DATA-ORIENTED DESIGN The hardware will thank you.

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Questions from a reader 03/01/2012:12:42:00

* Could a managed solution eventually draw closer to, or overtake, unmanaged code; can automatic detection of usage patterns help close the gap?

We've already seen how sometimes managed code can be faster than unmanaged with the controversial "Java faster than C" article. The reason behind this can only be attributed to performance tweaks that can be applied when the virtual machine's runtime compiler made data oriented optimisations. Data-oriented design can and will be leveraged by everyone that's interested in performance, not just those developing software products, but also the languages that other developers work with.
Usage patterns have often been seen as the holy grail of compiler analysis. The idea that a compiler can figure out what you meant, then implement it in the best way possible. This fairy tale is impossible in statically compiled code, as by definition it is compiled without access to the final data on which the process it run. Managed code can make runtime modifications to its processes, and thus has the opportunity to do realtime data directed optimisations.

Managed code is almost certainly going to to be faster than any naively written unmanaged implementation.

* How portable are these solutions? Are PS3, XBox, PC architectures too different to formulate a common solution? What additional development / maintenance cost does that introduce?

Part of the reason why data-oriented design has been seen as a controversial subject is that the x86 platform's design was influenced heavily by the object oriented programming paradigm. While the hardware we know and love/hate is reasonably good at processing object oriented branchy code, it's not because it's a good general purpose CPU design, but because it's been designed to run existing software faster than the competitor's CPUs.

If it were not for object-oriented design, we would probably be not having the "shift to parallel" discussions this late, nor would we have such large caches on our CPUs. We might not even have spent so much time developing technologies to their fullest such as the out-of-order processing hack, or the CPU resource sharing technique known as hyper-threading.

Because object oriented design tends to require some specific features of a computation system, it has decided the fate of many desktop CPU manufacturers design projects. Only outside desktop, where software is written for the hardware because there is only one customer, one platform, has there been good examples of leaving behind the features required by object-oriented design. The CELL BE is a good example of a piece of hardware that given the write technique, blows away the competition on a processing power per watt or dollar valuation. However, because the main target of the CELL BE was high business or games development, the bad press from the less adventurous developers caught on and has made the CELL out to be a failure.

As to portability of solutions, most data-oriented solutions are better because they concentrate on what needs to be done rather than what might be pretty to someone who likes to read abstractions or object-oriented code. This difference pays off on all platforms, from micro-controllers to quad core x86 machines. Most example code of how to write better code for the PPU or Xenon can be compiled for x86 and still see some marginal improvement. The reason the code isn't greatly better is that the x86 is very good at reducing the impact of Object-oriented code, but it can't fix it completely.

* Does DOD has a place outside game dev? Are games a special case, in that once released, the code is often not supported after a few patches? And you can't just throw more hardware at the solution with console dev.

Any place where resources are not limitless, DOD is preferable. Not only does it allow for simpler to reason about code, and also simpler to maintain due to an inherently transform oriented approach to problem solving, but it is faster and more efficient. There are hardly any areas of coding where being faster at no cost is something to be seen as a disadvantage.

DOD as a games development paradigm is a fallacy. Every minute you spend developing code to get around a language feature, or to make some code look nicer, or play well with others due to the unnecessary restrictions put in place by the object oriented language you are lumbered with, is a minute lost forever. There is no evidence that object-oriented design makes working on large projects easier, but there is evidence that it makes it harder (see large-scale C++ development for some eye-openers.) You only have to look to really big business to see that the 10Mloc+ projects are normally made up of smaller transforms, an inherently data-oriented approach to development, each with separate and specialised code, potentially running on separate and specialised hardware.

The only reason why DOD could be considered games development only is because games development doesn't naturally consider this standard way of solving data-transform problems in a sensible manner and needs a name to guide it back on track.