1. A distributed computing system, comprising: a plurality of compute nodes, each compute node comprising at least one processor and a memory; a data store that stores data objects of a data set, wherein the data store is accessible by the plurality of compute nodes; and a service interface; wherein the distributed computing system implements a distributed computing service; wherein the service interface is configured to receive a request to execute an application that processes the data set and that implements a message passing interface through which compute nodes participating in execution of the application exchange messages with each other; wherein in response to receiving the request, the distributed computing service is configured to: configure two or more of the plurality of compute nodes as a cluster of compute nodes according to a MapReduce distributed computing framework; configure a single compute node of the cluster of compute nodes to implement a single mapper process; and establish a plurality of communication channels for a plurality of pairs of compute nodes of the cluster of compute nodes, wherein each communication channel of the plurality of communication channels is established between a respective pair of compute nodes of the cluster of compute nodes, and wherein a same public-private key pair that is specific to the cluster of compute nodes is used to establish each communication channel; wherein the mapper process is configured to initiate execution of the application; wherein, during execution, the application is configured to spawn respective ones of a plurality of commands of the application to each compute node of the pair of compute nodes to process a respective portion of the data set; and wherein, during execution of the application, the compute nodes of the pair of compute nodes exchange messages with each other over the respective communication channel using the message passing interface.