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

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Message-ID: <20110722163335.2df4f6ca@notabene.brown>
Date:	Fri, 22 Jul 2011 16:33:35 +1000
From:	NeilBrown <neilb@...e.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Matt Mackall <mpm@...enic.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Nanosecond fs timestamp support: sad

On Thu, 21 Jul 2011 23:01:24 -0700 Andi Kleen <andi@...stfloor.org> wrote:

> Matt Mackall <mpm@...enic.com> writes:
> 
> 
> > This means I can touch a file something like 70k times per second and
> > get only 300 distinct timestamps on my laptop. And only 100 distinct
> > timestamps on a typical distro server kernel.
> 
> You should use the inode generation number if you really want
> to see every update.

I assume you mean i_version which gets incremented (under a spinlock) if the
filesystem asks for it.

This doesn't let you compare the ages of two files.  I wonder if that is
important.  Is it important to you Matt?


> 
> > Meanwhile, I can call gettimeofday 35M times per second and get ~1M
> > distinct responses.
> 
> They key word here is "I".
> 
> > Given that we can do gettimeofday three orders of magnitude faster than
> > we can do file transactions and it has four orders of magnitude better
> > resolution, shouldn't we be using it for filesystem time when
> > sb->s_time_gran is less than 1/HZ?
> 
> Some systems have a quite slow gettimeofday()
> That was the primary motivation for using jiffies. 
> 
> Also adding more granuality makes it more expensive,
> because there's additional work every time it changes.
> Even jiffies already caused regressions.
> 
> -Andi

I imagine a scheme where 'stat' would set a flag if it wasn't set, and
file_update_time would:
  - if the flag is set, use gettimeofday and clear the flag
  - if the flag is not set, use jiffies

so if you are looking, you will see i_mtime changing precisely but if not,
you don't pay the price.
This wouldn't allow precise ordering of distinct files either of course.

NeilBrown
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