modrails - rogue ruby processes consuming 100% cpu

Question

I'm having ruby instances from mod_rails go "rogue" -- these processes are no longer listed in passenger-status and utilize 100% cpu.

Other than installing god/monit to kill the instance, can anyone give me some advice on how to prevent this? I haven't been able to find anything in the logs that helps.

Answer 1

If you're using Linux, you can install the "strace" utility to see what the Ruby process is doing that's consuming all the CPU. That will give you a good low-level view. It should be available in your package manager. Then you can:

$ sudo strace -p 22710
Process 22710 attached - interrupt to quit
...lots of stuff...
(press Ctrl+C)

Then, if you want to stop the process in the middle and dump a stack trace, you can follow the guide on using GDB in Ruby at http://eigenclass.org/hiki.rb?ruby+live+process+introspection, specifically doing:

gdb --pid=(ruby process)
session-ruby
stdout_redirect
(in other terminal) tail -f /tmp/ruby_debug.(pid)
eval "caller"

You can also use the ruby-debug Gem to remotely connect to debug sockets you open up, described in http://duckpunching.com/passenger-mod_rails-for-development-now-with-debugger

There also seems to be a project on Github concerned with debugging Passenger instances that looks interesting, but the documentation is lacking: http://github.com/ddollar/socket-debugger/tree/master