Hm. Apparently, we don’t have any form of --help that lists all the knobs you can set, and it takes --dev-help for it to even print that it’s possible to set knobs.
You’re looking for --knob_server_mem_limit=, so you should be able to add knob_server_mem_limit = <2GB as an integer> to your foundationdb.conf, and it should work. Take a look at the various Knobs.h files (e.g. fdbserver/Knobs.h) for the full listings.
The storage server page cache knob is --knob_page_cache_4k, that describes the total number of bytes that should be held in the page cache (so divide by 4k to get the number of pages). It turns out that it defaults to 200MB, so it’s likely not going to be helpful to tweak down. (Though we should probably set the default higher in this day and age…)
SERVER_MEM_LIMIT only applies to proxies. I don’t think we have a generic way to limit any role to try and stay within 2GB, if you have the limit set as 4GB?