linux-kernel - Re: [PATCH 1/4] statx: Add a system call to make enhanced file info available

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Date:   Tue, 22 Nov 2016 07:43:35 +1100
From:   Dave Chinner <david@...morbit.com>
To:     One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:     David Howells <dhowells@...hat.com>,
        Andreas Dilger <adilger@...ger.ca>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] statx: Add a system call to make enhanced file info
 available

On Mon, Nov 21, 2016 at 02:30:13PM +0000, One Thousand Gnomes wrote:
> > > increase in timestamp resoultion of at least another 10e-3 is
> > > likely....  
> > 
> > Is it, though?  To be useful, surely you have to be able to jam quite a few
> > instructions into a 1ns block, including memory accesses.
> > 
> > Rather than providing:
> > 
> > 	struct timestamp {
> > 		__s64 seconds;
> > 		__s64 femtoseconds;
> > 	};
> > 
> > which would require 64-bit divisions to get nanosecond timestamps that we do
> > actually use, I would lean towards:
> > 
> > 	struct timestamp {
> > 		__s64 seconds;
> > 		__s32 nanoseconds;
> > 		__s32 femtoseconds;
> > 	};
> 
> Which gets silly. The nanosecond world is defined by the speed of light.
> Short of someone finding a way to change that digital computing as we
> know it today is going to be living in the nanoseconds world. You hit the
> point of 'can't measure the difference' before you hit the point of 'can
> usefully order things using'

We already have clock rates that are fractions of a nanosecond per
cycle. We have pmem storage here right now that is accessed at the
speed of the CPU - actual nanosecond resolution timestamp capability
is a reality at the bleeding edge of storage technology right now.

It doesn't take much vision to extend the current hardare
capabilities with coherent hardware accelerators (e.g. as has been
added to the Power platform) writing directly into pmem storage and
providing higher resolution timestamps than the CPU can generate.

Call me silly if you want - I don't care - but let's not ignore the
emerging storage technology trends that are there for everyone to
see...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com