Chapter 3.5 Memory and I/O Systems

2 Memory Management Memory problems are one of the leading causes of bugs in programs (60-80%) MUCH worse in languages with explicit memory management (C or C++) In garbage collecting languages (e.g. Java, Python, Unicon) you may still have leaks due to OS, memory reference cycles, or live-but-never used data

3 Memory Management We have three goals when working with memory Safety Knowledge Control

4 Memory Management Memory fragmentation Free physical memory is only available in chunks Can prevent an allocation from completing successfully even if there's plenty of free memory Virtual addressing helps with fragmentation problems

5 Memory Management Virtual addressing

6 Memory Management “Static” memory allocation Really means: globals + stack If all memory is statically allocated there will be very few problems. Mostly rules out: Leaks Fragmentation Running out of memory Very restrictive Lots of underutilized memory Many old games were done this way

7 Memory Management “Dynamic” allocation Really, this is talking about heap allocation Much more flexible than static allocations Lots of potential problems Often: override global new and delete to control allocations

8 Memory Management Memory manager Heaps are large collections of memory allocated for arbitrary mixed uses Managed by language runtime system Especially in C/C++: simple error-checking schemes reduce memory overrun problems Slow and wasteful of memory for lots of small objects

9 Memory Management Memory leaks A memory leak is a memory allocation that is no longer necessary and was not released by the program Memory leaks can cause the game to use up resources and run out of memory To find memory leaks, look for all the allocations that occurred between two particular points in time

10 Memory Management Memory pools Memory pools are contiguous blocks of memory used to allocate objects Allocations are extremely fast There is no fragmentation... Except for all the unused space We can allocate objects from specific classes with a simple macro

11 Memory Management Memory pools

12 File I/O Full file I/O (e.g. reading/writing local named files) not always available Many different API’s and speeds of accessing different devices Memory cards, network, etc Game developer usually has no control over physical disk allocation Which can lead to long load times

13 File I/O Unified file system Platform independent Provide same interface to different types of media Based around FileSystem class

14 File I/O File system Uses concept of streams Stream can be the source of data Possible sources File, memory, network Or it can be a layer that adds some properties Possible layers Buffering, compression, range, etc

15 File I/O File system

16 Game Resources A game resource (or asset) is anything that gets loaded that could be shared by several parts of the game A texture, an animation, a sound, etc We want to load and share resources easily There will be many different types of resources in a game

17 Game Resources Resource manager Uses registering object factory pattern Can register different types of resources All resource creation goes through the resource manager Requests for existing resources usually don't have to load it again Basically a giant in-memory cache

18 Game Resources Resources and instances Resource is the part of the asset that can be shared among all parts of the game Instance is the unique data that each part of the game needs to keep on its own This is exactly the flywheel design pattern

19 Game Resources Resource lifetime How do we know when we can destroy shared resources? All at once At the end of the level Explicit lifetime management Reference counting, garbage collection GPU resources may be managed differently than CPU resources

20 Serialization Every game needs to save and restore some game state Even for check point saves Level editing and creation could be implemented as a saved game

21 Serialization Saving [I]Serializable interface for read and write operations Each class implements a Write() (or writeObject(), etc) function Saving pointers is a problem They won't be the same when we load So save an ID of some kind Saving open OS handles is also no good

22 Serialization Loading Create object types through an object factory Read() function takes care of loading the data from the stream, assigning IDs stored in file with newly created objects/pointers Pointers are loaded into a translation table Second pass fixes up all the pointers