linux-kernel - Re: Nanosecond fs timestamp support: sad

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Message-ID: <20110729194903.GA1720@ucw.cz>
Date:	Fri, 29 Jul 2011 21:49:03 +0200
From:	Pavel Machek <pavel@....cz>
To:	Matt Mackall <mpm@...enic.com>
Cc:	NeilBrown <neilb@...e.de>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Andi Kleen <andi@...stfloor.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Nanosecond fs timestamp support: sad

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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