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Memory Management: the Details Sizing up Memory the gamedesigninitiative at cornell university Lecture 10 Memory Management: The Details Sizing Up Memory Primitive Data Types Complex Data Types byte: basic value (8 bits) Pointer: platform dependent 4 bytes on 32 bit machine char: 1 byte 8 bytes on 64 bit machine short: 2 bytes Java reference is a pointer int: 4 bytes Array: data size * length Not standard long: 8 bytes May change Strings similar (w/ trailing null) Struct: sum of struct fields float: 4 bytes IEEE standard Won’t change Same rule for classes double: 8 bytes Structs = classes w/o methods the gamedesigninitiative 2 Memory Details at cornell university Memory Example class Date { short year; 2 byte byte day; 1 byte byte month; 1 bytes } 4 bytes class Student { int id; 4 bytes Date birthdate; 4 bytes Student* roommate; 4 or 8 bytes (32 or 64 bit) } 12 or 16 bytes the gamedesigninitiative 3 Memory Details at cornell university Memory Alignment All data types should align Type starts at multiple of size class Date { Shorts at even addresses Ints/words at multiple of 4 short year; Longs at multiple of 8 byte day; Structs may require padding Field order matters! byte month; Pad between fields to align } Worse on 64 bit machines Rule: Order large to small the gamedesigninitiative 4 Memory Details at cornell university Memory Alignment All data types should align Struct w/ 3 shorts and an int: Type starts at multiple of size short short short int Shorts at even addresses Ints/words at multiple of 4 short int short short Longs at multiple of 8 Structs may require padding short short int short Field order matters! Pad between fields to align int short short short Worse on 64 bit machines Rule: Order large to small the gamedesigninitiative 5 Memory Details at cornell university Related Topic: Cache Lines All CPUs have caches A6 (iOS): 32k L1, 1Mb L2 Snapdragon (Nexus): 4k L0, 16k L2, 2Mb L2 Populate with cache lines Data block of fixed size Relative to cache size Fetch pulls in whole line Fetches the CPU whole line Can affect performance into cache Accessing neighbors is fast! Example: array scanning the gamedesigninitiative 6 Memory Details at cornell university Data Structures and Memory Collection types are costly Even null pointers use memory Common for pointers to use as much memory as the pointees Unbalanced trees are very bad Even true of (pointer) arrays Array uses additional memory Not so in array of structs Objects stored directly in array But memory alignment! the gamedesigninitiative 7 Memory Details at cornell university Data Structures and Memory Collection types are costly Even null pointers use memory Common for pointers to use as much memory as the pointees Unbalanced trees are very bad Even true of (pointer) arrays Array uses additional memory Not so in array of structs Objects stored directly in array But memory alignment! the gamedesigninitiative 8 Memory Details at cornell university Two Main Concerns with Memory Allocating Memory With OS support: standard allocation Reserved memory: memory pools Getting rid of memory you no longer want Doing it yourself: deallocation Runtime support: garbage collection the gamedesigninitiative 9 Memory Details at cornell university C/C++: Allocation Process malloc new Based on memory size Based on data type Give it number of bytes Give it a data type Typecast result to assign it If a class, calls constructor No initialization at all Else no default initialization Example: Example: char* p = (char*)malloc(4) Point* p = new Point(); Stack Heap Stack Heap sizeof ? n bytes 1 ? 0 (Class) … … ? 1 the gamedesigninitiative 10 Memory Details at cornell university C/C++: Allocation Process malloc new Based on memory size Based on data type Give it number of bytes Give it a data type Typecast result to assign it If a class, calls constructor No initialization at all Else no default initialization Preferred in C Preferred in C++ Example: Example: char* p = (char*)malloc(4) Point* p = new Point(); Stack Heap Stack Heap sizeof ? n bytes 1 ? 0 (Class) … … ? 