The key advantage of the doors IPC system is that it tries to be a synchronous IPC system, and whenver it can operate synchronously it can do things like: -

The key advantage of the doors IPC system is that it tries to be a synchronous IPC system, and whenver it can operate synchronously it can do things like: - immediately context switch to the server to reduce latency - transfer the caller's priority to the server to avoid priority inversion problems The downside of doors is that the server really has to be able to satisfy all requests very quickly, and if not then the client and server need to agree on an asynchronous completion notification mechanism (e.g., pass a fildes from the client to the server). And there's no guarantee that all requests can be handled synchronously. But there's a significant number of system services for which it is possible to have very fast, synchronous protocols, and for those doors really shines because of the thread shuttle and priority transference.