I find 2 related articles:
1. PID namespaces in the 2.6.24 kernel
"One of the new features in the upcoming 2.6.24 kernel will be the PID namespaces support developed by the OpenVZ team with the help of IBM. The PID namespace allows for creating sets of tasks, with each such set looking like a standalone machine with respect to process IDs. In other words, tasks in different namespaces can have the same IDs. "
" This feature is the major prerequisite for the migration of containers between hosts; having a namespace, one may move it to another host while keeping the PID values -- and this is a requirement since a task is not expected to change its PID.
Without this feature, the migration will very likely fail, as the processes with the same IDs can exist on the destination node, which will cause conflicts when addressing tasks by their IDs."
"PID namespaces are hierarchical; once a new PID namespace is created, all the tasks in the current PID namespace will see the tasks (i.e. will be able to address them with their PIDs) in this new namespace. However, tasks from the new namespace will not see the ones from the current. This means that now each task has more than one PID -- one for each namespace. "
2. see also : Notes from a container