This is to track the problem originally reported here: http://n4.nabble.com/Polling-hung-td1310838.html#a1310838
The referenced thread is relocated to http://jenkins.361315.n4.nabble.com/Polling-hung-td1310838.html

What the problem boils down to is that many remote operations are performed synchronously causing the channel object to be locked while a response returns. In situations where a lengthy remote operations is using the channel, SCM polling can be blocked waiting for the monitor on the channel to be released. In extreme situations, all the polling threads can wind up waiting on object monitors for the channel objects, preventing further processing of polling tasks.

Furthermore, if the slave dies, the locked channel object still exists in the master JVM. If no IOException is thrown to indicate the termination of the connection to the pipe, the channel can never be closed because Channel.close() itself is a sychronized operation.