Set your MP rendering to only use a portion of the CPUs and increase the per-CPU memory. You are simply running out of memmory and the BG instances shut down.
I would try to reduce the number of core at first. 2GB per core is the bare minimum for them to be usable. Use 4 cores 2.5 leaving 1Gb for other apps should be fine.
Also if you are using 8 cores it is possible that, because you are rendering to a single AVI/QT file, the file waits for the next frame to be rendered because not all file format allows frame to be written in a non sequential form (this is because some codecs needs to know the actual frame after them to do some math), so it slows down the whole process. Try rendering as sequence file (don't forget to check "ignore rendered files" in the output settings).
And last but not least, it's also possible that your bottleneck is your HDD. If you have 8 cores using the same file, it means that you have 8 simultaneous access on the samefile, and maybe this starves your HDD, so as it takes longer to get the data, AE shuts down some cores.
Tell me if any of this have been usefull for you.
Hope that helps,
RED footage can require a lot of memory, depending on the resolution.
If you go to Preferences > Memory and Multiprocessing and leave 1 or 2 CPUs free for other applications (so that the other cores can take more memory), do the rendering instances keep rendering to the end?
Thanks for the quick feedback. I'm trying different settings now with leaving some cores open and different amounts of RAM per core. I'll let you know what happens.
Also, yes, the instances do finish even if they go back to one core.
Another quick question, when changing mem settings, do you need to restart after effects each time for them to be applied. I have tried some different settings, and so far all the same results.
Right now my workaround is to render to DPX. when the processors drop to one, I stop the render and then restat it from where I left off. Works good except you have to baby sit it.
Well that seemed to do it. Thanks for the feedback. I saved one core for other applications and set the other 3 at 2.8GB and got a flawless multi core render all the way through.
Your feedback is much appreciated.
The fact that MP doesn't shut down when you leave one core free suggests that in your particular case, more RAM would help (if you really wanted to use all cores, I mean).
Usually, 2 GB per core is fine. But if you're using large source files or Comps, more would definitely help.
If you had an extra 4 GB, then you'd have 4 GB per core plus 4 GB for the main application and everything else.
Of course, you can keep using it with 3 cores to render multiple frames, which should still give you good performance.