http://stackoverflow.com/questions/21268808/profiling-data-tables-setkey-operation-with-rprof/

Short summary of problem:

Rprof timings are incorrect for at least one specific operation for a
function in the data.table package for a darwin machine running Snow
Leopard with the 64-bit kernel enabled.

Current fix:
Restart machine in (default) 32-bit kernel mode.

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