Getting the callstack of a running Ruby process

Sep 2011

Since we upgraded to Rails 3.1 at Virtual Manager we've seen our app servers' CPUs going into self-oscillation every two hours or so. And every time, it would be a Ruby process eating as much CPU power as it could.

If we did not manually kill these Ruby processes once in a while, the app server would eventually run of out CPU cores and become unavailable.

The problem was that I had no easy way of finding out what these Ruby process were doing. Our logs weren't telling us anything, presumably because the processes never really exited. I also tried adding Rack::Timeout to get our Rails app to throw an exception when requests took too long. No luck there.

What if I could somehow open up one of the strayed Ruby processes live while it was running, and look into its current callstack? Then I would be able to find out what was causing this.

Today, I did just that and located the culprit. I'd like to share how I did it.

I learned this technique from this blog post by Jamis Buck. However, his exact approach did not work for me. Perhaps because we're running Ruby 1.9.2. By combining what I learned from Jamis' blog post with what I learned by reading this blog post by Chris Heald I came up with my own solution.

First I installed GDB: apt-get install gdb.

Then I created this file in ~/.gdbinit:

define eval

define redirect_stdout
  call rb_eval_string("$_old_stdout, $stdout = $stdout,'/tmp/ruby-debug.' +, 'a'); $stdout.sync = true")

All credits for this little snippet goes to Chris Heald.

Next, I looked up the PID of the process I wanted to look into. I used top for this (PIDs are in the left most column per default I think). I then fired up GDB by running gdb and attached the process by typing attach [PID].

In order to redirect output into a log file you must then type redirect_stdout (defined in our GDB init script). After this, all output will be sent to a file called /tmp/ruby-debug.[PID].

Now, we can look into the Ruby process as we please. I called eval("Kernel.caller") to dump the entire callstack. I later found out, that you also have access to whatever variables are available in the current context - that means you can output arguments from the latest method call for instance. Pretty handy.

Bonus info

The culprit to our CPU problem was that Rack 1.3.2 utilizes URI.decode_www_form_component defined in Ruby's own uri/common.rb. With Ruby 1.9.2-p290 and Ruby 1.9.2-p180 this method goes nuts if called with certain input. It seems we are not the only one who have suffered from this.

My solution was to redefine the problematic methods in a Rails initializer. I simply copied over parts of uri/common.rb.

I found this related bug report in Ruby's issue tracker. The issue is already fixed in Ruby 1.9.3 and maybe backported to Ruby 1.9.2.

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