Emacs Buffer Allocation using mmap

Question

Why is mmap buffer allocation activated by default on cygwin, freebsd and irix6-5 but not on linux?

See USE_MMAP_FOR_BUFFERS in emacs/src/config.h. and use_mmap_for_buffers in emacs/configure.in.

Isn't mmap based access superior to normal buffer allocation?

This question seems very specific. Perhaps you'll better post it to emacs-devel@gnu.org? — Thomas, Jun 13 '11 at 08:18

janneb · Accepted Answer · 2011-06-13T15:05:09.487

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The default glibc malloc() uses mmap for large allocations; From the malloc(3) manpage. "When allocating blocks of memory larger than MMAP_THRESHOLD bytes, the glibc malloc() implementation allocates the memory as a private anonymous mapping using mmap(2). MMAP_THRESHOLD is 128 kB by default, but is adjustable using mallopt(3)."

Perhaps switching to mmap on those other platforms is to work around poor malloc() implementations that don't do it themselves?

edited Jun 13 '11 at 15:05

answered Jun 13 '11 at 08:46

janneb

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But how can malloc be a replacement for mmap? mmap maps file fd to memory but malloc just allocates memory not related to any file (descriptor). – Nordlöw Jun 13 '11 at 14:23
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@Nordlöw: See the "private anonymous mapping" part from what I quoted above. That is, the flags argument to mmap(2) is MAP_PRIVATE|MAP_ANONYMOUS. See the mmap(2) man page for more info. – janneb Jun 13 '11 at 15:04

score 2 · Answer 2 · edited May 23 '17 at 11:44

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mmap() allocation is easier to realize in a threadsafe fashion, but brk() allocation is about 10% slower on Linux. See this question.

edited May 23 '17 at 11:44

Community

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answered Jun 13 '11 at 08:31

Yannick Versley

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Emacs Buffer Allocation using mmap

2 Answers2