the gamedesigninitiative at cornell university Lecture 9 Memory Management

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1 Lecture 9

2 Gaming Memory Constraints Redux Wii-U Playstation 4 2GB of RAM 1GB dedicated to OS Shared with GPGPU 8GB of RAM Shared GPU, 8-core CPU OS footprint unknown 2

3 Two Main Concerns with Memory Getting rid of memory you no longer want Doing it yourself: deallocation Runtime support: garbage collection Having enough memory to function Reserved memory: memory pools Spatial locality: dynamic loading 3

4 Manual Deletion in C/C++ Depends on allocation malloc: free new: delete What does deletion do? Marks memory as available Does not erase contents Does not reset pointer int main() { cout << "Program started" << endl; int* a = new int[length]; delete a; for(int ii = 0; ii < LENGTH; ii++) { cout << "a[" << ii << "]=" << a[ii] << endl; Only crashes if pointer bad Pointer is currently NULL Pointer is illegal address cout << "Program done" << endl; 4

Can even happen in Java memoryarea = newarea; JNI supports native

5 Memory Leaks Leak: Cannot release memory Object allocated on heap Only reference is moved No way to reference object Consumes memory fast! Can even happen in Java memoryarea = newarea; JNI supports native libraries Method may allocate memory Need anor method to free Example: dispose() in JOGL 5

6 A Question of Ownership void foo() { MyObject* o = " new MyObject(); o.dosomething(); o = null; return; Memory Leak void foo(int key) { MyObject* o =" table.get(key); o.dosomething(); o = null; return; Not a Leak 6

7 A Question of Ownership void foo() { MyObject* o =" table.get(key); table.remove(key); o = null; return; Memory Leak? void foo(int key) { MyObject* o =" table.get(key); table.remove(key); ntable.put(key,o); o = null; return; Not a Leak 7

8 A Question of Ownership Thread 1 Thread 2 Owners of obj void run() { o.dosomething1(); void run() { o.dosomething2(); Who deletes obj? 8

9 Understanding Ownership Function-Based Object owned by a function Function allocated object Can delete when function done Ownership never transferred May pass to or functions But always returns to owner Really a stack-based object Active as long as allocator is But allocated on heap (why?) Object-Based Owned by anor object Referenced by a field Stored in a data structure Allows multiple ownership No guaranteed relationship between owning objects Call each owner a reference When can we deallocate? No more references References unimportant 9

Understanding Ownership Function-Based Object owned by a function Function allocated object Can delete when function done Ownership never transferred May pass to or functions But always returns to

10 Understanding Ownership Function-Based Object owned by a function Function allocated object Can delete when function done Ownership never transferred May pass to or functions But always returns to owner Easy: Will ignore Really a stack-based object Active as long as allocator is But allocated on heap (why?) Object-Based Owned by anor object Referenced by a field Stored in a data structure Allows multiple ownership No guaranteed relationship between owning objects Call each owner a reference When can we deallocate? No more references References unimportant 10

11 Reference Strength Strong Reference Reference asserts ownership Cannot delete referred object Assign to NULL to release Else assign to anor object Can use reference directly No need to copy reference Treat like a normal object Standard type of reference Weak Reference Reference!= ownership Object can be deleted anytime Often for performance caching Only use indirect references Copy to local variable first Compute on local variable Be prepared for NULL Reconstruct object? Abort computation? 11

12 Weak Reference Example in Java public class CombatComponent { WeakReference<NPC> target; public void attacktarget(aicontroller ai) { NPC Target = target.get(); if (Target == null) { // Be prepared for NULL Target = ai.pickcombattargetfor(this); target = new WeakReference<NPC>(Target); // Do stuff with Target Class in package java.lang.ref 12

Weak Reference Example in Java public class CombatComponent { WeakReference<NPC> target; public void attacktarget(aicontroller ai) { NPC Target = target.

13 Weak Reference Example in Java public class CombatComponent { WeakReference<NPC> target; public void attacktarget(aicontroller ai) { NPC Target = target.get(); if (Target == null) { // Be prepared for NULL WeakReference Target = ai.pickcombattargetfor(this); GC if no standard references left target = new WeakReference<NPC>(Target); Encourages eager GC policies // Do stuff with Target Class in package java.lang.ref Reference Managers in Java SoftReference GC possible if no references left But only if space is needed 13

Reference Counting Every object has a counter Tracks number of owners No owners = memory leak Increment when assign reference Can be explicit method

14 Reference Counting Every object has a counter Tracks number of owners No owners = memory leak Increment when assign reference Can be explicit method call can we do it automatically? Decrement when remove reference Can be explicit or automatic If makes count 0, delete it Ref 1 Ref 2 Object Count = 12 14

15 When to Adjust Count? On object allocation Initial allocator is an owner Even if in a local variable When added to an object Often handled by setter Part of class invariant When removed from object Also handled by setter Release before reassign Any or time? public class Container { RCObject object; public Container() { // Initial allocation; ownership Object = new RCObject(); public void setobject(rcobject o) { if (object!= null) { object.decrement(); o.increment(); object = o; 15

