I was reading this article, and this guy goes on talking about how everyone can greatly benefit from mixing in data oriented design with OOP. He doesn't show any code samples, however.
I googled this and couldn't find any real information as to what this is, let alone any code samples. Is anyone familiar with this term and can provide an example? Is this maybe a different word for something else?
First of all, don't confuse this with data-driven design.
My understanding of Data-Oriented Design (DOD) is that it is about organizing your data for efficient processing. Especially with respect to cache misses etc. Data-Driven Design on the other hand is about letting data control a lot of the behavior of your program (described very well by Andrew Keith's answer).
Say you have ball objects in your application with properties such as color, radius, bounciness, position, etc.
In OOP you would describe balls like this:
class Ball {
Point position;
Color color;
double radius;
void draw();
};
And then you would create a collection of balls like this:
vector<Ball> balls;
In Data Oriented Design, however, you are more likely to write the code like this:
class Balls {
vector<Point> position;
vector<Color> color;
vector<double> radius;
void draw();
};
As you can see there is no single unit representing one Ball anymore. Ball objects only exist implicitly.
This can have many advantages, performance-wise. Usually, we want to do operations on many balls at the same time. The hardware usually wants large contiguous chunks of memory to operate efficiently.
Secondly, you might do operations that affect only part of the properties of a ball. For E.g. if you combine the colors of all the balls in various ways, then you want your cache to only contain color information (which DOD allows). However, when all ball properties are stored in one unit (like OOP) you will pull in all the other properties of a ball as well. Even though you don't need them.
Say each ball takes up 64 bytes and a Point takes 4 bytes. A cache slot takes, say, 64 bytes as well. If I want to update the position of 10 balls, then:
These numbers are arbitrary - I assume a cache block is bigger.
But it illustrates how memory layout can have a severe effect on cache hits and thus performance. This will only increase in importance as the difference between CPU and RAM speed widens.
In my ball example, I simplified the issue a lot, because usually for any normal app you will likely access multiple variables together. E.g. position and radius will probably be used together frequently. Then your DOD structure should be:
class Body {
Point position;
double radius;
};
class Balls {
vector<Body> bodies;
vector<Color> color;
void draw();
};
The reason you should do this is that if data used together are placed in separate arrays, there is a risk that they will compete for the same slots in the cache. Thus loading one will throw out the other.
So compared to Object-Oriented programming, the classes you end up making are not related to the entities in your mental model of the problem. Since data is lumped together based on data usage, you won't always have sensible names to give your classes in Data-Oriented Design.
The thinking behind Data-Oriented Design is very similar to how you think about relational databases. Optimizing a relational database can also involve using the cache more efficiently, although in this case, the cache is not CPU cache but pages in memory.
Just like with Data-Oriented Design these choices are made by looking at what the data access patterns are and where the performance bottleneck is.
struct balls {vector<vec3> pos; vector<vec3> velocity;}, wouldn't updating the position of each ball actually thrash the cache since you'd move back and forth between the velocity vector and the position vector (yes modern machines and cache-lines and all that, this is also just an illustration)? –
Nanine Mike Acton gave a public talk about Data oriented design recently:
My basic summary of it would be: if you want performance, then think about data flow, find the storage layer that is most likely to screw with you and optimize for it hard. Mike is focusing on L2 cache misses, because he's doing realtime, but I imagine the same thing applies to databases (disk reads) and even the Web (HTTP requests). It's a useful way of doing systems programming, I think.
Note that it doesn't absolve you from thinking about algorithms and time complexity, it just focuses your attention at figuring out the most expensive operation type that you then must target with your mad CS skills.
I just want to point out that Noel is talking specifically about some of the specific needs we face in game development. I suppose other sectors that are doing real-time soft simulation would benefit from this, but it is unlikely to be a technique that will show noticeable improvement to general business applications. This set up is for ensuring that every last bit of performance is squeezed out of the underlying hardware.
A data oriented design is a design in which the logic of the application is built up of data sets, instead of procedural algorithms. For example
procedural approach.
int animation; // this value is the animation index
if(animation == 0)
PerformMoveForward();
else if(animation == 1)
PerformMoveBack();
.... // etc
data design approach
typedef struct
{
int Index;
void (*Perform)();
}AnimationIndice;
// build my animation dictionary
AnimationIndice AnimationIndices[] =
{
{ 0,PerformMoveForward }
{ 1,PerformMoveBack }
}
// when its time to run, i use my dictionary to find my logic
int animation; // this value is the animation index
AnimationIndices[animation].Perform();
Data designs like this promote the usage of data to build the logic of the application. Its easier to manage especially in video games which might have thousands of logic paths based on animation or some other factor.
I first heard about Data-Oriented Design in the Our Machinery podcast, episode "S3: EP4 Data-oriented Design". https://www.owltail.com/podcast/Atvr2-Our-Machinery
Maybe this has changed, but a while ago finding information about Data-Oriented Design was difficult. The only book I found is: https://www.manning.com/books/data-oriented-programming
If you want to take advantage of modern processor architecture, you need to lay out your data in memory in a certain way. CPUs are really good at processing simple types that are laid out sequentially in memory. Any other layout has a much higher processing cost.
In object-oriented approach, you always think about one instance, and then you are extending it to several instances by grouping objects into collections. But from the hardware point of view, this comes with the added cost.
In data-oriented approach, you don't have an "instance" in the same way you have in object-oriented programming. Your instance can have an identifier, similar to data in relational databases, but apart from that, data related to your instance can be split over several tables (tables are implemented as vectors), to allow efficient processing.
An example: imagine you have class Student { int id; std::string name; float average; bool graduated; }. In case of OOP, you would put all your students in a single vectors.
In data-oriented design, you will first ask yourself what kind of processing you want to do to this data. Say you want to calculate an average mark for all students that still haven't graduated. So you will create a table which contains only students that have graduated, and another that haven't. You won't keep the student name in that table since it is not used for processing. But you will keep a student ID and an average mark in the table.
Now calculating average mark for non-graduated students will mean iterating through the non-graduated table and performing the calculation. Since average marks are neighboring in memory, your CPU will use SIMD and process the data in the most efficient way possible. Since we are not querying the bool graduated to test if the student has graduated, there are no data cache misses.
This sounds nice in theory but I have never done this kind of development on a real-world project. If anybody have any experience, please contact me, I have many questions.
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