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A general description is here.
It incorporates (slightly revised) copies of packages ‘multicore’ and ‘snow’ (excluding MPI, PVM and NWS clusters). Code written to use the higher-level API functions in those packages should work unchanged (apart from changing any references to their namespaces to a reference to ‘parallel’, and links explicitly to ‘multicore’ or ‘snow’ on help pages).
It also contains support for multiple RNG streams following L'Ecuyer _et al_ (2002), with support for both ‘mclapply’ and ‘snow’ clusters. This replaces functions like ‘clusterSetupRNG()’ from ‘snow’ (which are not in ‘parallel’).
Using ‘fix()’ or ‘edit()’ on a R object (except perhaps a matrix or data frame) writes its temporary file with extension ‘.R’ so editors which select their mode based on the extension will select a suitable mode.
Java detection (‘R CMD javareconf’) works around bogus ‘java.library.path’ property in recent Oracle Java binaries.