the gamedesigninitiative at cornell university Lecture 8 Memory: The Basics

Transcription

1 Lecture 8 Memory: The Basics

2 Gaming Memory Constraints Playstation MB Ram for system 256 MB for graphics card X-Box MB Ram for system Shared w/ graphics card iphone/ipad 1GB Ram for system Shared w/ graphics card Nintendo Wii 88 MB Ram for system 24 MB shared w/ graphics card 2

3 Graphics Memory Separate pool for graphics card Faster than general memory Generally stores texture info Also includes frame buffer But CPU has slower access Access via general memory Moving data to graphics memory requires explicit I/O Standard structure for PCs Game consoles are different! Consoles: single memory pool Must Load Textures 3

4 Multicore and Memory Shared Memory X-Box 360 Cores access same memory Not Coherent Often have private caches Coherency a big problem: caches may not agree Nonuniform Memory Playstation 3 All memory is local only Communicate w/ messages Coherency less of an issue, but programming is hard Explicit Memory Transfer 4

5 The Challenge of Memory Major challenge for gaming hardware Must minimize memory footprint Must ship memory between processors Must ensure consistency between processors Less of an issue with PCs Lots and lots of memory Relatively fast access Very few cores 5

6 The Challenge of Memory Major challenge for gaming hardware Must minimize memory footprint Must ship memory between processors Must ensure consistency between processors Less of an issue with PCs Lots and lots of memory Relatively fast access Very few cores But Will Become an Issue in Future 6

7 Why Not Virtual Memory? Secondary storage exists Consoles have 250 GB HD idevices have 64 GB Flash But access time is slow HDs transfer at ~160 MB/s Best SSD is ~500 MB/s Recall 16 ms per frame At best, can access 8 MB Yields uneven performance 7

8 Memory Usage: Images Pixel color is 4 bytes 1 byte each for r, b, g, alpha More if using HDR color Image a 2D array of pixels 1280x1024 monitor size 5,242,880 bytes ~ 5 MB MipMaps Original Image More if using mipmaps Graphic card texture feature Smaller versions of image Cached for performance But can double memory use 8

9 But My JPEG is only 8 KB! Formats often compressed JPEG, PNG, GIF But not always TIFF Must uncompress to display Need full pixels to uncompress In RAM or graphics card Only load when needed Texture loading is primary I/O operation in high-end games Many tricks to optimize The cause of texture popping 9

10 Traditional Memory Organization High Address Function parameters Local variables Return values Easy to Track Free Space Heap Objects created via new Allocations with malloc Problems! 10 Low Address Program Data Program Code Static Variables Easy to Track

11 Traditional Memory Organization High Address Function parameters Local variables Return values Dedicated to process. Free Space Consists of machine addressable space. Heap Objects created via new Allocations with malloc Leverages Virtual Memory 11 Low Address Program Data Program Code Static Variables

12 Device Memory Mobile Memory Organization Program Data Heap Program Data Program Data Program Data 12

13 How Do Apps Compete for Memory? Active app takes what it can Cannot take anything from OS OS may suspend inactive apps App Suspension App quits and memory freed ios: 5 minutes (or so) on exit Android: If memory needed Suspend apps can recover OS allows limited paging Page out state on suspension Page back in when restart app 13

14 How Do Apps Compete for Memory? Active app takes what it can Cannot steal from OS OS may suspend inactive apps App Suspension App quits and memory freed ios: 5 minutes (or so) on exit Android: If memory needed Suspend apps can recover OS allows limited paging Page out state on suspension Page back in when restart app You must code this! Orwise, data is lost. 14

15 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 8 bytes on 64 bit machine Java reference is a pointer int: long: 4 bytes 8 bytes Not standard May change Array: data size * length Strings similar (w/ trailing null) float: 4 bytes double: 8 bytes IEEE standard Won t change Struct: sum of struct fields Same rule for classes Structs = classes w/o methods 15

16 Memory Example class Date { } short year; byte day; byte month; 2 byte 1 byte 1 bytes 4 bytes class Student { int id; Date birthdate; Student* roommate; 4 bytes 4 bytes 4 or 8 bytes (32 or 64 bit) 16 } 12 or 16 bytes

17 Memory Alignment All data types should align Type starts at multiple of size class Date { short year; Shorts at even addresses Ints/words at multiple of 4 Longs at multiple of 8 } byte day; byte month; Structs may require padding Field order matters! Pad between fields to align Worse on 64 bit machines Rule: Order large to small 17

18 Memory Alignment Struct w/ 3 shorts and an int: short short short int short int short short All data types should align Type starts at multiple of size Shorts at even addresses Ints/words at multiple of 4 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 18

19 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 Can affect performance Accessing neighbors is fast! Example: array scanning Big deal for data structures CPU Fetches whole line into cache 19

20 Data Structures and Memory Collection types are costly Even null pointers use memory Common for pointers to use as much memory as 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! 20

21 Data Structures and Memory Collection types are costly Even null pointers use memory Common for pointers to use as much memory as 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! 21

22 C/C++: Allocation Process malloc Based on memory size Give it number of bytes Typecast result to assign it No initialization at all Example: char* p = (char*)malloc(4) Heap new Based on data type Give it a data type If a class, calls constructor Else no default initialization Example: Point* p = new Point(); Heap 22??? n bytes sizeof(class)

23 C/C++: Allocation Process malloc Based on memory size Give it number of bytes Typecast result to assign it No initialization at all Example: char* p = (char*)malloc(4) Preferred in C Heap new Based on data type Give it a data type If a class, calls constructor Else no default initialization Preferred in C++ Example: Point* p = new Point(); Heap 23??? n bytes sizeof(class)

24 C/C++: Allocation Process malloc Based on memory size Give it number of bytes Typecast result to assign it No initialization at all Example: char* p = (char*)malloc(4) Heap new Can emulate malloc Create a char (byte) array Arrays not initialized Typecast after creation Example: char* p = (char*)malloc(4) Heap??? n bytes??? n bytes 24

25 C++: Objects vs. Allocation Based Object Call with () after variable Calls constructor with args Puts object entirely on stack Deleted when stack popped Example: Point p(1,2); 1 2 Heap Based Object Call with new syntax Pointer on stack Object in heap Must be manually deleted Example: Point p* = new Point(1,2) Heap

26 C++: Objects vs. Allocation Based Object Call with () after variable Calls constructor with args Not in Java, C#, Obj-C Puts object entirely on stack But Deleted C#/Obj-C when have stack structs popped Classes without any methods Can exist on stack Example: Point p(1,2); 1 2 Heap Based Object Call with new syntax Pointer on stack Object in heap Must be manually deleted Example: Point p* = new Point(1,2) Heap

27 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; 27

28 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 28

29 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 29

30 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 30

31 A Question of Ownership Thread 1 Thread 2 Current owners of obj void run() { } o.dosomething1(); void run() { o.dosomething2(); } Who deletes obj? Topic of Next Lecture 31