Re: Nanosecond fs timestamp support: sad

Pavel Machek
2011-07-29T19:49:16Z

Hi!

> > If not, we probably should tell NFSv4 to use timestamps and focus on making
> > them work well.
> > ??
> >
> > The timestamp used doesn't need to update ever nanosecond. I think if it
> > were just updated on every userspace->kernel transition (or effective
> > equivalents inside kernel threads) that would be enough capture all
> > causality. I wonder how that would be achieved.. I wonder if RCU machinery
> > could help - doesn't it keep track of when threads schedule ... or something?
>
> Sort of.
>
> Some observations:
>
> - we only need to go to higher resolution when two events happen in the
> same time quantum
> - this applies at both the level of seconds and jiffies
> - if the only file touched in a given quantum gets touched ago, we don't
> need to update its timestamp if stat wasn't also called on it in this
> quantum

parse error aroound 'ago'.

> - we never need to use a higher resolution than the global
> min(s_time_gran)
>
>
> For instance, if a machine is idle, except for writing to a single file
> once a second, 1s resolution suffices.

Are you sure? As soon as you get network communication...

> Any time two files are touched in the same second, the second one (and
> later files) needs jiffies resolution. Similarly, any time two files are
> touched in the same jiffy, the second one should use gtod().

For make. I don't see how this is globally true.

I do

( date; > stamp; date ) | ( sleep 5; cat > counterexample )

I know timestamp should be between two dates, but it is not.

Pavel

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