I have a small RoR app that generates images using RMagick and the performance is not anywhere near where I thought it would be. The server configuration is a 64-bit Ubuntu 11.04 image on EC2 running Rails 3.1RC4 / Ruby 1.9.2 / Nginx / Passenger. I've tried several different Amazon instance types, from small (with a 32-bit image) to c1.xlarge and the results are very similar.
Firing up ApacheBench (ab -n 10 -c 1) gives me an acceptable average response time of 300ms and the test finishes in about 2 seconds, but increasing this to 5 concurrent or even 2 concurrent requests drops the performance to 1500ms and the test takes longer. Even on the big instance type a ab run (ab -n 10 -c 10) slows the system down to 5000ms. I would expect that response times would stay pretty much consistent but the the total time would drop. Is this an incorrect assumption? On every test the memory never climbs very high at all (< 1GB) but the CPUs are working 100%. My MacBook Pro running development mode can match these numbers.
It almost seems like something is single threaded somewhere. The app is almost as simple as you can get and the only complex thing is the RMagick calls. Is there something with RMagick that is causing threading problems? Is there a better RoR app server for this type of thing (Unicorn, Mongrel cluster, etc.)? Am I using ApacheBench incorrectly?
I have added some new text-only routes to the config and they perform very well. Returning 32K of plain text barely causes the CPU any problems and I can reach 72 req/sec (which is probably limited by my internet connection and not the EC2 server). Returning 5 bytes gets me over 250 req/sec.