Re: Proposal: Use hi-res clock for file timestamps

Andi Kleen
2010-08-17T18:29:33Z

> OK, so that leaves us with the race, even on newer filesystems:
>
> 1. File is modified, mtime updated
> 2. Client fetches mtime to revalidate cache
> 3. File is modified again, mtime updated
> 4. Client fetches new mtime to revalidate cache

You'll always have a race window with time, the only way around
that would be a version number.
> - Tell everyone to use NFSv4 (and make sure we have
> changeattr/i_version working correctly).
> - Use a finer-grained time source. (I believe you when you say
> the TSC is too slow, but maybe we should run some tests to
> make sure.)

It depends on the CPU too.

> - Increment mtime by a nanosecond when necessary.

You cannot be more precise than the backing file system: this causes
non monotonity when the inodes are flushed (has happened in the past)

-Andi