linux-ext4 - Re: page fault scalability (ext3, ext4, xfs)

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Date:	Wed, 14 Aug 2013 23:28:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Lang <david@...g.hm>
Cc:	Dave Chinner <david@...morbit.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Dave Hansen <dave.hansen@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	xfs@....sgi.com,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: page fault scalability (ext3, ext4, xfs)

On Wed, Aug 14, 2013 at 11:18 PM, David Lang <david@...g.hm> wrote:
> On Wed, 14 Aug 2013, Andy Lutomirski wrote:
>
>>> The big problem with this approach is that not doing the
>>> timestamp update on page faults is going to break the inode change
>>> version counting because for ext4, btrfs and XFS it takes a
>>> transaction to bump that counter. NFS needs to know the moment a
>>> file is changed in memory, not when it is written to disk. Also, NFS
>>> requires the change to the counter to be persistent over server
>>> failures, so it needs to be changed as part of a transaction....
>>
>>
>> NFS can do whatever it wants, although I suspect that even NFS can get
>> away with deferring cmtime updates.
>
>
> NFS already has to do syncs to make sure the data is safe on disk, have a
> flag that NFS can use to make the ctime safe, everyone else can get the
> performance improvement and NFS can have it's slow-but-safe approach.
>

I don't see the current code that updates times for NFS.  I'm not
planning on making any changes that'll affect NFS at all (i.e. I don't
think any flag will be needed), but I'd be more confident if I
understand why it worked in the first place.

(For filesystems that provide page_mkwrite, there hasn't been a
file_update_time call in the core code for several kernel versions.)

--Andy
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