阅读0.11版的内核源码时，在linux-0.11/fs/pipe.c中，函数sys_pipe()里面出现了2个宏定义，NR_OPEN 与 NR_FILE。下面说明一下它们的区别：
    1. NR_OPEN is the maximum number of files that can be opened by process。
    NR_OPEN是一个进程可以打开的最大文件数**。
    A process cannot use more than NR_OPEN file descriptors.
     一个进程不能使用超过NR_OPEN文件描述符。 **限制的是单个进程
    The kernel also enforces a dynamic bound on the maximum number of file descriptors in the signal->rlim[RLIMIT_NOFILE] structure of the process descriptor; this value is usually 1,024, but it can be raised if the process has root privileges.

    1. NR_FILE is the limit on total number of files in the system at any given point in time
      NR_FILE 是系统在某一给定时刻，限制的文件总数 **限制的是整个系统**

    While initializing the kernel we setup the vfs cache with
    start_kernel
    vfs_caches_init(num_physpages);
    files_init(mempages);
    fs/file_table.c says
    / One file with associated inode and dcache is very roughly 1K.
     Per default don’t use more than 10% of our memory for files.
    n = (mempages * (PAGE_SIZE / 1024)) / 10;
    this n can never be greater than NR_FILE

    若要修改这些可以参考：系统ulimit限制进程占用资源