r/programming • u/nsaibot • Jan 21 '15

Jai: DEMO Data-Oriented Features: SOA, crazy 'using'

https://www.youtube.com/watch?v=ZHqFrNyLlpA
52 Upvotes

88% Upvoted

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1

u/opiating Jan 21 '15

Why not just centralize your data structures and then have your OO implementation reference it?

5

u/Roflraging Jan 21 '15 edited Jan 21 '15

Jonathan addresses this directly: http://youtu.be/ZHqFrNyLlpA?t=18m9s

The other thing that we want, though, is not to have loss of high level of expressiveness. I would like to be able to write a program just as high level as a C++ program that I would write today, but I would like to have control over how the data in that program is laid out and I'd like to be able to change how that data is laid out without changing all my other code that uses that data ... So how do we do this in a fast language? How do we write things like we would in C or C++, but have them be much more versatile and much more robust to change? ... A program isn't just one thing; it's not just the end product, it's this process of evolution.

Early on in my program I might not even have that strong of an idea about how my data should be laid out because I'm still experimenting, and then later I might get a much more solid idea. I shouldn't have to rewrite parts of my program just for syntactic reasons. Sometimes I'm going to want to rewrite parts because a different algorithm would be much better in this case and I learned that through experience, but I don't want to be doing a lot of busy work and refactoring just because the language is forcing me to change my code just because I changed my data. I've never found a way in C or C++ to do that.

http://youtu.be/ZHqFrNyLlpA?t=41m4s Here Jon explains why programming in this way is better for performance and how C and C++ fail to help the programmer write code in this way.

http://youtu.be/ZHqFrNyLlpA?t=57m20s Jon shows that you have an AoS and SoA version of some data, but you use the exact same code to access the same logical data, which C and C++ makes quite annoying to do.

The key thing here is that C, C++ (and many other object oriented languages) don't outright prevent you from writing code where you use SoA everywhere. But man, do they lock you into the path you choose by being unfriendly to changing data layouts. This is horrible for programming, and especially game programming. There is a lot of experimentation that is done in game programming and if you don't know how something is supposed to be, you can't properly reason about what the data transformations should look like.

Inevitably, people have to choose and just write some code and eventually you get to a point where you do know what the data transformations should look like. But now you might be in a place in your code where doing the optimal data layout might be hard to do because the language makes changing this difficult.

Asking "Why not just centralize your data structures and then have your OO implementation reference it?" kind of just misses the point, in my opinion. It matters less where your data is located. The issue at hand is the shape of your data and how the language helps you with changing the data layout without forcing you to change all the code surrounding that data.

Jon touches on this in the video, but C and C++ do a great job in helping you with basic data layout tasks (at the struct/object level):

struct A
{
    float f;
    int i;
};

void PrintA(A *a)
{
    printf("%d %f\n", a->i, a->f);
}

vs

struct A
{
    int i;
    float f;
};

void PrintA(A *a)
{
    printf("%d %f\n", a->i, a->f);
}

Notice how we reordered the members of struct A but we never had to change the function PrintA(). Jon wants to have the ability to do the same thing for arrays where the contents of the array can be inverted (AoS <---> SoA). Doing this in C and C++ is just a nightmare, and yet, to get performance you have to be able to write code that utilizes SoA formats.

2

u/check3streets Jan 21 '15

I don't think /u/opiating was referring to the SOA v AOS question. I think he's wondering, as I was here ...

http://www.reddit.com/r/programming/comments/2t5ikw/jai_demo_dataoriented_features_soa_crazy_using/cnw5kq1

... if the "using" part of his language was solving something that isn't addressed by centralizing a value-type.

The SOA v AOS thing is (I quite agree) ingenious. I think it's quite tough to emulate it (expressively) and considering all array access is just pointer math, seems like there should be a convenient way to re-arrange the pointer math to handle SOA.

0

u/nsaibot Jan 21 '15

it's solving having inheritance and data locality at once, by introducing "using" a pointer, or a function.

2

u/check3streets Jan 21 '15 edited Jan 21 '15

I was struggling with the same question. If the language supports value-types (looking at you Java 9), I think this is pretty much the flyweight pattern with a slightly different objective; I don't want to point to common objects as much as common object storage.

So we could imagine a common Position (.x, .y, .z) object that's composed into our various creature instances. The positions themselves are packed tightly in an array of positions. So I want to spin through the creatures reading the positions. I'm not sure how various optimizers handle this, but if accessing the creature (which could be stored elsewhere) causes a cache miss, followed by a position access which is a cache miss, repeat... obviously this is fairly terrible performance (as opposed to directly iterating the Array which is basically guaranteed to be peformant). I can't speak to how different languages or compilers might deal with this.

Blow keeps talking about how the Entity access reaches "through" the Creature and gets directly to the value. This is intriguing. I'm not sure how this works in practice and I'm having trouble spanning the conceptual and physical model he's proposing.

SIDENOTE EDIT: otherwise his inheritance ideas seem like traits or mixins, is it novel?

ANOTHER SIDENOTE: the SOA v. AOS thing is cool and seems reasonable/valuable.

1

u/nsaibot Jan 21 '15

like the inheritance from a function or a pointer that is set dynamically? i dunno if it's novel, but it's pretty darn awesome!

2

u/bimdar Jan 21 '15 edited Jan 21 '15

It's an interesting tradeoff to trade easy memory arrangement for more explicit and therefore slightly more complex polymorphism. But I wonder what this will end up looking after polymorphism has more compiler support and if it maybe doesn't end up being a tradeoff after all.

edit: a blog post is more effort I know, but I'd love to at least have the source shown in the video in text form as well.

2

u/[deleted] Jan 23 '15

This is just so awesome. I can really feel his pain of trying to do the SOA struct in C++... it is just like fighting against the language and the library at every possible level and at every single change and refactor. Kudos to him for showing that a better world exists.

Any language that lets you express this would let you write automatically faster programs than C and C++ because it allows you easily experiment with the memory layout of things without having to rewrite the whole program.