我们来分析下pipe的这部分代码:

//pipe.c:L349
static ssize_t
pipe_read(struct kiocb *iocb, const struct iovec *_iov,
               unsigned long nr_segs, loff_t pos)
{
...
   if (ret > 0)
        file_accessed(filp);
    return ret;
}

    我们可以看到在pipe读的时候要设置 file_accessed时间的,接着:

//fs.h:L1761
extern void touch_atime(struct vfsmount *mnt, struct dentry *dentry);
static inline void file_accessed(struct file *file)
{
        if (!(file->f_flags & O_NOATIME))
                touch_atime(file->f_path.mnt, file->f_path.dentry);
}

    如果文件没设置 O_NOATIME就真正动手设置atime,接着:

//inode.c:L1493
void touch_atime(struct vfsmount *mnt, struct dentry *dentry)
{
        struct inode *inode = dentry->d_inode;
        struct timespec now;
        if (inode->i_flags & S_NOATIME)
                return;
        if (IS_NOATIME(inode))
                return;
        if ((inode->i_sb->s_flags & MS_NODIRATIME) && S_ISDIR(inode->i_mode))
                return;
        if (mnt->mnt_flags & MNT_NOATIME)
                return;
        if ((mnt->mnt_flags & MNT_NODIRATIME) && S_ISDIR(inode->i_mode))
                return;
        now = current_fs_time(inode->i_sb);
        if (!relatime_need_update(mnt, inode, now))
                return;
        if (timespec_equal(&inode->i_atime, &now))
                return;
        if (mnt_want_write(mnt))
                return;
        inode->i_atime = now;
        mark_inode_dirty_sync(inode);
        mnt_drop_write(mnt);
}

    我们可以看出上面的流程还是比较复杂的,开销也很大.

     我们来演示下:

$ cat > pipe_test.c
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
static int fds[2];
static pthread_t rp;
static void *rp_entry(void *arg) {
  char c[1];
  while (1 == read(fds[0], c, 1)) {
    if (*c == \'Q\') break;
  }
  fprintf(stderr, "pipe read ok\n");
  return NULL;
}
int main(int argc, char *argv[]) {
  long i, n;
  int rc;
  if (argc < 2) {
    fprintf(stderr, "usage: pipe_test NNNNNN\n");
    return -1;
  }
  n = atol(argv[1]);
  pipe(fds);
  //fcntl(fds[0], F_SETFL, O_NOATIME);
  pthread_create(&rp, NULL, rp_entry, NULL);
  fprintf(stderr, "pipe write %ld...", n);
  for (i = 0; i < n; i++) {
    write(fds[1], "A", 1);
  }
  write(fds[1], "Q", 1);
  fprintf(stderr, "ok\n");
  pthread_join(rp, NULL);
  close(fds[0]);
  close(fds[1]);
  return 0;
}
CTRL+D
$ gcc -D_GNU_SOURCE pipe_test.c -lpthread
$ sudo opcontrol --setup --vmlinux=/usr/lib/debug/lib/modules/2.6.18-164.el5/vmlinux
$ sudo opcontrol --init && sudo opcontrol --reset && sudo opcontrol --start
$ ./a.out 10000000
pipe write 10000000...ok
pipe read ok
$ sudo opcontrol --shutdown
$ opreport -l|less
samples  %        app name                 symbol name
378654   92.7742  vmlinux                  .text.acpi_processor_idle
12978     3.1797  vmlinux                  current_fs_time
2530      0.6199  vmlinux                  thread_return
2345      0.5745  vmlinux                  touch_atime
2253      0.5520  vmlinux                  .text.acpi_safe_halt
1597      0.3913  vmlinux                  timespec_trunc
1368      0.3352  vmlinux                  file_update_time
1253      0.3070  vmlinux                  __mark_inode_dirty
901       0.2208  vmlinux                  pipe_writev
768       0.1882  vmlinux                  __mutex_lock_slowpath
763       0.1869  vmlinux                  try_to_wake_up
270       0.0662  vmlinux                  copy_user_generic_unrolled
254       0.0622  vmlinux                  acpi_set_register
254       0.0622  vmlinux                  system_call
233       0.0571  vmlinux                  pipe_readv
188       0.0461  vmlinux                  dnotify_parent
167       0.0409  vmlinux                  mutex_unlock
...

    我们可以看到touch_atime的开销很大,远比pipe的读写大.

     这次把这行注释去掉: fcntl(fds[0], F_SETFL, O_NOATIME); 指示pipe在读的时候不更新atime,看下效果:

$ opreport -l|less
samples  %        app name                 symbol name
599018   95.2466  vmlinux                  .text.acpi_processor_idle
4140      0.6583  vmlinux                  .text.acpi_safe_halt
3281      0.5217  vmlinux                  thread_return
2812      0.4471  vmlinux                  current_fs_time
2615      0.4158  vmlinux                  file_update_time
1790      0.2846  vmlinux                  __mutex_lock_slowpath
1657      0.2635  vmlinux                  timespec_trunc
1341      0.2132  vmlinux                  try_to_wake_up
1281      0.2037  vmlinux                  mutex_unlock
1080      0.1717  vmlinux                  mutex_lock
1001      0.1592  vmlinux                  pipe_readv
925       0.1471  vmlinux                  pipe_writev

    这下看不到touch_atime了,开销省了,对于高性能服务器是很重要的.

     小结: 细节很重要!

建议继续学习：

1. Golang socket 里面奇怪的 pipe 使用    (阅读：730)