1 the gamedesigninitiative 11 Memory Details at cornell university C/C++: Allocation Process malloc new Based on memory size Can emulate malloc Give it number of bytes Create a char (byte) array Typecast result to assign it Arrays not initialized No initialization at all Typecast after creation Example: Example: char* p = (char*)malloc(4) Point* p = (Point*)(new char[8]) Stack Heap Stack Heap n bytes n bytes ? ? n bytes ? ? … … ? ? the gamedesigninitiative 12 Memory Details at cornell university Recall: Custom Allocators Pre-allocated Array (called Object Pool) Start Free End Instead of new, get object from array Just reassign all of the fields Easy if only Use Factory pattern for constructor one object Delete all objects at frame end type to Just reset free pointer to start allocate Do not worry about freeing mid frame the gamedesigninitiative 13 Memory Details at cornell university Anatomy of an Objective-C Constructor NSDate* d1 = [NSDate dateWithString:"2014-03-05"]; Static Method Allocates & Initializes the gamedesigninitiative 14 Memory Details at cornell university Anatomy of an Objective-C Constructor NSDate* d1 = [NSDate dateWithString:"2014-03-05"]; Static Method Allocates & Initializes NSDate* d2 = [[NSDate alloc] initWithString:"2014-03-05"]]; Static Method Instance Method Allocates Initializes the gamedesigninitiative 15 Memory Details at cornell university Custom Allocation in Objective-C @implementation GObject static char* mempool = malloc(sizeof(GObject)*AMOUNT); static int pointer = 0; + (id)alloc { if (pointer >= AMOUNT) { return Nil; } pointer += sizeof(GObject); return (id)mempool[pointer-sizeof(Gobject)]; } the gamedesigninitiative 16 Memory Details at cornell university Custom Allocation in Objective-C @implementation GObject static char* mempool = malloc(sizeof(GObject)*AMOUNT); static int pointer = 0; + (id)alloc { if (pointer >= AMOUNT) { return Nil; } Fail gracefully pointer += sizeof(GObject); return (id)mempool[pointer-sizeof(Gobject)]; } the gamedesigninitiative 17 Memory Details at cornell university Object Pools In Java public class GObjectFactory { private Gobject mempool = new Gobject[AMOUNT]; private int pointer = 0; public GObjectFactory() { for(int ii = 0; ii < AMOUNT; ii++) { mempool[ii] = new GObject(); } } Initialize Pool … the gamedesigninitiative 18 Memory Details at cornell university Object Pools In Java public class GObjectFactory { … public MakeGObject() { if (pointer >= AMOUNT) { return null; } GObject o = mempool[pointer++]; // Initialize object here return o; Initialization } & Allocation Combined the gamedesigninitiative 19 Memory Details at cornell university C++: Objects vs. Allocation Stack Based Object Heap Based Object Call with () after variable Call with new syntax Calls constructor with args Pointer on stack Puts object entirely on stack Object in the heap Deleted when stack popped Must be manually deleted Example: Example: Point p(1,2); Point p* = new Point(1,2) Stack Stack Heap 1 1 2 2 the gamedesigninitiative 20 Memory Details at cornell university C++: Objects vs. Allocation Stack Based Object Heap Based Object Call with () after variable Call with new syntax Calls constructor with args Pointer on stack Not in Java, C#, Obj-C Puts object entirely on stack Object in the heap But Deleted C#/Obj when-C have stack structs popped Must be manually deleted Classes without any methods Example: Example: Can exist on the stack Point p(1,2); Point p* = new Point(1,2) Stack Stack Heap 1 1 2 2 the gamedesigninitiative 21 Memory Details at cornell university Two Main Concerns with Memory Allocating Memory With OS support: standard allocation Reserved memory: memory pools Getting rid of memory you no longer want Doing it yourself: deallocation Runtime support: garbage collection the gamedesigninitiative 22 Memory Details at cornell university Manual Deletion in C/C++ Depends on allocation int main() { cout << "Program started" << endl; malloc: free int* a = new int[LENGTH]; new: delete What does deletion do? delete a; Marks memory as available for(int ii = 0; ii < LENGTH; ii++) { Does not erase contents cout << "a[" << ii << "]=" Does not reset pointer << a[ii] << endl; Only crashes if pointer bad } Pointer is currently NULL cout << "Program done" << endl; Pointer is illegal address } the gamedesigninitiative 23 Memory Details at cornell university Memory Leaks Leak: Cannot release memory Object allocated on the heap Only reference is moved No way to reference object Consumes memory fast! memoryArea = newArea; Can even happen in Java JNI supports native libraries Method may allocate memory Need another method to free Example: dispose() in JOGL the gamedesigninitiative 24 Memory Details at cornell university A Question of Ownership ! void foo() { void foo(int key) {! MyObject* o = " !MyObject* o =" new MyObject();! table.get(key);! !o.doSomething();! !o.doSomething();! !o = null;! !o = null;! Memory Not a !return;! Leak !return;! Leak }! }! the gamedesigninitiative 25 Memory Details at cornell university A Question of Ownership ! void foo() {! void foo(int key) {! !MyObject* o =" !MyObject* o =" table.get(key);! table.get(key);! !table.remove(key);! !table.remove(key);! !ntable.put(key,o);! !o = null;! Memory !o = null;! Leak? !return;! Not a !return;! Leak }! }! the gamedesigninitiative 26 Memory Details at cornell university A Question of Ownership Thread 1 Thread 2 “Owners”
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