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Cycles : CUDA Rendering : Environment with MIS uses double memory usage
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System Information
Windows 7, x64, gtx580

Blender Version
Broken: b33d83b
Worked: 2.70a

Short description of error
Blender uses approximately double RAM usage on the GPU when using MIS with environment light

Please find attached example blend file,
Ram usage under 2.70a : 689mb
Ram usage under b33d83b : 1405mb

Event Timeline

By the way this was tested via the utility GPU-Z

from further testing, there was quite a big increase between between builds 685316b and 362b25b3 (~500mb) and about another 100mb or so between c08c931 and ba1acec

Please double check these hash values! If the hash is valid, it will become grey and you can click on it, like the last one. Best to get the hash from the Splash Screen.. Would save us some time figuring out the correct range. :)

The first increase of memory (2-5 May) could be caused by the baking commit:

Just clarify... it really is only effecting the MIS option... without MIS, the increase between 2.70a and b33d83b is about 14 mb... which isnt too bad...

Also, that the Map Resolution doesnt really matter...

@Brecht Van Lommel (brecht): Blind guess, maybe related to the added Baking function call in kernel_shader_evaluate()? Maybe kernel_bake_evaluate() gets inlined here.

Yes, I suppose it keeps the memory reserved after using the bake kernel. Probably it can be split up to avoid that.

Since we have another Baking related issue here, it might be best to disable GPU baking for 2.71.

@Dalai Felinto (dfelinto)