These are commonly used in most profilers where all invocations of a routine are recorded. The name and group registration takes place when the timer is created (typically the first time a routine is entered). A given timer is started and stopped at routine entry and exit points. A user defined timer can also measure the time spent in a group of statements. Timers may be nested but they may not overlap. The performance data generated can typically answer questions such as: what is the total time spent in MPI_Send() across all invocations?