让erlang支持Huge TLB

压榨linux2.6最后点内存分配性能,使用hugetlb.

Huge TLB Filesystem
   ===================

   Most modern architectures support more than one page size. For example,
   the IA-32 architecture supports 4KiB pages or 4MiB pages but Linux only
   used large pages for mapping the actual kernel image. As TLB slots are a
   scarce resource, it is desirable to be able to take advantages of the
   large pages especially on machines with large amounts of physical memory.

   In 2.6, Linux allows processes to use large pages, referred to as huge
   pages. The number of available huge pages is configured by the system
   administrator via the /proc/sys/vm/nr_hugepages proc interface. As the
   success of the allocation depends on the availability of physically
   contiguous memory, the allocation should be made during system startup.

   The root of the implementation is a Huge TLB Filesystem (hugetlbfs) which
   is a pseudo-filesystem implemented in fs/hugetlbfs/inode.c and based on
   ramfs. The basic idea is that any file that exists in the filesystem is
   backed by huge pages. This filesystem is initialised and registered as an
   internal filesystem at system start-up.

   There is two ways that huge pages may be accessed by a process. The first
   is by using shmget() to setup a shared region backed by huge pages and the
   second is the call mmap() on a file opened in the huge page filesystem.

   When a shared memory region should be backed by huge pages, the process
   should call shmget() and pass SHM_HUGETLB as one of the flags. This
   results in a file being created in the root of the internal filesystem.
   The name of the file is determined by an atomic counter called
   hugetlbfs_counter which is incremented every time a shared region is
   setup.

   To create a file backed by huge pages, a filesystem of type hugetlbfs must
   first be mounted by the system administrator. Once the filesystem is
   mounted, files can be created as normal with the system call open(). When
   mmap() is called on the open file, the hugetlbfs registered mmap()
   function creates the appropriate VMA for the process.

   Huge TLB pages have their own function for the management of page tables,
   address space operations and filesystem operations. The names of the
   functions for page table management can all be seen in $<$linux/hugetlb.h
   $>$ and they are named very similar to their ``normal'' page equivalents.

   ======================================


把erl_mseg.c稍稍改造下是可以支持的！


void
erts_mseg_init(ErtsMsegInit_t *init)
{
...
#if HAVE_MMAP && !defined(MAP_ANON)
    mmap_fd = open("/dev/zero", O_RDWR);
    if (mmap_fd < 0)
erl_exit(ERTS_ABORT_EXIT, "erts_mseg: unable to open /dev/zero\n");
#endif
...
}

mmap_fd改成指向hugetlbfs的文件句柄就好了或者最好的方法就个getenv("MMAP_FILENAME")来获取。