This has been a long term annoyance in MR.
A distributed render will hang at 99 percent, for say 2hrs out of a 5hr render. If I'm using 5 machines to distribute the render, each is an 8 core MacPro, so 16 hyper threaded cores. The render will be using about 2 cores, on 2 machines, (1 core on each) out of a possible 80 during these 2 hours, so what's not hyper threaded?
This isn't a one off it happens on a regular basis. It's not every scene, but there is something, either materials or combination of materials that's causing the machines to calculate using only a single core.
Basic details are:
Mental ray 3.8
MacPro 2 x 2.93 Quad-Core Intel Xeon
OS X 10.6.4