I am currently doing some measurements of a scene containing about 80k vertices that implements soft shadow mapping using PCF filter.
When im measuring the frame time with SLI enabled (using force alternate frame rendering 1) both GPUs are utilized but the performance drops. I can understand that behavior when using a low resolution where I achieve 1000+ FPS due the GPU synchronization overhead. However i dont understand that the performance is still lower when I increase the rendering load and the resolution. I have already took a look into the NVIDIA SLI optimization guide here:http://http.download.nvidia.com/developer/presentations/2005/GDC/OpenGL_Day/OpenGL_SLI.pdfbut there should be no issue concerning the facts from the presentation.
Can anyone tell me what I may be missing? Do I maybe need a SLI profile for my application?
I am using 2 GF 970GTX in SLI with the latest drivers installed.
PS: I am currently trying to look into this using NVIDIA Nsight, however it does not record any GPU frames when I enable force alternate frame rendering 1.
PS2: In the meanwhile i figured out that glGetQueryObjectui64v takes more time when SLI is enabled. I am now double buffering my results which gives me likely more accurate results (http://www.lighthouse3d.com/tutorials/opengl-short-tutorials/opengl-timer-query/). I figured that out because the FPS increases well but the frame time does not when using glGetQueryObjectui64v.
In the meanwhile I am assuming that the problem is just related to my way of measuring stuff:
So I am actually interested in how to correctly measuring frame time with AFR enabled without stalling the GPU/CPU using the Opengl timer_query object.