J3/06-132 Date: January 31, 2006 To: J3 From: Craig Rasmussen Subject: Team Formation in Co-Array Fortran Reference: WG5 worklist item, Co-Array Fortran ----------------------------------------------------------------------- The current CAF proposal specifies a team of images as a set of images that are collaborating in a computation. The members of the team are specified by an rank-one default integer array that holds the image indices, with no duplication. Teams are used in collective operations such as synchronization. The specification of a team in the current CAF proposal works fine as long as the number of members of a team is small. Unfortunately the current proposal does not scale to large numbers of processors. Team formation can require significant resources, as communication schedules must be calculated, stored and the results shared across team members. In the current CAF proposal, team formation must be done each time a team is used. This will likely mean that collectives operations cannot take advantage of O(log n) algorithms (where n is the number of team members). Because processor counts now exceed 150 K and will certainly exceed at least one million in the lifetime of CAF, it is proposed that the cost of team formation be amortized by providing intrinsic functions to both form and destroy a team.