# 11. Run-time operation and tuning MPI applications

Once your MPI application is running, there are many different components that can be tuned to affect behavior at run time.

- 11.1. Environment variables set for MPI applications
- 11.2. Networking support
- 11.3. Running multi-threaded MPI applications
- 11.4. Dynamically loading `libmpi` at runtime
- 11.5. Calling fork(), system(), or popen() in MPI processes
- 11.6. Fault tolerance
- 11.7. Large Clusters
- 11.8. Processor and memory affinity
- 11.9. MPI-IO tuning options
- 11.10. Benchmarking Open MPI applications
- 11.11. Building heterogeneous MPI applications