Fencing data transfers in a parallel active messaging interface of a parallel computer

A method of fencing data transfers 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 coupled for data communications through the PAMI and through data communications resources including at least one segment of shared random access memory through which data communications are delivered to target endpoints in the same order in which the communications are transmitted from origin endpoints;
• the method comprising:
• initiating execution through the PAMI of an ordered sequence of active SEND instructions for SEND data transfers between two endpoints, an origin endpoint and a target endpoint, each SEND instruction effecting a deterministic SEND data transfer through a segment of shared memory in which the SEND data transfers are effected according to the ordered sequence of the SEND instructions;
• and
• executing through the PAMI, with no FENCE accounting for SEND data transfers, an active FENCE instruction, the FENCE instruction completing execution only after completion of all SEND instructions initiated prior to execution of the FENCE instruction for SEND data transfers between the two endpoints.