id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
9735	No need to memset to 0 after mmap to /dev/zero	Tianlong Wu <rockuw@…>	olli	"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."	Bugs	closed	To Be Determined	coroutine	Boost 1.55.0	Problem	fixed