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    SubjectRe: Proposal: Use hi-res clock for file timestamps
    On Wed, 18 Aug 2010 13:32:03 -0400
    "J. Bruce Fields" <> wrote:

    > On Wed, Aug 18, 2010 at 03:53:59PM +1000, Neil Brown wrote:
    > > I'm not sure you even want to pay for a per-filesystem atomic access when
    > > updating mtime. mnt_want_write - called at the same time - seems to go to
    > > some lengths to avoid an atomic operation.
    > >
    > > I think that nfsd should be the only place that has to pay the atomic
    > > penalty, as it is where the need is.
    > >
    > > I imagine something like this:
    > > - Create a global struct timespec which is protected by a seqlock
    > > Call it current_nfsd_time or similar.
    > > - file_update_time reads this and uses it if it is newer than
    > > current_fs_time.
    > > - nfsd updates it whenever it reads an mtime out of an inode that matches
    > > current_fs_time to the granularity of 1/HZ.
    > We can also skip the update whenever current_nfsd_time is greater than
    > the inode's mtime--that's enough to ensure that the next
    > file_update_time() call will get a time different from the inode's
    > current mtime.

    Yes, I agree with you and Patrick - very sensible optimisation.

    > Would the extra expense rule out treating sys_stat() the same as nfsd?
    > It would be nice to be able to solve the same problem for userspace
    > nfsd's (or any other application that might be using mtime to save
    > rereading data).

    It would be nice, but I would be loathe to add any cost to 'stat' unless we
    knew it was needed.
    If we had an xstat() which could explicitly ask for
    high-precision-time-stamps, then yes - otherwise maybe not.

    (or maybe define a system:linux.xxxx xattr which would read as a
    high-precision time stamp... I seem to be warming to the idea of using the
    xattr interface for enhancing stat).


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