Data communications in a parallel active messaging interface of a parallel computer

A method of algorithm selection for data communications in a parallel active messaging interface (‘PAMI’) of a parallel computer, the parallel computer comprising a plurality of compute nodes that execute a parallel application, the PAMI comprising data communications endpoints, each endpoint comprising a specification of data communications parameters for a thread of execution on a compute node, including specifications of a client, a context, and a task, the compute nodes and the endpoints coupled for data communications through the PAMI and through data communications resources, the method comprising:

• associating in the PAMI data communications algorithms and ranges of message sizes so that each algorithm is associated with a separate range of message sizes;
• receiving in an origin endpoint of the PAMI a data communications instruction, the instruction specifying transmission of a data communications message from the origin endpoint to a target endpoint, the data communications message characterized by a message size;
• selecting, from among the associated algorithms and ranges, a data communications algorithm in dependence upon the message size, a number of ranges, an upper boundary value and an index identifying a range containing the message size;
• transmitting, according to the selected data communications algorithm from the origin endpoint to the target endpoint, the data communications message.