Ask /r/ethereum: How to learn geth internals?

I'm trying to learn the internals of geth, particularly in regards to the sync code. My short-term goal is to make a change so that in snap/fast sync, the last N full blocks are downloaded instead of the last 64.

I edited downloader.go and changed fsMinFullBlocks to 2_000_000 as a first guess, and it seems to work for a few blocks, but then geth rapidly consumes all my system memory as the sync progresses.

Can anyone give advice either on this specific issue, or more generally how to start learning the geth codebase?