Various examples are directed to systems and methods in which a flow controller of a first synchronous flow may receive an instruction to execute a first loop using the first synchronous flow. The flow controller may determine a first iteration index for a first iteration of the first loop. The flow controller may send, to a first compute element of the first synchronous flow, a first synchronous message to initiate a first synchronous flow thread for executing the first iteration of the first loop. The first synchronous message may comprise the iteration index. The first compute element may execute an input/output operation at a first location of a first compute element memory indicated by the first iteration index.