Custom pseudo-instancing faster than real instancing on Nvidia cards?

theagentd · 2013-06-22T17:06:09

EDIT: This seem to be caused by glDrawElementsInstanced() being extremely CPU intensive. See my 4th post further down. Hello. I have this situation where I need to render a large number of instances of a few different very simple meshes (100-300 triangles). Rendering them one and one turned out to be too CPU intensive, so instancing seemed like the perfect solution except for the fact that it requires OGL3. Therefore I came up with a "pseudo"-instancing method where I duplicated and stored my model 128 times in a VBO (instead of just once) and could therefore render up to 128 tiles in a single draw call by uploading instance positions to a uniform vec3[] which was used by a shader to position each instance. Now I've also implemented an OGL3 version where I upload my per-instance data using super efficient manually synchronized VBO mapping instead of using glUniform3f and render the geometry using real instancing. However, this turned out to be remarkably slower on Nvidia hardware up to the point where my pseudo-instancing was 40% faster than real instancing. On the other hand, on AMD and Intel hardware real instancing is (sometimes much) faster. Here's my test result data. Test1 = real instancing, Test2 = pseudo-instancing. AMD HD5500 Test1: 27 Test2: 22 AMD HD6970 Test1: 225 Test2: 57 AMD HD7790 Test1: 195 Test2: 35 Nvidia GTX 295 Test1: 68 Test2: 89 Nvidia GTX 460M (laptop) Test1: 83 Test2: 87 Intel HD5000* Test1: 60 Test2: 54 GT 630 (rebranded 500 series GPU) Test1: 54 Test2: 55 *Tested at much lower rendering resolutions to reduce the fragment bottleneck, so the FPS numbers on this test is much higher than it should be compared to the other cards. My pseudo-instancing uses 128x more bandwidth (and memory), around 10x more draw calls and much less efficient memory uploading to the GPU than real instancing. I cannot for the love of god fathom why in the world this would be faster on Nvidia cards.

Graphics and GPU Programming Programming

Started by theagentd June 15, 2013 10:44 PM

10 comments, last by theagentd 10 years, 10 months ago

Hodgman

52,717

June 22, 2013 04:06 PM

So... How do I report this so Nvidia actually listens?

I'm not quite sure, but making an account at developer.nvidia.com would probably be a good start, and then either on their forums, or via the "contact" form.
The small, self-contained reproduction test is great for them to be able to see exactly what you're on about.

I haven't looked at your code, but simply adding an index buffer shouldn't double your CPU-side frame-times I would think...
I ran your test on my PC (Q6600 CPU, GTX460, driver 9.18.13.2018 12/05/2013) and got (from left to right buttons): 77 43 31 83 42

. 22 Racing Series .

theagentd

990

Author

June 22, 2013 05:06 PM

Thank you very much! That means that the performance quirks exist (at least) on the following cards:

GTX 295

GTX 460m (laptop)

GTX 460

GT 630

GTX 680

The only difference between the indexed renderer and the array renderer is that one uses 6 indices to form 2 GL_TRIANGLES and the other effectively does the same thing internally using GL_QUADS but without an index buffer. This isn't a problem when just rendering quads, but everything I'm rendering in my engine uses indexed triangles.

EDIT: I've posted this on the Nvidia developer forums: https://devtalk.nvidia.com/default/topic/548150/opengl/performance-bug-in-gldrawelementsinstanced/

Custom pseudo-instancing faster than real instancing on Nvidia cards?

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Custom pseudo-instancing faster than real instancing on Nvidia cards?

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