16 Reference Counting in Obj-C // create a new instance" Fraction *frac = [[Fraction alloc] init];! [frac retain]; Reference count 2 // set values and print " [frac setnumerator: 1]; " [frac setdenominator: 3]; Reference count 1 printf( "The fraction is: " ); " [frac print]; " printf( "\n" ); 16 // free memory " [frac release];! [frac release]; Reference count 1 Reference count 0 frac is deleted

17 Reference Counting in Classic Obj-C // create a new instance" Fraction *frac = [[Fraction alloc] init];! [frac retain]; Reference count 2 // set values and print " [frac setnumerator: 1]; " [frac setdenominator: 3]; Reference count 1 printf( "The fraction is: " ); " [frac print]; " printf( "\n" ); // free memory " [frac release];! Reference count 1 Memory Leak! 17

18 Which Is Correct? Fraction* foo(int n, int d) { // create a new instance" Fraction *frac =" [[Fraction alloc] init]; // set values " [frac setnumerator: n]; " [frac setdenominator: d]; // free memory " [frac release]; // return it " return frac; Fraction* foo(int n, int d) { // create a new instance" Fraction *frac =" [[Fraction alloc] init]; // set values " [frac setnumerator: n]; " [frac setdenominator: d]; // Do nothing" // return it " return frac; 18

$Fraction* foo(int n, int d) { // create a new instance" Fraction *frac =" [[Fraction alloc] init]; Fraction* foo(int n, int d) { // create a new instance" Fraction *frac =" [[Fraction alloc] init];$

19 Which Is Correct? Fraction* foo(int n, int d) { // create a new instance" Fraction *frac =" [[Fraction alloc] init]; Fraction* foo(int n, int d) { // create a new instance" Fraction *frac =" [[Fraction alloc] init]; // set values " [frac setnumerator: n]; " [frac setdenominator: d]; // free memory " [frac release]; Trick Question! // set values " [frac setnumerator: n]; " [frac setdenominator: d]; // Do nothing" // return it " return frac; // return it " return frac; 19

$Neir is Optimal Fraction* foo(int n, int d) { // create a new instance" Fraction *frac =" [[Fraction alloc] init]; // set values " [frac setnumerator:$ $make ownership transfer part of // set values " [frac setnumerator: specification n]; " [frac setdenominator: d]; // free memory " [frac release]; //$

20 Neir is Optimal Fraction* foo(int n, int d) { // create a new instance" Fraction *frac =" [[Fraction alloc] init]; // set values " [frac setnumerator: n]; " [frac setdenominator: d]; Fraction* foo(int n, int d) { // create a new instance" Fraction One *frac possibility: =" [[Fraction alloc] init]; make ownership transfer part of // set values " [frac setnumerator: specification n]; " [frac setdenominator: d]; // free memory " [frac release]; // return it " return frac; Object freed. Nothing left to return. // Do nothing" // return it " return frac; Reference kept. Who will release this reference? 20

21 Objective-C: An Alternate Solution Fraction* foo(int n, int d) { Autorelease // create a new instance" Fraction *frac =" [[Fraction alloc] init]; // set values " [frac setnumerator: n]; " [frac setdenominator: d]; // free memory " [frac autorelease]; // return it " return frac; Delay release until later. Handled by OS. Places object in a pool OS releases all in pool At end of event handler Games are not event driven! Game loop runs continuously Pool is not released You can release it manually At end of loop iteration? 21

$Objective-C: An Alternate Solution Fraction* foo(int n, int d) { // create a new instance" Fraction *frac =" [[Fraction alloc] init]; // set values " [frac setnumerator: n]; " [frac setdenominator:$

22 Objective-C: An Alternate Solution Fraction* foo(int n, int d) { // create a new instance" Fraction *frac =" [[Fraction alloc] init]; // set values " [frac setnumerator: n]; " [frac setdenominator: d]; // free memory " [frac autorelease]; // return it " return frac; Delay release until later. Handled by OS. Autorelease Al Demers sez: Places object in a pool This is a hack. OS releases Bad Apple, all in bad. pool At end of event handler Games are not event driven! Game loop runs continuously Pool is not released You can release it manually At end of loop iteration? 22

$Reference Counting in ios 5 // create a new instance" Fraction *frac = [[Fraction alloc] init];!$

23 Reference Counting in ios 5 // create a new instance" Fraction *frac = [[Fraction alloc] init];! [frac retain]; // set values and print " [frac setnumerator: 1]; " [frac setdenominator: 3]; printf( "The fraction is: " ); " [frac print]; " printf( "\n" ); // free memory " [frac release];! No-op (does nothing) No-op (does nothing) Automated Reference Counting Handled by compiler Inserts retain/release for you Still reference counting, not GC Old methods are deprecated Backwards compatibility only No-ops if ARC is turned on 23

24 C++ Analogue: Smart Pointers C++ can override anything Assignment operator = Dereference operator -> Use special object as pointer A field to reference object Also a reference counter Assignment increments What about decrementing? When smart pointer deleted Delete object if count is 0 void foo(){ // Create smart pointer" smart_ptr<myobject> p(); // Allocate & assign it" p = new MyObject(); p->dosomething(); // NO LEAK 24

$A field to reference object Also a reference counter Assignment increments void foo(){ // Create smart pointer" smart_ptr<myobject> p(); // Allocate & assign it" p = new MyObject(); p->dosomething();$

25 C++ Analogue: Smart Pointers C++ can override anything Assignment operator = Dereference operator -> Use special object as pointer Stack object; not in heap. A field to reference object Also a reference counter Assignment increments void foo(){ // Create smart pointer" smart_ptr<myobject> p(); // Allocate & assign it" p = new MyObject(); p->dosomething(); What about decrementing? 25 When smart pointer deleted Delete object if count is 0 // NO LEAK Stack released; Smart pointer is disposed.

