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    SubjectRe: [RFC v2 03/83] Add super.h.
    On Fri, Mar 16, 2018 at 3:59 AM, Theodore Y. Ts'o <tytso@mit.edu> wrote:
    > On Thu, Mar 15, 2018 at 09:38:29PM +0100, Arnd Bergmann wrote:
    >>
    >> You could also have a resolution of less than a nanosecond. Note
    >> that today, the file time stamps generated by the kernel are in
    >> jiffies resolution, so at best one millisecond. However, most modern
    >> file systems go with the 64+32 bit timestamps because it's not all
    >> that expensive.
    >
    > It actually depends on the architecture and the accuracy/granularity
    > of the timekeeping hardware available to the system, but it's possible
    > for the granularity of file time stamps to be up to one nanosecond.
    > So you can get results like this:
    >
    > % stat unix_io.o
    > File: unix_io.o
    > Size: 55000 Blocks: 112 IO Block: 4096 regular file
    > Device: fc01h/64513d Inode: 19931278 Links: 1
    > Access: (0644/-rw-r--r--) Uid: (15806/ tytso) Gid: (15806/ tytso)
    > Access: 2018-03-15 18:09:21.679914182 -0400
    > Modify: 2018-03-15 18:09:21.639914089 -0400
    > Change: 2018-03-15 18:09:21.639914089 -0400

    Note how the nanoseconds only differ in digits 2, 7, 8, and 9 though:

    The atime update happened 4 jiffies (at HZ=100) after the mtime,
    the low digits are presumably jitter or ntp adjustments.

    This is the result of current_time() using the plain tk_xtime
    rather than reading the highres clocksource as ktime_get_real_ts64()
    does.

    This was a performance optimization a long time ago. We could
    make the current_time() behavior configurable if we want though,
    e.g. at compile time, or as a per-mount option. It's probably more
    common these days to have a highres clocksource that can
    be read efficiently than it was back when current_fs_time()
    was first introduced.

    Arnd

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