[racket-dev] Questions about the GC code

I've been reading the code of the files gc/os_dep.c and 
gc/include/private/gcconfig.h. I've some questions related to OpenBSD.

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In the X86_64 config, the file defines STACKBOTTOM and HEURISTIC2 
(unused because racket picks STACKBOTTOM). What's the preferred?. I'm 
asking because I don't know if the adding of HEURISTIC2 was an error or 
really there a good reason to select one or other.

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There is some recommended benchmark to test the performance of MMAP vs 
sbrk? I've tried both with generic code and I don't see the difference.

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OpenBSD only supports MIPS64. Should I add ELFCLASS64 to the MIPS block 
within gcconfig.h?