Hi Rod,
Yes it's a 64-bit system, I'm running slackware 14.1 with linux kernel 3.18.4. The machine is a Dell T5500 workstation, dual Xeon processors with 72 GB of main memory and 240 GB of swap space, so address space shouldn't be an issue. I'm also doing these tests running gimp/gmic as root so there shouldn't be any user limits affecting me.
I did another test, I just created an image that's 16817x9596 in size, white on one side, black on the other. Here's a scaled down version so you can see what it looks like:
I then ran upscale diffusion at 200% (default settings) on this image and zoomed into the transition between white and black (note: red arrows added by me obviously!):
Notice the ghosting line, what's interesting is that it has a color to it. It's not a gray, it's a bluish color. So I did the same thing to the starting image with the upscale scale2x filter:
It shows even more colors being added to the transition area. This could be completely normal to the algorithm but it just seems a little odd since there's only black and white that are being upscaled.