[Bf-cycles] OpenCL and AMD GPUs

Kyriazis, George George.Kyriazis at amd.com
Mon Oct 27 21:34:14 CET 2014

Greetings bf-cycles,

I work for AMD, and we have been thinking about working in the OpenCL kernel (read: have started working on it).  It is, I presume, a well-known fact that the OpenCL implementation has "issues" on AMD.

Our current approach is to split up the OpenCL kernel into multiple (smaller) kernels, in order to get better utilization of the GPU.  I've had brief discussions with Martijn, Brecht and Ton, and they all seem eager to finally "fix" (for a lack of a better term)  OpenCL, which is a good sign.

Technical details have not been discussed yet, but an open forum like bf-cycles is a better place for that.

As a starter point of discussion, I'd like to comment about the main motivation of the kernel split.  As it is well known, the AMD OpenCL implementation has some problems compiling the current OpenCL kernel.  This has been mainly attributed to the length of the kernel, and problems with register allocation.  Although the above is correct, those causes fail to address the main issue, which is the fact that a huge kernel (like cycles) that is a straight-forward port of CPU code, does have a lot of code divergence.  Code divergence causes a lot of workitems go idle during kernel execution, which is not a good thing.

Splitting the kernel allows for each (sub)-kernel to have better GPU utilization, and hence better performance.  As a side-effect, it decreases the size of each kernel, and makes things easier for the register allocator.  So, the current problems that the AMD OpenCL implementation has will not express themselves in a split kernel.  Our current thought is to have those individual kernels communicate via queues.

Any questions / comments / etc. about our approach is welcome, of course.


George Kyriazis

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