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

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Date:	Wed, 18 Aug 2010 20:17:14 -0700
From:	john stultz <johnstul@...ibm.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	"Patrick J. LoPresti" <lopresti@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andi Kleen <andi@...stfloor.org>,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Proposal: Use hi-res clock for file timestamps

On Wed, 2010-08-18 at 22:31 -0400, J. Bruce Fields wrote:
> On Wed, Aug 18, 2010 at 06:41:02PM -0700, john stultz wrote:
> > On Wed, Aug 18, 2010 at 11:12 AM, J. Bruce Fields <bfields@...ldses.org> wrote:
> > > I'm completely ignorant about higher-resolution time sources.  Any
> > > recommended reading?  What resolution do they actually provide, what's
> > > the expense of reading them, how reliable are they, and how do the
> > > answers to those questions vary across different hardware and kernel
> > > versions?  A quick look at drivers/clocksource/ doesn't suggest
> > > simple answers.
> > 
> > Yea, there aren't simple answers. Clocksource hardware varies
> > drastically in resolution and access time across systems and
> > architectures. Further, clocksources may change while the system is
> > up, so we don't really expose the hardware resolution.
> > 
> > On x86, access latency varies from ~50ns (TSC) to ~1.3us (ACPI PM).
> > (And that is ignoring the PIT, which can be 18us per call - luckily
> > almost no hardware uses that). The resolution similarly scales from
> > sub-ns (TSC @ > 1ghz cpus) to ~279ns (ACPI PM). Of course, across
> > architectures you will see even more variance.
> 
> The race in question occurs when you manage to check mtime between two
> file data updates, with all three operations occurring within a clock
> tick.
> 
> No idea if that's feasible in hundreds of nanoseconds.

I think this is what Andi meant that you'll always race with time and
that version counters are the only real solution here.

> I'm also not sure how to judge the access latency.  Certainly a
> microsecond is a lot compared to just reading a cached mtime value.
> 
> Will we ever see them go backwards?  (So if I know I wrote to file B
> after writing to file A, is there ever a case where I could end up with
> an earlier mtime on B than A?)

You should not. However, there have been bugs in the past, and there
will probably be a few more in the future.

There are also theoretical issues with SMP systems where the TSCs are
not perfectly synced, but the window for those races should be small
(ie: smaller then can be detected - otherwise we'll throw out the TSC).


thanks
-john


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