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

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Date:	Fri, 13 Aug 2010 12:09:39 -0700
From:	john stultz <johnstul@...ibm.com>
To:	"Patrick J. LoPresti" <lopresti@...il.com>
Cc:	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 Fri, 2010-08-13 at 11:57 -0700, Patrick J. LoPresti wrote:
> On Fri, Aug 13, 2010 at 11:45 AM, john stultz <johnstul@...ibm.com> wrote:
> > On those TSC broken systems that use the hpet or acpi_pm, a
> > getnstimeofday call can take 0.5-1.3us, so the penalty can be quite
> > severe.
> 
> So you are saying my proposal is a bad idea forever?  (But then why
> even bother having nanosecond resolution on ext4?)
> 
> Or that it is a bad idea for now?

I'm not judging the idea as good/bad, just providing information for
context.

> Or that it needs to be refined?  Maybe use hi-res precision on systems
> where it is known to be fast?
> 
> > And even with the TSC, expect some performance impact, as
> > reading hardware and doing the multiply is more costly then just
> > fetching a value from memory.
> 
> Relative to file system operations?  Seriously?  What performance hit
> would you expect on real-world applications?
> Something like 0.1% (10 nsec / 10 usec) worst case?

If you can show this does not affect performance in benchmarks, etc, I'm
sure it will be easier to push the patch. As outside of performance, I
don't think there's much of an issue with the change.

So other then "show some numbers", my only thought that might make the
patch more attractive is that rather than a global change, or a static
CONFIG_ option, would it maybe make more sense as a mount option?

thanks
-john

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