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The BioConductor group has put together a Cloud Formation stack for doing interactive parallel computing in R. Follow those instructions, selecting the number of workers and size of the EC2 instances. Once the stack comes up, which took about 10 minutes for me, you log into RStudio on the head node. You'll start R processes on the worker nodes and send commands to the workers.
stack name: StartBioCParallelClusterWithSSH
template url: https://s3.amazonaws.com/bioc-cloudformation-templates/parallel_cluster_ssh.json
Note that the parallel package is perfectly happy starting up several copies of R on a single machine, which can be helpful for testing.
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