Witness:

3.9G    /run/hive/comments-rocksdb-storage
3.8G    /run/hive/shared_memory.bin
7.7G    total

Exchange:

1.6G    /run/hive/account-history-rocksdb-storage
3.9G    /run/hive/comments-rocksdb-storage
4.9G    /run/hive/shared_memory.bin
11G total

Could be, I've tested it, but on machines with bigger amount of RAM. As long as the kernel has enough memory you don't even need to explicitly tie shared_memory.bin to RAM, as it will handle cache pretty well. As for the rocksdb (both account history in case of exchanges, and comments for all the nodes), it usually makes a better use of that memory than "wasting" it for database storage. To a large extent it depends on a performance ratio between RAM and storage.