Unable to obtain a thread dump when Jenkins is stuck coming up.
According to https://wiki.jenkins-ci.org/display/JENKINS/Obtaining+a+thread+dump
you should be able to obtain a thread dump by sending SIGQUIT on the master node. While the PID of Jenkins changes (indicating that SIGQUIT was acted upon), no thread dump is observed on stdout/stderr or /var/log/jenkins/jenkins.log.

Since I am trying to diagnose why the Jenkins takes 2hrs to start up, the /threadDump method in the GUI is not suitable in this case.