Memory: The Basics

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Memory: The Basics

Lecture 8


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�  Playstation 3 �  256 MB Ram for system �  256 MB for graphics card

�  X-Box 360 �  512 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

Memory Basics

Gaming Memory Constraints

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�  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

Memory Basics

Graphics Memory

Must Load Textures

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�  Shared Memory �  Cores access same memory �  Often have private caches �  Coherency a big problem:

caches may not agree

�  Nonuniform Memory �  All memory is local only �  Communicate w/ messages �  Coherency less of an issue,

but programming is hard

Memory Basics

Multicore and Memory

X-Box 360

Playstation 3

Not Coherent

Explicit Memory Transfer

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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

Memory Basics 5


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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

Memory Basics

But Will Become an Issue in Future

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�  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

Memory Basics 7

Why Not Virtual Memory?


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�  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

�  More if using mipmaps �  Graphic card texture feature �  Smaller versions of image �  Cached for performance �  But can double memory use

Memory Basics

Memory Usage: Images

Original Image

MipMaps

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�  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”

Memory Basics

But My JPEG is only 8 KB!

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Memory Basics 10

Traditional Memory Organization

Program Data

Heap

Stack

Free Space

Program Code Static Variables

Objects created via new Allocations with malloc

Function parameters Local variables Return values

Easy to Track

Easy to Track

Problems!

High Address

Low Address


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Memory Basics 11

Traditional Memory Organization

Program Data

Heap

Stack

Free Space

Program Code Static Variables

Objects created via new Allocations with malloc

Function parameters Local variables Return values

High Address

Low Address

Dedicated to process.

Consists of machine addressable space.

Leverages Virtual Memory


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Memory Basics 12

Mobile Memory Organization

Program Data

Stack

Program Data

Stack

Program Data

Stack

Program Data

Stack

Device Memory

Heap


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�  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

Memory Basics 13

How Do Apps Compete for Memory?


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�  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

Memory Basics 14

How Do Apps Compete for Memory?

You must code this! Otherwise, data is lost.


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Primitive Data Types

�  byte: basic value (8 bits)

�  char: 1 byte

�  short: 2 bytes

�  int: 4 bytes

�  long: 8 bytes

�  float: 4 bytes

�  double: 8 bytes

Memory Basics

Sizing Up Memory

Complex Data Types

�  Pointer: platform dependent �  4 bytes on 32 bit machine

�  8 bytes on 64 bit machine �  Java reference is a pointer

�  Array: data size * length �  Strings similar (w/ trailing null)

�  Struct: sum of struct fields �  Same rule for classes �  Structs = classes w/o methods

Not standard May change

IEEE standard Won’t change

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class Date {

short year;

byte day;

byte month;

}

class Student {

int id;

Date birthdate;

Student* roommate;

}

2 byte

1 byte

1 bytes

4 bytes

4 bytes

4 bytes

4 or 8 bytes (32 or 64 bit)

12 or 16 bytes Memory Basics

Memory Example

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class Date {

short year;

byte day;

byte month;

}

�  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 �  Field order matters!

�  Pad between fields to align �  Worse on 64 bit machines

�  Rule: Order large to small

Memory Basics

Memory Alignment

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�  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 �  Field order matters!

�  Pad between fields to align �  Worse on 64 bit machines

�  Rule: Order large to small

Memory Basics

Memory Alignment

Struct w/ 3 shorts and an int: short short short int

short short short int



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Memory Basics 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 the whole line into cache


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�  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!

Memory Basics

Data Structures and Memory

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�  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!

Memory Basics

Data Structures and Memory

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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)

Memory Basics 22

C/C++: Allocation Process

new

�  Based on data type �  Give it a data type �  If a class, calls constructor �  Else no default initialization

�  Example: Point* p = new Point();

Stack

? ?

… ?

Heap

n bytes

sizeof(Class)

Stack

1 0

… 1

Heap


the

malloc



Memory Basics 23


new

�  Based on data type �  Give it a data type �  If a class, calls constructor �  Else no default initialization

�  Example: Point* p = new Point();

Stack

? ?

… ?

Heap

n bytes

sizeof(Class)

Stack

1 0

… 1

Heap

Preferred in C Preferred in C++


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malloc



Memory Basics 24


new

�  Can emulate malloc �  Create a char (byte) array �  Arrays not initialized �  Typecast after creation


Stack

? ?

… ?

Heap

n bytes

Stack

? ?

… ?

Heap

n bytes


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Stack Based Object

�  Call with () after variable �  Calls constructor with args �  Puts object entirely on stack �  Deleted when stack popped

�  Example: Point p(1,2);

Memory Basics 25

C++: Objects vs. Allocation

Heap Based Object

�  Call with new syntax �  Pointer on stack �  Object in the heap �  Must be manually deleted

�  Example: Point p* = new Point(1,2)

1 2

Stack Stack

1 2

Heap


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Stack Based Object

�  Call with () after variable �  Calls constructor with args �  Puts object entirely on stack �  Deleted when stack popped

�  Example: Point p(1,2);

Memory Basics 26

C++: Objects vs. Allocation

Heap Based Object

�  Call with new syntax �  Pointer on stack �  Object in the heap �  Must be manually deleted

�  Example: Point p* = new Point(1,2)

1 2

Stack Stack

1 2

Heap

�  Not in Java, C#, Obj-C

�  But C#/Obj-C have structs �  Classes without any methods �  Can exist on the stack


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�  Depends on allocation �  malloc: free �  new: delete

�  What does deletion do? �  Marks memory as available �  Does not erase contents �  Does not reset pointer

�  Only crashes if pointer bad �  Pointer is currently NULL �  Pointer is illegal address

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;

}

cout << "Program done" << endl;

}

Memory Basics 27

Manual Deletion in C/C++


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Memory Leaks

Memory Basics

�  Leak: Cannot release memory �  Object allocated on the heap

�  Only reference is moved �  No way to reference object

�  Consumes memory fast!

�  Can even happen in Java �  JNI supports native libraries �  Method may allocate memory

�  Need another method to free �  Example: dispose() in JOGL

memoryArea = newArea;

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A Question of Ownership

void foo() {

MyObject* o = � new MyObject();

o.doSomething();

o = null;

return;

}

void foo(int key) {

MyObject* o =� table.get(key);

o.doSomething();

o = null;

return;

}

Memory Basics

Memory Leak

Not a Leak

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void foo() {

MyObject* o =� table.get(key);

table.remove(key);

o = null;

return;

}

void foo(int key) {

MyObject* o =� table.get(key); table.remove(key); ntable.put(key,o); o = null;

return; }

Memory Basics

Memory Leak?

Not a Leak

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Thread 1

void run() {

o.doSomething1();

}

Memory Basics 31


Thread 2

void run() {

o.doSomething2();

}

Current owners of obj

Who deletes obj?

Topic of Next Lecture

Documents

Memory: The Basics