MENTAL RAY MEMORY SETTINGS
You must set the memory limit manually by turning off the Render Settings -> Batch Render -> Options -> Memory -> Auto Memory Limit. Then you can specify the memory limit manually. If you are rendering under Windows XP 64bit there is another issue with automatic memory which maxes out at the limit of the 32 bit version of the OS.
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Either on the render Options or (when you use the commandline renderer with Render -r mr) by doing this:
add this line to the mrRenderer.xml file in the Maya8.5/bin/rendererDesc/ directory (C:\Program Files\Autodesk\Maya8.5\bin\rendererDesc on Windows)
Now you can specify the limit in the commandline:
"C:\Program Files\Autodesk\Maya8.5\bin\Render.exe" -r mr -v 5 -jl 4000 "C:\Shared\maya\projects\myProject\scenes\myScene.m b"
*TIP : Under MR globals/translation/performance you can check EXPORT OBJECTS ON DEMAND which apparently only loads objects if they are hit by a light ray.
*TIP : Type select mentalrayGlobals to select them. Under it's Extra Attributes you will find Memory Zone slider. It is set at 20 which means MR will let the render go 20% over your memory limit. Crank it if needed.