Thursday, January 5, 2012

Components system + Flow Graph

 Like lots of other games, my game uses a component system.

This system is designed to allow for improved cache usage and easy parallelization.
It is loosely based on this article published by Insomniacs.

For cache friendly behavior all components of a given type live in contiguous memory, and are all updated together.

To specify attributes for components I have a Lua file,  which for each component type indicates, among other things, how they should update(parallel, serial), and which other component types they depend on.

 I use TBB Flow Graph to express dependencies, any component that has a dependency on another being updated prior to it, just lists that component as a dependency in the Lua file.

For instance this is the declaration of my Occlusion Rasterizer component in Lua.

reg("occlusion_rasterizer",    no_inteface,   TM.MULTI,  0.0,  3, {"commands"}) 

The parameters in order are:

name      - name in C++, this matches them up
interface - specifies which interface, if any, the component implements, if Occlusion Rasterizer had implemented an interface this would be the name of that component.  This allows for things like finding all components that implement a given interface.
parallel      - either SINGLE or MULTI, SINGLE means each component of this type is updated sequencially, MULTI means they are all updated in parallel
time delay - how long between updates to this type, so if you want a given type to only update  4 times per second make this value 0.25 etc
count      - how many components to reserve, this allocates a contiguous block of memory. It will grow as needed, but this allows for the equivalent of a vector.reserve()
dependencies - list of any components that must update prior to this component, TBB flow graph guarantees they will finish updating prior to this component updating. Occlusion Rasterizer depends on a command list being generated, so it lists "commands" as a dependency.

 Each component type derives from a base component type in C++, currently this adds 12 bytes of overhead to each instance in 32 bit builds, and 16 bytes in 64 bit builds.

The base component contains this:
vtable ptr -  4/8 bytes. Removing this is possible but makes the system somewhat clumsier to use.
chain index  - 4 bytes, which chain it is on, chains are a series of components
type            - 2 bytes, each component type has a unique ID
offset          - 2 bytes,  index into the pool containing components of its type

 I've been pretty happy with this system, my components are small self contained little objects,  they are cache friendly, and can be allocated and destroyed in large numbers. Adjusting dependencies as new types are added is also a very simple since no recompilation is needed, just a chance to a Lua script file.

 A major advantage of component systems, which I think does not get stated clearly enough, is how much glue code they save you from writing.  Having to create classical C++ game objects and populate them with appropriate member data, worry about all the fragile and arbitrary hierarchies, and then writing all the systems that control them is a huge amount of unnecessary code.

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