The MPI_COMM_SPAWN and MPI_COMM_SPAWN_MULTIPLE routines provide an interface between MPI and the runtime environment of an MPI application. The difficulty is that there is an enormous range of runtime environments and application requirements, and MPI must not be tailored to any particular one.
MPI assumes, implicitly, the existence of an environment in which an application runs. It does not provide ``operating system'' services, such as a general ability to query what processes are running, to kill arbitrary processes, to find out properties of the runtime environment (how many processors, how much memory, etc.). Complex interaction of an MPI application with its runtime environment should be done through an environment-specific API.
At some low level, MPI must be able to interact with the runtime system, but the interaction is not visible at the application level and the details of the interaction are not specified by the MPI standard.
In many cases, it is impossible to keep environment-specific information out of the MPI interface without seriously compromising MPI functionality. To permit applications to take advantage of environment-specific functionality, many MPI routines take an info argument that allows an application to specify environment-specific information. There is a tradeoff between functionality and portability: applications that make use of environment-specific info are not portable.
MPI does not require the existence of an underlying ``virtual machine'' model, in which there is a consistent global view of an MPI application and an implicit ``operating system'' managing resources and processes. For instance, MPI processes spawned by one MPI process may not be visible to another; additional hosts added to the runtime environment by one MPI process may not be visible in another MPI process; MPI processses spawned by different processes may not be automatically distributed over available resources.
Interaction between MPI and the runtime environment is limited to the following areas: