There's once I was running some jruby stuff in jenkins during a build, the job kept on hanging on some stage, I thought there must be bug somewhere, I forced kill it a couple of times with no success, but kept the last one running before I head home. Then after a couple of hours, I found out an email said the built was passed... eventually, I had figured out that jruby was using /dev/random, since jenkins was running in vm, so no enough entropy was generated. after force mounting /dev/random to urandom, the hanging issue just disappeared.