I spoke too soon. While the originally observed misbehavior followed platform… So I started rendering the scene on my Windows box, mostly because I hate abusing my laptop with multi-day renders. It took 4 days to render out… Then I decided to update my render node—it runs in a vm on my linux box so I got that running, updated to the latest beta, added it and… the first network tile finally completed and it has one corner in a region affected by caustics. In just that region there are sharp lines demarking how that tile rendered the caustics. The rest of the tile is fine, just not the area with caustics.
Its a VM so “processor” is not quite the same as real hardware, but this is kvm (linux kernel virtual machine) with only a very thin layer separating it from the hardware. Both the windows and linux machines are Athlon x2 processors. The windows box, being 32 bit, is running the CPU in 32 bit mode. The linux, being linux, is running the CPU in 64 bit mode (and a modest 8GB RAM to go with that). 64-bit mode opens up more registers and I don’t remember what all else. I don’t see how it would impact rendering, however, unless something like SSE worked differently in 64 bit mode.
But this has two Windows 32-bit instances participating in the batch render giving different results for the caustics. It’ll probably be four or five more hours at least before another tile completes (currently all are working on regions that are predominately affected by caustics. When that happens I’ll be able to save the result to separate network from local rendering of caustics. Because it has already hosed the caustics, I’m tempted to pull the windows render node out and fire up the OS X render node just to see how it plays.
What I really need, though, is a monster rig with 24 cores —I’ve been tempted by the (now discontinued, I think) 12-core (not the bulldozer design, true 12-cores) AMD processor. For what it was $1000 each wasn’t bad with $400 for a dual-CPU motherboard that could go to 64/256GB. I just haven’t had (and still don’t have) the money for such a beast. But the pressure to do network rendering would drop off significantly with such a rig… that last 4 day render would have been ~8 hours? wheeee!