On 29th of June, Andres Freund committed patch:
Add cluster_name GUC which is included in process titles if set. When running several postgres clusters on one OS instance it's often inconveniently hard to identify which "postgres" process belongs to which postgres instance. Add the cluster_name GUC, whose value will be included as part of the process titles if set. With that processes can more easily identified using tools like 'ps'. To avoid problems with encoding mismatches between postgresql.conf, consoles, and individual databases replace non-ASCII chars in the name with question marks. The length is limited to NAMEDATALEN to make it less likely to truncate important information at the end of the status. Thomas Munro, with some adjustments by me and review by a host of people.