1. A parallel computer system comprising a network of interconnected compute nodes that operates a global message-passing application for performing communications across the network, wherein each of the compute nodes comprises one or more individual processors with memories, wherein local instances of the global message-passing application operate at each compute node to carry out local processing operations independent of processing operations carried out at other compute nodes, and wherein each compute node further comprises: a DMA engine constructed to interact with the application via Injection FIFO Metadata describing an Injection FIFO containing an arbitrary number of message descriptors in order to process messages with a fixed processing overhead irrespective of the number of message descriptors comprising the Injection FIFO.