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I've been trying to make use of the GPU as part of a project of mine. I've looked into both CUDA and OpenCL, but the lack of information showing you how to introduce these into a project is shocking. Even their dedicated forum groups are dead. So now, I'm looking into DirectCompute.

From what I can tell, it's simply a new type of shader file that makes use of HLSL. My question is this, does my program (aside from being DirectX 10 / 11 ) need its structure changed?

I mean, is it simply a case of creating the CS file, setting in the project like I would any other shader, and watch the magic happen?

Any information on this would be appreciated.

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What is so dead about the official OpenCL forum? It seems to have recent posts in it. – Nicol Bolas Mar 4 '12 at 2:16
"lack of information", "shocking"? There are tons of examples. The OpenCL wikipedia page even shows a Fast Fourier Transformation example, its obvious how to integrate that into a project. – Maik Semder Mar 4 '12 at 10:35
Great OpenCL tutorial by Tim Mattson explaining the basic principles and the paradigm shift behind parallel distributed computing: "Replace loops with functions", coming with a bunch of easy to follow small examples – Maik Semder Mar 4 '12 at 10:53
up vote 1 down vote accepted

Yes, you initialize it like every other shader type using it's specific CreateComputeShader call with the device handle. And you bind it and it's required resources just like every other shader type being executed.

Now the important thing is to understand the threading model. You need to establish how you want to distribute your computation across threads. And then how many threads you want per a thread group and then finally how many thread groups to dispatch with the device context on the CPU.

The number of threads per a thread group is declared in the shader file. It's also important to note that you logically distribute the thread groups and threads across a three dimensional array. So if I were to dispatch (2, 8, 1) thread groups where the shader declares each thread group as (1, 8, 1) threads. This would execute 128 instances of the kernel.

Some other important things to note, buffers that are going to be written to within the compute shader must be bound as unordered access views to the pipeline while read only buffers can be bound as a resource views. There are also some nifty new buffer features. An important one to note are the Append/Consume buffers. These allow you to pop and push data to and from a buffer with out having to worry about synchronization with other threads.

For general synchronization, compute shaders offer some atomic functions and what are called memory barriers. Memory barriers are essentially synchronization mechanisms for shared memory among thread groups.

Also some peformance tips, minimize context switching of the pipeline from computational use to standard rendering use. Also avoid having to read data from the GPU as much as possible. Locking and reading buffers from the CPU is expensive.

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The overall structure of the program will be similar to that of a graphics application. You'll need to initialize Direct3D the same way and load / set up all your shaders/buffers/textures the same way. You'll call the Dispatch method to kick off a compute shader job, instead of one of the Draw methods.

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These videos will help you get started using DirectCompute I suggest you don't skip the first couple of videos as they cover important concepts about the GPU and the threading model as pointed out by KlashnikovKid and Nathan Reed.

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