MPI_T is an interface for tools introduced in the 3.0 version of MPI.
The interface provides mechanisms for tools to access and set
performance and control variables that are exposed by an MPI
implementation. For example, a tool may be able to record the length
of the unexpected message queue over the course of the execution; or a
tool could set the value of the "eager limit" for better message
passing performance.  The number and types of variables that are
exposed is completely determined by the MPI implementation and need to
be discovered by tools via the query interface. Thus, tool writers
need to take care to code in such a way that they are not dependent on
a single implementation's variables.  Major implementations, including
Open MPI , MPICH and MVAPICH, are working to support this interface,
so the functionality will be available to tools in the near future.

We have developed a set of simple MPI_T tools to get tool writers
started on the path to more sophisticated support of the new interface
and make them available here. The tools are Gyan and VarList.