使用和不使用 %M 选项显示权限的命令 find 的巨大性能差异

Question

在我的 CentOS 7.6 上，我通过运行创建了一个包含 3,000,000 个文件的文件夹（称为 many_files）：

for i in {1..3000000}; do echo $i>$i; done;

我正在使用该命令find将有关此目录中文件的信息写入文件。这工作得非常快：

$ time find many_files -printf '%i %y %p\n'>info_file

real    0m6.970s
user    0m3.812s
sys     0m0.904s

现在，如果我添加%M以获取权限：

$ time find many_files -printf '%i %y %M %p\n'>info_file

real    2m30.677s
user    0m5.148s
sys     0m37.338s

该命令需要更长的时间。这对我来说非常令人惊讶，因为在 C 程序中我们可以使用它struct stat来获取文件的 inode 和权限信息，而在内核中struct inode保存了这些信息。

我的问题：

1. 是什么导致了这种行为？
2. 有没有更快的方法来获得这么多文件的文件权限？

Answer 1

第一个版本只需要readdir(3)/getdents(2)目录，当在支持此功能的文件系统上运行时（ext4：filetype显示为 的功能tune2fs -l /dev/xxx，xfs：ftype=1显示为xfs_info /mount/point...）。

此外，第二个版本还需要stat(2)每个文件，需要额外的 inode 查找，因此在文件系统和设备上进行更多搜索，如果它是旋转磁盘并且没有保留缓存，则可能会相当慢。这stat仅适用于名称，索引节点和文件类型看时，因为目录项足够不需要：

  The linux_dirent structure is declared as follows:

       struct linux_dirent {
           unsigned long  d_ino;     /* Inode number */
           unsigned long  d_off;     /* Offset to next linux_dirent */
           unsigned short d_reclen;  /* Length of this linux_dirent */
           char           d_name[];  /* Filename (null-terminated) */
                             /* length is actually (d_reclen - 2 -
                                offsetof(struct linux_dirent, d_name)) */
           /*
           char           pad;       // Zero padding byte
           char           d_type;    // File type (only since Linux
                                     // 2.6.4); offset is (d_reclen - 1)
           */
       }

相同的信息可用于readdir(3)：

struct dirent {
    ino_t          d_ino;       /* Inode number */
    off_t          d_off;       /* Not an offset; see below */
    unsigned short d_reclen;    /* Length of this record */
    unsigned char  d_type;      /* Type of file; not supported
                                   by all filesystem types */
    char           d_name[256]; /* Null-terminated filename */
};

怀疑但通过比较（在较小的样本上......）两个输出得到证实：

strace -o v1 find many_files -printf '%i %y %p\n'>info_file
strace -o v2 find many_files -printf '%i %y %M %p\n'>info_file

这在我的 Linux amd64 内核 5.0.x 上仅显示为主要区别：

[...]

 getdents(4, /* 0 entries */, 32768)     = 0
 close(4)                                = 0
 fcntl(5, F_DUPFD_CLOEXEC, 0)            = 4
-write(1, "25499894 d many_files\n25502410 f"..., 4096) = 4096
-write(1, "iles/844\n25502253 f many_files/8"..., 4096) = 4096
-write(1, "096 f many_files/686\n25502095 f "..., 4096) = 4096
-write(1, "es/529\n25501938 f many_files/528"..., 4096) = 4096
-write(1, "1 f many_files/371\n25501780 f ma"..., 4096) = 4096
-write(1, "/214\n25497527 f many_files/213\n2"..., 4096) = 4096
-brk(0x55b29a933000)                     = 0x55b29a933000
+newfstatat(5, "1000", {st_mode=S_IFREG|0644, st_size=5, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "999", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "998", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "997", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "996", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "995", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "994", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "993", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "992", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "991", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "990", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0

[...]

+newfstatat(5, "891", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+write(1, "25499894 d drwxr-xr-x many_files"..., 4096) = 4096
+newfstatat(5, "890", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0

[...]