Disadvantage: Combining with raw pointers (and hence any stdlib code) Intruisive

26 Where Does Count Go? Non-Intruisive Pointers Count inside smart pointer Advantage: Works with any class Disadvantage: Combining with raw pointers (and hence any stdlib code) Intruisive Pointers Count inside referred object Advantage: Easy to mix with raw pointers Disadvantage: Requires custom base object [Images courtesy of Kosmas Karadimitriou] 26

27 References vs. Garbage Collectors Reference Counting Advantages Deallocation is immediate Works on non-memory objects Ideal for real-time systems Disadvantages Overhead on every assignment Cannot easily handle cycles (e.g. object points to itself) Requires training to use Mark-and-Sweep Advantages No assignment overhead Can handle reference cycles No specialized training to use Disadvantages Collection can be expensive Hurts performance when runs Usually triggered whenever memory is close to full 27

28 Incremental Mark-and-Sweep Objects have multiple colors Indicate where in GC process At GC, change some colors Free objects of certain color Natural for game loops Give GC a time budget Do at end of game loop Stop when done or time up See online reading for more Lua 3.0: Quad-Color M&S 28

memory is always close to full, incremental mark and sweep is better than all or options (even manual

29 Incremental Mark-and-Sweep Objects have multiple colors Indicate where in GC process At GC, change some colors Free objects of certain color Natural for game loops Give GC a time budget Al Demers sez: Unless memory is always close to full, incremental mark and sweep is better than all or options (even manual deallocation) Do at end of game loop Stop when done or time up See online reading for more Lua 3.0: Quad-Color M&S 29

30 Incremental Mark-and-Sweep Problem is availability Not available in Java Objective-C uses ARC No good C/C++ libraries You need to implement! Create a singleton allocator Has (weak) references to all objects that it allocates Popular interview question Insomniac uses this to test gameplay programmers Lua 3.0: Quad-Color M&S 30

31 Two Main Concerns with Memory Getting rid of memory you no longer want Doing it yourself: deallocation Runtime support: garbage collection Having enough memory to function Reserved memory: memory pools Spatial locality: dynamic loading 31

32 Custom Allocators Subdivide memory into functional units Physics memory, AI memory, etc If area is full, that function must cut back Works as a form of budgeting (hard or soft) Enforce budgets with a custom allocator Objects allocated for a function go in that region C++ and Objective-C support this functionality Example: [[Fraction alloc] init] 32 Allocation Initialization

custom allocator Objects allocated for a function go in that region C++ and Objective-C support this

33 Custom Allocators Subdivide memory into functional units Physics memory, AI memory, etc If area is full, that function must cut back Works as a form of budgeting (hard or soft) Enforce budgets with a custom allocator Objects allocated for a function go in that region C++ and Objective-C support this functionality Example: [[Fraction alloc] init] Advanced and Difficult to Write 33 Allocation Initialization

cells Different from a memory pool Track player visibility radius Load/unload

34 Dynamic Loading Most game data is spatial Only load if player nearby Loading Radius Unload as player moves away Minimizes memory used Arrange memory in cells Different from a memory pool Track player visibility radius Load/unload via outer radius Alternative: loading zones Visibility Radius Elevators in Mass Effect 34

35 Dynamic Loading in Assassin s Creed 35

36 Implementing Dynamic Loading Part of serialization model Level/save file has cells Cell addresses in memory Load/page on demand On Disk Sort of like virtual memory But paging strategy is spatial In RAM 36

Dynamic Loading Challenges Not same as virtual memory Objects unloaded do not exist Do not save state when unload Objects loaded are new created Can lead to unexpected states Forgetful NPCs Creative

37 Dynamic Loading Challenges Not same as virtual memory Objects unloaded do not exist Do not save state when unload Objects loaded are new created Can lead to unexpected states Forgetful NPCs Creative Assassin s Creed kills Workaround: Global State Track major game conditions Example: Guards Alerted Use to load objects in standard, but appropriate, configurations See Piazza for There is No Spoon 37

38 Summary Memory management can be a major challenge Manual deallocation is difficult Garbage collection is better on paper But not all techniques easily available Custom allocators common in AAA games Organize memory in pools and budget use Could oretically support GC as well Dynamic loading most relevant to you 38

Memory Management: The Details

Lecture 10 Memory Management: The Details Sizing Up Memory Primitive Data Types Complex Data Types byte: char: short: basic value (8 bits) 1 byte 2 bytes Pointer: platform dependent 4 bytes on 32 bit machine