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A general description is here.
(‘For experts only’, and the introductory manual says.) The use of environment variables ‘R_NSIZE’ and ‘R_VSIZE’ to control the initial (= minimum) garbade collection trigger for number of cons cels and size of heap has been restored: they can be overridden by the command-line options ‘--min-nsize’ and ‘--min-vsize’; see ‘?Memory’.