Executing an allgather operation on a parallel computer, including executing an alltoallv operation with a list of send displacements, where each send displacement is a send buffer segment pointer, each send displacement points to the same segment of a send buffer, the parallel computer includes a plurality of compute nodes, each compute node includes a send buffer, the compute nodes are organized into at least one operational group of compute nodes for collective operations, each compute node in the operational group is assigned a unique rank, and each send buffer is segmented according to the ranks.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 This invention was made with Government support under Contract No. B519700 awarded by the Department of Energy. The Government has certain rights in this invention.