Spawning new processes is expensive, because it involves setting up TCP communication, clock synchronization, configuration broadcasting, amongst others. Its flexibility is also restricted because it needs to map the new processes into the model of communicators that provide the context for all communication events. This model follows the one used in MPI and implies that only processes inside the same communicator can communicate at all.
For spawned processes, the following model is currently supported: one of the existing processes starts one or more new processes. These processes need to know the contact string of the spawning process and call VT_clientinit() with that information; in contrast to the startup model from the previous section, no prefix is used. Then while all spawned processes are inside VT_clientinit(), the spawning process calls VT_attach() which does all the work required to connect with the new processes.
The results of this operation are:
a new VT_COMM_WORLD which contains all of the spawned processes, but not the spawning process
a communicator which contains the spawning process and the spawned ones; the spawning process gets it as result from VT_attach() and the spawned processes by calling VT_get_parent()
The first of these communicators can be used to log communication among the spawned processes, the second for communication with their parent. There's currently no way to log communication with other processes, even if the parent has a communicator that includes them.