What we're all trying to say is that DOD is just an idea and shouldn't be taken as some methodology to blindly follow into the depths of every piece of code. It's just an idea, just another way to look at a problem. Though the thing about "problems" in computer science is that each one is quite unique from the next. Such specific problems will usually require a specific solution. Specific solutions to specific problems does not sound like a good thing to apply generic step-by-step instructions upon!
Just a couple of Data-Oriented Design questions.
I'm not trying to use it for every piece of code. I'm trying to use it to speed up entity/component updates. I must say, what I'm asking seems like a totally reasonable request. That is, help me understand how data-oriented design is used in games. So far, I've gotten some good replies along with at least two lectures about how I shouldn't be using data-oriented design like a golden hammer. Is that what I'm doing? Because most of the reading I've done on the subject uses this sort of entity-component update as an example of precisely where DOD comes in handy. Is there some way in which I'm misapplying the concept? If so, it would most helpful if you would be explicit about it.
Point taken, definitely. A lot times I don't feel that I understand a concept unless I've implemented it, though. So the point of this exercise is "Understand Data-Oriented Design". Once I've done that, I'll feel ready to make decisions about where DOD makes sense.
Learning when not to use a tool is as important as learning how to use it.
There are many parts of your game that could use a healthy dose data-oriented design. Look into those. The areas those are will be somewhat game-specific. Some of the more important areas end up being deep in physics code or graphics code that are either inside a complex third-party library (Box2D, Havok, etc.) or custom-coded for your game (rendering).
A good option, if you're trying to learn, is to use a tool like VTune or Cachegrind to find out where performance is suffering due to bad data access behavior, then focus your gaze at that location.
The BitSquid guys have some excellennt use of DOD and a great blog that may give you better ideas about how to use it. http://bitsquid.blogspot.com/ You should just start at the oldest blog post and read forward from there. I don't agree with everything they do but most of it is great and it's very illustrative.
Personally, I don't find a huge amount of value of trying to apply DOD to components. Components glue together different modules, and those individual modules end up being where you need to worry about efficiency the most. Threading also has a bigger impact on modern computer architectures than maximizing single-threaded performance, which is another aspect of DOD that seems to get too little attention: designing data structures in ways that allow trivial concurrency with little contention overhead. Contiguous arrays make that easier since you can divide them up and operate on slices of them without touching other slices. Or if you need to modify bits outside the slice the thread "owns," write changes to a secondary buffer and then merge them after the thread jobs complete so that each thread sees a consistent read-only view of all data without any locks or atomics or anything being necessary.
Keeping your contiguous arrays sorted (if you can, cheaply; often you can't) also helps because it reduces the need to keep indices matched up between different arrays. If you know that entity 3 always has its component before entity 4 then you can iterate over arrays of components and trivially ignore missing ranges.
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