The setup they're using is an nginx serving 8 unicorn workers with a backlog of 256. They then throw requests at that with a concurrency of 20. DB pool is 256 too. It seems to me quite likely that the unicorn queue fills up very quickly and it starts rejecting requests, which could be an error. It's hard to see how a maximum of 8 workers would ever get close to the 256 available DB connections.

At first glance the unicorn setup is totally inadequate for the amount of traffic being thrown at it. The first thing to do would be massively increase both the number of workers and the backlog, otherwise this almost instantly turns into an overflowing request queue and literally millions of errors.

There's no denying, though, that this kind of request flood is not exactly rails' strong point and if you're expecting massive numbers of fairly simple requests you're probably better off with something else.