Hi, then please attach the scene. :)

Mib

I'll be happy to oblige, though this project represents nearly a year's worth of work and I would very much prefer to not circulate it publicly. Is there any way that I can post it somewhere privately?

Almost always it's possible to isolate the problem in a simplified scene. You can make a copy of the .blend and keep removing things, replacing meshes, materials, etc, while ensuring the memory increase after the first tile still happens. Usually you then end up with a .blend that has no value besides demonstrating the bug. This is how us developer would start debugging the problem anyway.

If it's not possible to isolate the problem like that, you could send the scene to brecht@blender.org.

That makes sense.

I've tried removing elements but there doesn't appear to be any common thread to the memory swell. If the initial tile weighs in around 2gb memory, then the memory grows violently when it attempts to start the second tile. If the memory for the first tile is less than 2gb it stays low the whole time.

I'll keep tinkering with it to see if I can further triangulate but in the meantime I'll shoot a copy your way. I'm curious to learn if it misbehaves for you as well.

I tried the file on Windows 10 with a GTX 960, NVidia driver version 364.72. But I can't reproduce the problem. GPU memory usage stays fixed at 2047 MB during the entire render, both in Blender 2.76b and 2.77.

For memory usage, are you looking at what Blender reports in the render window, or using another application like GPU-Z?

Can you try rendering with File > Load Factory Settings (only re-selecting the GPU render again in the user preferences), to see if there is any addon or setting that influences this?

The pixel memory usage for a single tile in this file is much smaller than 500MB (around 15MB), so even if two tiles were in memory at the same time, that should not cause such a big spike. I'm not sure where else Cycles could be allocating GPU memory, normally only memory for tile pixels is allocated once rendering has started.

If we are talking about total GPU memory instead of only memory used by this one specific render (only visible in something like GPU-Z, not reported by Cycles), then there are some other things that could causes more memory usage during the render. Though still I don't know where that would be right when Cycles transitions to the second tile.