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

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Message-ID: <20161121143013.79373b5e@lxorguk.ukuu.org.uk>
Date:   Mon, 21 Nov 2016 14:30:13 +0000
From:   One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:     David Howells <dhowells@...hat.com>
Cc:     Dave Chinner <david@...morbit.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

> > 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'

Alan