rails dev logs

How does rails compute the timing/summary information in the dev logs?

e.g.
    Completed in 0.84545 (1 reqs/sec) | Rendering: 0.00029 (0%) | DB:
0.11569 (13%) | 200 OK [http://localhost/installations/123]

-Shawn

simple benchmarking tool

Any idea where in the codebase that is done?