My single-VPS-hosted app spends a lot of time waiting on DB queries, which themselves spend a lot of time waiting on disk I/O. I've therefore found that adding

My single-VPS-hosted app spends a lot of time waiting on DB queries, which themselves spend a lot of time waiting on disk I/O. I've therefore found that adding RAM and doing nothing with it so it can be used as disk cache (or, for Postgres, allocating some of it as shared_buffers) is the best non-non-blocking solution. (I'd originally allocated almost all RAM to running up to 16 or so Passenger instances. Once more than 3 or 4 of these were serving requests, everything ground to a halt waiting on the DB. Now I'm down to about 4 instances, with the rest of the RAM for cache, and they cope fine with the same load).