Linux – List current inotify watches (pathname, PID)

Is it safe to raise that value and what would be the consequences of a too high value?

Yes, it's safe to raise that value and below are the possible costs [source]:

• Each used inotify watch takes up 540 bytes (32-bit system), or 1 kB (double - on 64-bit) [sources: 1, 2]
• This comes out of kernel memory, which is unswappable.
• Assuming you set the max at 524288 and all were used (improbable), you'd be using approximately 256MB/512MB of 32-bit/64-bit kernel memory.
  • Note that your application will also use additional memory to keep track of the inotify handles, file/directory paths, etc. -- how much depends on its design.

To check the max number of inotify watches:

cat /proc/sys/fs/inotify/max_user_watches

To set max number of inotify watches

Temporarily:

• Run sudo sysctl fs.inotify.max_user_watches= with your preferred value at the end.

Permanently (more detailed info):

• put fs.inotify.max_user_watches=524288 into your sysctl settings. Depending on your system they might be in one of the following places:
  • Debian/RedHat: /etc/sysctl.conf
  • Arch: put a new file into /etc/sysctl.d/, e.g. /etc/sysctl.d/40-max-user-watches.conf
• you may wish to reload the sysctl settings to avoid a reboot: sysctl -p (Debian/RedHat) or sysctl --system (Arch)

Check to see if the max number of inotify watches have been reached:

Use tail with the -f (follow) option on any old file, e.g. tail -f /var/log/dmesg: - If all is well, it will show the last 10 lines and pause; abort with Ctrl-C - If you are out of watches, it will fail with this somewhat cryptic error:

tail: cannot watch '/var/log/dmsg': No space left on device

To see what's using up inotify watches

find /proc/*/fd -lname anon_inode:inotify |
   cut -d/ -f3 |
   xargs -I '{}' -- ps --no-headers -o '%p %U %c' -p '{}' |
   uniq -c |
   sort -nr

The first column indicates the number of inotify fds (not the number of watches though) and the second shows the PID of that process [sources: 1, 2].

When you read() an inotify fd, the name field of the returned struct tells you which file was modified relative to the directory being watched, so you shouldn't have to stat every file in a directory after the event.

See http://linux.die.net/man/7/inotify

Specifically:

struct inotify_event {
    int      wd;       /* Watch descriptor */
    uint32_t mask;     /* Mask of events */
    uint32_t cookie;   /* Unique cookie associating related
                          events (for rename(2)) */
    uint32_t len;      /* Size of 'name' field */
    char     name[];   /* Optional null-terminated name */
};

The name field is only present when an event is returned for a file inside a watched directory; it identifies the file pathname relative to the watched directory. This pathname is null-terminated, and may include further null bytes to align subsequent reads to a suitable address boundary.