1. An apparatus for performing a global barrier operation in a parallel computer, the parallel computer comprising a plurality of compute nodes, the compute nodes coupled for data communications, each compute node executing a plurality of tasks, with one task on each compute node designated as a master task, the apparatus comprising a computer processor, a computer memory operatively coupled to the computer processor, the computer memory having disposed within it computer program instructions that, when executed by the computer processor, cause the apparatus to carry out the steps of: for each task on each compute node until all master tasks have joined a global barrier: determining whether the task is a master task; if the task is not a master task, joining a single local barrier; if the task is a master task: determining whether all other tasks on the compute node have joined the single local barrier; and if all other tasks on the compute node have joined the single local barrier, joining the global barrier and the single local barrier.