArray add/removeObjectsInArray 1.223.791
CCArray add/removeObjectsInArray 2.732.601

Object Creation

These tests tell you how long it takes to allocate memory, initialize the object, and deallocate it. The longer this takes for an object, the higher the chance that doing this during gameplay will negatively affect performance. Note that these tests do not give any indication whatsoever of the runtime/rendering performance of these objects.

Name Each (ns)
NSAutoreleasePool alloc/init/release 225
NSObject alloc/init/release 1.868
CCMoveTo alloc/init/release 2.869
CCSequence alloc/initOne/release 3.864
CCNode alloc/init/release 4.663
CCSprite alloc/initWithFile/release 21.914
CCParticleSystemQuad 25 particles alloc/init/release 284.135
CCParticleSystemQuad 250 particles alloc/init/release 675.596
CCLabelBMFont alloc/initWithString/release 897.208
CCLabelTTF alloc/initWithString/release 1.385.200
CCTMXTiledMap small alloc/init/release 6.494.550
CCTMXTiledMap large alloc/init/release 492.783.615
CCSprite GCD alloc/initWithFile/release 12.424.037.083

Messaging / Function Calls

Low-level overhead for calling C++ functions respectively sending Objective-C messages in various ways.

Name Each (ns)
ObjC class @public variable 3
C++ cached virtual method call 11
C++ virtual method call 11
IMP-cached message send 11
Objective-C message send 34
Objective-C performSelector 115
ObjC class nonatomic property dot notation 127
ObjC class atomic property dot notation 141
ObjC class nonatomic property message send 160
ObjC class atomic property message send 164
NSInvocation message send 2.101
Objective-C CCArray message send 43.796
Objective-C NSArray message send 43.829
Objective-C NSArray enumerateWithBlock msg send 70.741
Objective-C NSArray enumerateWithBlock concurrent msg send 100.265
Objective-C NSArray makeObjectsPerformSelector 121.572
Objective-C CCArray makeObjectsPerformSelector 138.597

Object Comparison

Compare objects with various methods, and testing if it makes any difference if the test fails (mismatch) or succeeds (match).

Name Each (ns)
NSObject hash 35
NSObject is not Equal 50
NSString isEqual 50
NSString isEqualToString 50
NSObject isEqual 52
isMemberOfClass, class cached 144
is not MemberOfClass, class cached 144
NSString hash 148
isMemberOfClass 199
is not MemberOfClass 200
isKindOfClass, class cached 224
is not KindOfClass, class cached 260
NSString is not EqualToString 277
NSString is not Equal 277
isKindOfClass 278
is not KindOfClass 338

Arithmetic Tests

Simple calculations done frequently in an app, using various data types.

Name Each (ns)
Double square root -0
Float division with int conversion -0
Float square root -0
Double division with int conversion -0
Integer multiplication 0
Integer division 0
Float multiplication 5
Double multiplication 9
Float division 20
Accelerometer Highpass filter 20
Double division 33

Memory Tests

Allocating and releasing memory.

Name Each (ns)
16MB malloc/free -0
1MB memcpy 0
16 byte memcpy 0
16 byte malloc/free 0

File IO

Name Each (ns)
Read 16-byte file 109.127
Read 16MB file 110.008
Write 16-byte file 202.430
Write 16MB file 219.193
Write 16-byte file (atomic) 359.137
Write 16MB file (atomic) 362.201

Miscellaneous Tests

Name Each (ns)
Zero-second delayed perform 7
pthread create/join 158.470
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