Forum OpenACS Q&A: Re: Need some tips about memory allocation
slightly off-topic, but we're doing something very similar to what you describe (exec calls to generate PDFs). Would you mind briefly describing your ns_proxy setup? I'd be very interested in that.
On current configuration I am running on out of the box setting for Naviserver and OpenAcs installed from the two automatic scripts provided here.
About Aolserver, I found out that version 4.5.1 provided by Debian disables ns_proxy because of a well known segfault problem which has never made it to the upstream source. This is the reference to a Google group discussing the issue: https://groups.google.com/forum/#!topic/aolserver/KBE8UySmdH4
I think nothing special should be configured on a fresh Naviserver instance. If you are running on Aolserver you will need either to patch the ns_proxy module or be sure to have enough memory, if you require to 'exec' frequently.