Splitting fragment shader work across multiple nvidia gpus / SLI question
I realize this is an Nvidia specific problem, but the developer forums are down at the moment so I thought I'd ask here, just in case anyone has had the same issue.
I'm trying to modify a raytracer to take advantage of a multiple gpu setup (two Nvidia GTX 570s) in Windows. So far the problem has been convoluted by 1) the NV_gpu_affinity extension unavailable on consumer cards, and 2) the common Alternate Frame Rendering SLI mode being incompatible with the rendering technique used (each frame builds on the result of the previous frame, thus rendering two frames simultaneously is not an option). The obvious approach given the constraints would be to split the work so that one GPU works on a percentage of the image while the other tackles the remaining percentage.
As far as I'm aware, this means that my only option is Split Frame Rendering mode with SLI. This should solve the problem, though it doesn't appear to be working correctly / as I imagined. I can force Split Frame Rendering through the Nvidia control panel (actually, with Nvidia Inspector as the SFR option is no longer available in Nvidias own control panel). This results in both cards being fully utilized when rendering, however there is no increase in frame rate (either windowed or fullscreen). Also, the onscreen SLI indicator only displays when double buffering is enabled - unfortunate as I need access to the backbuffer, thus I have to create a FBO which acts as one and disable double buffering. With double buffering enabled, the SLI indicator displays and shows a horizontal line to indicate the split in the workload, though it traverses pretty quickly to the bottom of the image, so it's probably not working as intended.
I can't find much documentation on how to optimize for SFR, and any I can isn't really applicable to my problem, so I can't be sure if there's something I'm missing. Does SFR even take fragment shaders into account (I'm rendering onto a fullscreen quad so there's very little direct geometry work going on)? Seeing as the SLI indicator only shows when double buffering is enabled, does that mean the SFR process is somehow tied to the backbuffer? Can anyone shed any light on this?