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

A method of eager send 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:

• receiving in an origin endpoint of the PAMI an eager send data communications instruction that specifies a transmission of transfer data from the origin endpoint to a target endpoint, the transfer data disposed in a send buffer, the send buffer characterized by a read/write send buffer memory address in a read/write virtual address space of the origin endpoint;
• determining by the origin endpoint for the send buffer a read-only send buffer memory address in a read-only virtual address space, the read-only virtual address space shared by both the origin endpoint and the target endpoint, the read-only virtual address space non-overlapping with respect to the read/write virtual address space, the read-only virtual address space established by an operating system of the parallel computer with all frames of physical memory mapped to pages of virtual memory in the read-only virtual address space;
• and
• communicating by the origin endpoint to the target endpoint an eager send message header that includes the read-only send buffer memory address.