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A general description is here.
R on Windows uses again the system memory allocator. Previously, R shipped with Doug Lea's allocator as a replacement to overcome performance limitations seen with system allocators on earlier versions of Windows. Like on Unix-alike, once using the system allocator, R on Windows no longer provides ‘memory.limit()’, ‘memory.size()’, command-line option ‘--max-mem-size’ and environment variable ‘R_MAX_MEM_SIZE’. The allocation limit was important on 32-bit Windows, but R no longer supports 32-bit builds.