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From: David Howells <dhowells@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: dhowells@redhat.com, Andreas Dilger <adilger@dilger.ca>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/4] statx: Add a system call to make enhanced file info available
Date: Fri, 18 Nov 2016 22:54:02 +0000	[thread overview]
Message-ID: <11317.1479509642@warthog.procyon.org.uk> (raw)
In-Reply-To: <20161118220744.GC31101@dastard>

Dave Chinner <david@fromorbit.com> wrote:

> And when we start thinking in those timeframes, an
> 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;
	};

where the fields are, in effect, additive.  Which means I could represent this
as:

	__s64	stx_atime_s;	/* Last access time */
	__s64	stx_btime_s;	/* File creation time */
	__s64	stx_ctime_s;	/* Last attribute change time */
	__s64	stx_mtime_s;	/* Last data modification time */
	__s32	stx_atime_ns;	/* Last access time (ns part) */
	__s32	stx_btime_ns;	/* File creation time (ns part) */
	__s32	stx_ctime_ns;	/* Last attribute change time (ns part) */
	__s32	stx_mtime_ns;	/* Last data modification time (ns part) */

and then add:

	__s32	stx_atime_fs;	/* Last access time (fs part) */
	__s32	stx_btime_fs;	/* File creation time (fs part) */
	__s32	stx_ctime_fs;	/* Last attribute change time (fs part) */
	__s32	stx_mtime_fs;	/* Last data modification time (fs part) */

later.

If we *really* do want to allow for atto- or femto- second resolution
timestamps (and you've still not entirely convinced me that it's going to be
necessary - the speed of signal propagation still has an ungetroundable
limit), then we could stick the space in now - but I think it's likely to
remain dead space.

Maybe we should switch to Windows-style timestamp resolution:

	struct timestamp {
		__s64 hundred_ns;	/* Time in 100ns increments */
		__s32 femtoseconds;	/* Additional fs component */
	};

> > Using the existing FS_*_FL flags as initial values is not worse than
> > starting with any other arbitrary values for the flags.
>
> Except it starts with a sparse set of flags for no good reason.

Actually, a very good reason.  You can map those flags, on ext4 at least, with
a load, an AND and an OR.  Three instructions[*].  If the bits don't
correspond, it gets more expensive (4-5 instructions per bit + 1).

[*] Leastways, it *should* be three instructions, but gcc fails to optimise it
    correctly.  I have a bz logged for this.

David

  parent reply	other threads:[~2016-11-18 22:54 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-11-17 13:34 [RFC][PATCH 0/4] Enhanced file stat system call David Howells
2016-11-17 13:35 ` [PATCH 1/4] statx: Add a system call to make enhanced file info available David Howells
2016-11-17 18:39   ` Jeff Layton
2016-11-18  2:32     ` Andreas Dilger
2016-11-18  8:59     ` David Howells
2016-11-18  8:59       ` David Howells
2016-11-18  9:25       ` Andreas Dilger
2016-11-18  9:25         ` Andreas Dilger
2016-11-17 23:40   ` Dave Chinner
2016-11-18  3:28     ` Andreas Dilger
2016-11-18 22:07       ` Dave Chinner
2016-11-18 22:54       ` David Howells [this message]
2016-11-19 22:43         ` Dave Chinner
2016-11-21 14:30         ` One Thousand Gnomes
2016-11-21 20:43           ` Dave Chinner
2016-11-22 10:39         ` David Howells
2016-11-22 13:55           ` Jeff Layton
2016-11-22 20:58           ` Dave Chinner
2016-11-18  9:53     ` David Howells
2016-11-18  8:48   ` David Howells
2016-11-18 12:01     ` Jeff Layton
2016-11-18  9:36   ` David Howells
2016-11-18 17:17     ` Jeff Layton
2016-11-18 18:04     ` David Howells
2016-11-18 18:54       ` Jeff Layton
2016-11-18 19:08       ` David Howells
2016-11-18  9:43   ` David Howells
2016-11-18 21:41     ` Dave Chinner
2016-11-18 22:24     ` David Howells
2016-11-18 10:29   ` David Howells
2016-11-18 10:29     ` David Howells
2016-11-18 21:27     ` Dave Chinner
2016-11-18 21:48     ` David Howells
2016-11-18 21:48       ` David Howells
2016-11-18 22:17       ` Dave Chinner
2016-11-18 22:17         ` Dave Chinner
2016-11-19 10:21         ` Michael Kerrisk (man-pages)
2016-11-17 13:35 ` [PATCH 2/4] statx: Ext4: Return enhanced file attributes David Howells
2016-11-18  3:30   ` Andreas Dilger
2016-11-17 13:35 ` [PATCH 3/4] statx: NFS: " David Howells
2016-11-17 13:35 ` [PATCH 4/4] statx: AFS: " David Howells
2016-11-18  3:34   ` Andreas Dilger
2016-11-18  8:47   ` David Howells
2016-11-17 14:39 ` [RFC][PATCH 0/4] Enhanced file stat system call One Thousand Gnomes
2016-11-17 15:10 ` Michael Kerrisk
2016-11-17 16:33 ` David Howells
2016-11-17 16:45 ` David Howells
2016-11-17 20:00   ` J. Bruce Fields
2016-11-18  2:30     ` Andreas Dilger
2016-11-18  4:29       ` NeilBrown
2016-11-18 13:41   ` One Thousand Gnomes
2016-11-18 13:49   ` David Howells

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