Hi everyone!

I'm digging into open source engines to understand their memory management but the more I look in it, the more confused I get.

For example Unigine ( it's a nice engine ) uses different headers for all kind of allocations.

struct FixedChunk;
struct FixedAllocator;
struct HeapChunk;
struct HeapPool;
struct HeapAllocator;
struct SystemAllocator;
struct Allocator;
struct Memory;


Why are they using so many structs(all in different headers)?
An allocator is an allocator so why don't they just write 1 struct: Allocator. and use it for everything.
I don't get it why they uses so many structs. So if someone could explain this for me, thank you!


~EngineProgrammer

An allocator is an allocator so why don't they just write 1 struct: Allocator. and use it for everything.
I don't get it why they uses so many structs. So if someone could explain this for me, thank you!

Each allocator likely has a mix of common and unique/specific needs based on a unique purpose or scenario. This probably means the structs have slightly differing members based around that need, which likely means the structs have varying sizes. If you packed all structs into one you'd be stuck with the same size struct all the time and the size would always be equivalent to the largest of them all, also always the worst case scenario.

Structs in memory management are often used as headers for blocks of allocations (or sometimes with each allocation) so this would mean that every time you deploy the header it will use the same memory as this worst case scenario, regardless of the other memory you're looking to allocate. If you are using one of the smaller structs that uses only a subset of the members, needing only to occupy just a few bytes then you are wasting memory.

While there are other ways to boil this egg, using different structs allows you to more easily access only the members you need and only use only the memory those members require.

Two blogs that describe some of the available allocation strategies very well:
http://molecularmusings.wordpress.com/
http://jfdube.wordpress.com/

You might want to take a look at Hoard (formerly known as HeapLayers), an open-source C++ allocator (collection/library), which enables you to use both general-purpose allocators as well as special-purpose ones. Hoard's real strength is that most of its allocation layers can be composed and extended in nearly countless ways via C++ mix-ins (brilliant idea, IMHO) and that its general-purpose allocator is very efficient for multithreaded applications.
One of the authors, Emery Berger, have also published several, in-depth articles online (most available from his site) about allocation strategies and implementation.