Networked computer

A computer comprising a plurality of processing nodes, each processing nodes having at least one processor configured to process input data to generate output data in the form of an array of data items;

• the plurality of processing nodes arranged in cliques in which each processing node of a clique is connected to each other processing node in the clique by first and second clique links, the cliques being inter-connected in rings such that each processing node is a member of a single clique and a single ring, the processing nodes being configured to exchange data items in respective exchange steps of a machine learning collective, wherein the processing nodes of all cliques are configured to exchange in each exchange step via the respective first and second clique links at least two data items with the other processing node(s) in its clique, and all processing nodes are configured to reduce each received data item with the data item in the corresponding position in the array on that processing node,
• wherein the machine learning collective is an Allreduce collective and each processing node is configured to exchange data items in exchange steps of an Allgather phase, following a reduce-scatter phase of the Allreduce collective, wherein in each step of the Allgather phase reduced data items are exchanged between processing nodes in a clique, and
• wherein the processing nodes are each configured to transmit data items in a forwards direction to its adjacent processing node in the ring in at least some of the exchange steps in the reduce-scatter phase.