aerender does not respect mem_usage flags
among the many flags provided by aerender, there is a mem_usage flag. However it does not seem to make any difference, and while I used to be able to render 12 processes simultaneously on a machine with 32gb ram, I can't run more than 4 on a machine nowadays with 64gb without the entire machine grinding to a halt.
Perhaps the "percentage of memory" could be updated to be a fixed value to more accurately reflect modern needs.

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Chris Zwar commented
Hey Andrew, I've been trying to find out more about this for about 10 YEARS! The mem% flag dates to when AE was 32bit and the app could only access a maximum of 4 gig ram. Since AE went to 64bit, it would suggest the Max Mem Usage and Max Ram Cache % are redundant. How do they work? I don't know. Hopefully, AErender now respects the global performance cache. It would be great to get some clarity from the Adobe engineers on how AE render works with memory.