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CS3 or CS4?
If you're trying to use all 8 cores, then you need to be sure that you have enough RAM to support all of those cores. If you try to use too many cores for the RAM that you have installed, then the background processes will run out of memory. For After Effects CS3, see the comments on the "Render Multiple Frames Simultaneously" section of After Effects CS3 Help. Controlling the number of cores to use and the allocation of memory to them is a lot easier in After Effects CS4; see the "Render multiple frames simultaneously" section of After Effects CS4 Help.
I believe CS3 caps it's ram at 3 gigs period, and in CS4 AE can use up to 3 gigs of ram per processor. One of the more overlooked improvements in my opinion.
> I believe CS3 caps it's ram at 3 gigs period, and in
> CS4 AE can use up to 3 gigs of ram per processor. One
> of the more overlooked improvements in my opinion.
No, in that way After Effects CS3 and After Effects CS4 are not different. The main change in this regard is that it's easier to configure memory usage in After Effects CS4. See the "Memory usage and storage" and "Memory & Multiprocessing preferences" sections of After Effects Help for details.
To Navarro's original question: Read the "When After Effects temporarily disables multiprocessing with Render Multiple Frames Simultaneously" section of After Effects Help. That section does what its name suggests; it lists the conditions under which After Effects temporarily switches to not using the background processes for rendering. The reason that I asked about CS3 versus CS4 is that this switch-over is handled much more gracefully by CS4.
Thanks Todd! The comment on how to access more memory in the "Memory usage and storage" part was very helpful.
Thanks for all the replies. It must be that I was running out of memory. I "only" have 16GB, so I have to allot less than 2GB per core -- and my source files were 6K rez!
FWIW, I had to tweak the settings (on Vista 64 with 4 cores and 8 GB ram) to get optimal performance. Initially I had it using all cores and I noticed that after I start a render, it would run all cores at 100%, but as the RAM usage approached 100% it suddenly throttled back and released over 4GB of ram, and then ran each core at 30%. The render time estimate quickly built up (tripled) - from < 2 hours to 4 and then it wound up taking 5 hours. Aak! I was thinking my investment of more RAM and a quad-core and switching to 64 bit would be a waste of time and money.
BUT, happily... I found that by telling AE to give other apps about 2GB ram, and leaving 1 core free, it still throttled back when RAM filled up, but it ran the first three cores at an average 50% and the 4th core at 75% and the most important part, the render time estimate didn't change, and steadily dropped.
So it sounds like AE likes you to leave the rest of Windows (and probably its main thread) with about 25% of your memory and cpu resources so it can "think". I could probably optimize even more but now I'm happy with the render speed.
Hopefully this info is useful.