Cycles - Cuda rendertimes???

Hi all!

I have a question about how to use cycles - Cuda - GPU in order to reduce rendertimes.

I was under the impression that Cuda and GPU processing would reduce rendertimes drastically.
But I seem to be missing something.

My specs are:
MacBookPro
500 Gb
4 Gb ram (2x2)
ie7 2,3 GHz (Quad core 64 bit)
Nvidia Geforce GT650M
Cuda 6.5.45 (compute capability 3.0)

Now this is what I've done:
Downloaded a test blend from BlenderArtist from this link: [You must be registered and logged in to see this link.]
and here are the times I got.

(1) Time: 8:44:40 (GPU Cuda) Device GPU Compute
(2) Time: 5:15:70 (GPU Cuda) device CPU
(3) Time: 5:26:08 (CPU) Device Supported

In this test it seems that the illogical combination (2) is the fastest, but why is that? I expected (1) to be the correct setting, but apparently not.

Any ideas?

cheers

Chapel

Just to clarify:

1. Is GPU enabled in the user settings, Feature Set = Supported, and Device = GPU Compute
2. Is GPU enabled in the user settings, Feature Set = Supported, and Device = CPU
3. Is GPU disabled in the user settings

Correct?

So, the GT650M has the same number of CUDA cores as the  GTX560 ti, which is what I use and is pretty decent for my needs.  The GT650M uses a lot less power than the 560ti and it sits on a single chip rather than a card, so it's perfect for a MacBook.  The downside is that it has limited texture rates, texture mapping units, and render outputs (bringing this up to explain my results that show up in a bit).  So it may be that using Cycles on GPU forces all the textures and render elements to pass through the GPU in addition to performing the rendering calculations--and when you do the GPU via CPU option (#2) it offloads the textures to the GPU but performs the render via CPU (but that's just a guess and doesn't jive well with what we should expect to see).

But here's another kicker:  I got the exact same times as you (8:40 and 5:45), but my differences were the GPU was 5:45 and the CPU was 8:40--so I'm also wondering if you may have swapped the results during your testing?

Maybe this is a better way to illustrate the issue:
I made some screenshots of the settings in each case, but for some reason the screenshots are hard to read, so I added the "important" information beneath each image in the following order:
User Interface;
Render-properties setting;
Rendertime


[You must be registered and logged in to see this image.]
CPU;
Feature set: supported;
rendertime 5:18:03


[You must be registered and logged in to see this image.]
Cuda GeForce GT 650M;
Feature set: Supported, Device CPU;
rendertime 5:18:74


[You must be registered and logged in to see this image.]
Cuda GeForce GT 650M;
Feature set: Supported, Device: GPU Compute;
rendertime 8:43:90

Sorry, Chapel--I'm at a loss. Are you using a developmental Blender version, or the standard release? Do the render times have any significant differences using a different Blender version (2.72 vs 2.71)?

Standard Blender release 2.73a

I haven't tried any other version, but since you mention it, I will.
It may take a while, as I need to download those versions first. I don't have them on the mac.