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You didn't say which version of After Effects you're using, so I'm going to assume After Effects CS4 and remind you to make sure that you've updated using the After Effects CS4 (9.0.2) patch.
Regarding RAM and processor usage, see this page:"Performance tip: Don't starve your software of RAM". Following the guidelines there for your 7GB computer, you'd leave 2GB for other applications, maybe 1 GB for the foreground proces to hold RAM preview frames---leaving you with 2GB each for two background rendering processes.
The apparent doubling of the number of processor cores is likely because of hyperthreading---a technology that in some cases allows each processor core to act like two. Don't be fooled. You four-core computer is just a four-core computer as far as After Effects is concerned. The purported doubling of processor cores that hyperthreading creates isn't relevant for Render Multiple Frames Simultaneously multiprocessing.
Regarding the Keylight out of memory errors: I don't know a lot about the cause of this error, but I do know that it's a known issue. If you want some help investigating, I might be able to help, but for now I'll just give you the same search link that I would be using to start my investigation.
We are indeed on CS4 9.0.2. Thanks for the explanations, I'll look into those suggestions. Somewhere in my searching, I'd seen that the multiprocessing should become an option to think about only when renders are working fine without it - is this something you recommend, or do you recommend enabling it now and configuring it as you described? If enabled, and keeping in mind the hyper-threaded core count, would there be a most correct number of CPUs to leave for other applications? There are many combinations of options that place the "Actual number of CPUs that will be used" at more than four - given that there are only four actual CPUs, would this present a problem?
I'll also look into the Keylight errors to see if there's a good reliable way to patch that up - it does seem to be an issue that others are having even on much more powerful machines.
Thank you for your help.
> I'd seen that the multiprocessing should become an option to think about only when renders are working fine without it - is this something you recommend...?
That's fine advice.
I don't think that you'll have to worry about manually setting the CPUs to leave for other applications. If you set the RAM to leave for other applications to a reasonably high number (2GB) and also set the minimum allocation for each After Effects process to a relatively high number (2GB), then you already have made it so that your computer with 7GB won't be using all of its processor cores for After Effects.
Makes sense. So with all of this in mind, I currently have:
AE RAM usage 5GB, with 2GB left for other applications.
Multiprocessing enabled, with 2GB minimum allocated per CPU.
Foreground memory usage 0.99GB, Background 4.01, this gets 2GB per CPU. Actual CPUs that will be used becomes 2, with 0 CPUs manually left for other applications. Unless you have any further suggestions, we'll give this a try and see how it goes, while still investigating Keylight.
Thanks again for your help.
Gave those settings a try - one 20-second clip rendered just fine, but when trying a second clip, about 40 seconds long, the render hung about 90% of the way through. The RAM use indicator at the bottom stayed around 25%, with no noticeable spike. The program did not freeze, it allowed me to click the Pause/Continue buttons and then the Stop. However, after stopping the render, the program became unresponsive and a Force Quit was necessary. The render to the point of failure was viewable, though. There was no error message.
Any ideas as to what might be a cause?