Jeremy Kemper (aka bitsweat) committed a very useful tool into the Rails trunk not too long ago: a request profiler! It's a human-friendly wrapper script around the ruby-prof library, a nice ruby code profiler, that lets you run multiple requests against a URI in your application and get a detailed code profile report in text and HTML.
Wanna try it out? Update your vendor/rails to at least revision 8016, run
rake rails:update to copy the new
script/performance/request script into your app's
script directory, install the ruby-prof gem (
sudo gem install ruby-prof), then let it loose on your app.
The options are pretty simple, just pass the script the URI you wanna profile. There're options for specifying how many requests you want to process, and for profiling POST requests, there's one for
specifying a POST fixture file.
USAGE: script/performance/request uri [options] -u, --uri [URI] Request URI. Defaults to http://localhost:3000/ -n, --times  How many requests to process. Defaults to 1000. -b, --benchmark Benchmark instead of profiling --method [GET] HTTP request method. Defaults to GET. --fixture [FILE] Path to POST fixture file --open [CMD] Command to open profile results. Defaults to "open %s &" -h, --help Show this help
At the end of the run, you get a text and HTML report with the methods called and the time spent in them. If you haven't seen a code profile before, it looks something like this:
Very nice stuff! It's extremely convenient to have profiling like this built into Rails itself - personally I've not even run any profiling on my Rails code because it seemed like a hassle (though I do run httperf benchmarks and take note the number of requests per second). With code profiling, you can easily see which parts of your code are the bottlenecks and optimize away - it's a godsend that it's so easy to do it in Rails!
Anyway, this is all still new stuff I imagine and subject to change (and improvements), but still very exciting for those of us who are hitting performance bottlenecks in our Rails apps (and are not already doing code profiling).