[Boost-bugs] [Boost C++ Libraries] #9735: No need to memset to 0 after mmap to /dev/zero

Subject: [Boost-bugs] [Boost C++ Libraries] #9735: No need to memset to 0 after mmap to /dev/zero
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2014-03-04 07:22:01

#9735: No need to memset to 0 after mmap to /dev/zero
------------------------------------+-----------------------
 Reporter:  Tianlong Wu <rockuw@…>  |      Owner:  olli
     Type:  Bugs                    |     Status:  new
Milestone:  To Be Determined        |  Component:  coroutine
  Version:  Boost 1.55.0            |   Severity:  Problem
 Keywords:                          |
------------------------------------+-----------------------
 In boost source
 /libs/coroutine/detail/standard_stack_allocator_posix.cpp:120, it has the
 following snippet:

 {{{#!c
     void * limit =
 # if defined(macintosh) || defined(__APPLE__) || defined(__APPLE_CC__)
     ::mmap( 0, size_, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1,
 0);
 # else
     ::mmap( 0, size_, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
 # endif
     ::close( fd);
     if ( ! limit) throw std::bad_alloc();

     std::memset( limit, '\0', size_);
 }}}

 Since mmap() with the MAP_PRIVATE flag implies a copy-on-write mechanism
 which ensures that memory pointed by limit will be initialized to all-
 zero. The additional memset slows down coroutine creation and makes
 coroutine not that lightweight. In this case of memseting ~64KB memory
 takes ~40us on my machine. And creating a coroutine takes ~50us.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/9735>
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