The 8.0.2 update which mainly adds OSX “Leopard” support on mac among many bug fixes also adds a new preference which lets you indicate how many processors should be used when the Multiprocessing “Render Multiple Frames Simultaneously” is on. This is a very welcome addition for BG Renderer users that didn’t have enough RAM to handle all their processors, especially on  8-proc machines which should have a minimum of 16gb of RAM installed. Here’s the official blurb from Adobe:

Multiprocessing performance issues with multi-core machines, especially high-end 8-core hardware. When all 8 cores try to render a complex or memory intensive composition with Render Mutliple Frames Simultaneously, the application can become memory starved if each process has insufficient memory. To remedy this, there is a text preference to control the maximum number of cores that Render Multiple Frames Simultaneously will use. Open the text preference file and look in the ["MP"] section for: “MaxNumberOfProcesses” = “0″. “0″ is the default setting, which disables this preference setting. For better performance on an 8-core machine with 8GB RAM, for example, change the “0″ setting to “4″ and save the preference file and restart After Effects. This restricts multiprocessing with RenderMultiple Frames Simultaneously to 4 cores, each getting approximately 2GB of RAM.

This was such a big issue that I had updated BG Renderer to check how many processors and memory a machine had (mac only) and issued a warning if it was less than 1.5 gb per proc although I think it really should be at least 2gb since things usually render slower not faster (as explained above).  I plan on updating BG Renderer to facilitate changing this pref, but I want to do some testing first to see how changing it affects the background render processes since the pref controls the whole app not just the bg render.  Stay tuned.