I'm noticing a lot "Status: Cleaning Up" when rendering a certain segment of my project (it has 12 4K layers in 3D space with motion blur. No lights). The cleaning up stage is taking longer than any of the other stages by a significant amount. AE also beachballs during cleaning up.
Running OS X 10.6.8, 32GB RAM, CS5.5, multiprocessing on, 3GB per process, 3 active background renderers. With MP on or off, the Activity Monitor shows just the AE foreground app doing any work (but one core rarely goes over 100%).
Yes, it's flushing and re-organizing the buffers and in your scenario it surely has to do a lot of that. Simple computational math: 12x4K*8bpc~768MB per frame and those numbers explode if you are working in 16bpc or 32bpc modes... Now multiply that with the number of motion blur samples and any intermediate buffers that effects and nested comps may require and you quickly arrive at quite daunting numbers even on a beefy systems such as yours. Or in simple speak: Probably it cannot hold more than 5 frames in memory at any given time, if at all and purging and re-loading specific frames outweighs the actual processing by a huge amount. This may even become more compolicated if you are using clips rather than images/ image sequences because there is additional frame look-up overhead...