You might want to look at the last commits in our PR: #10567

I just added an auto-tuning feature for cache_size in it based on what is theoretically needed to make the garbage collection tasks hit the cache reliably for their index usage.

This is in test on our large folders system (just restarted Syncthing with this code). If it works properly I was still wondering if it should be :

• automatic, assuming that large instances should have the RAM for it,
• behind an option : "autotune the cache_size yes/no",
• behind an option and overriding is possible manually by the user.

As cache_size is effectively max_cache_size I'll monitor the RAM usage of our instance (using 7GiB for one folder and 2GiB for the other) to see if it is used and report here if you are interested.