1. A computing method, comprising: providing a computing system that includes a plurality of compute nodes interconnected by a communication network, formed of network switching elements; wherein the switching elements are separate from the compute nodes; accepting, by a processor serving as an offload manager, a notification of a computing task for execution by the computing system, the notification specifying a designated partial group of the compute nodes assigned by a job scheduler, separate from the offload manager, to execute the computing task; identifying, by the processor, based on the given interconnection topology and on a criterion derived from the computing task, a subset of the network switching elements to be configured to connect the compute nodes in the designated partial group to one or more root switching elements; and configuring the network switching elements in the subset, by the processor, to forward node-level results of the computing task produced by the compute nodes in the designated partial group to the one or more root switching elements through the subset, so as to cause the one or more root switching elements to calculate and output an end result of the computing task based on the node-level results.