Message-Id: <20130115163359.16d64ab4.akpm@linux-foundation.org>
Date:	Tue, 15 Jan 2013 16:33:59 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	axboe@...nel.dk, lucho@...kov.net, jack@...e.cz, ericvh@...il.com,
	tytso@....edu, viro@...iv.linux.org.uk, rminnich@...dia.gov,
	martin.petersen@...cle.com, neilb@...e.de, david@...morbit.com,
	gnehzuil.liu@...il.com, linux-kernel@...r.kernel.org,
	hch@...radead.org, linux-fsdevel@...r.kernel.org,
	adilger.kernel@...ger.ca, bharrosh@...asas.com, jlayton@...ba.org,
	linux-ext4@...r.kernel.org, hirofumi@...l.parknet.co.jp
Subject: Re: [PATCH v2.4 0/3] mm/fs: Remove unnecessary waiting for stable
 pages

On Tue, 15 Jan 2013 16:22:46 -0800
"Darrick J. Wong" <darrick.wong@...cle.com> wrote:

> > > This patchset has been tested on 3.8.0-rc3 on x64 with ext3, ext4, and xfs.
> > > What does everyone think about queueing this for 3.9?
> > 
> > This patchset lacks any performance testing results.
> 
> On my setup (various consumer SSDs and spinny disks, none of which support
> T10DIF) I see that the maximum write latency with these patches applied is
> about half of what it is without the patches.  But don't take my word for it;
> Andy Lutomirski[1] says that his soft-rt latency-sensitive programs no longer
> freak out when he applies the patch set.  Afaik, Google and Taobao run custom
> kernels with all this turned off, so they should see similar latency
> improvements too.
> 
> Obviously, I see no difference on the DIF disk.

We're talking 2001 here ;) Try leaping into your retro time machine and
run dbench on ext2 on a spinny disk and I expect you'll see significant
performance changes.

The problem back in 2001 was that we held lock_page() across the
duration of page writeback, so if another thread came in and tried to
dirty the page, it would block on lock_page() until IO completion.  I
can't remember whether writeback would also block read().  Maybe it did,
in which case the effects of this patchset won't be as dramatic as were
the effects of splitting PG_lock into PG_lock and PG_writeback.

> > For clarity's sake, please provide a description of which filesystems
> > (and under which circumstances) will block behind writeback when
> > userspace is attempting to dirty a page.  Both before and, particularly,
> > after this patchset.  IOW, did everything get fixed?
> 
> Heh, this is complicated.
> 
> Before this patchset, all filesystems would block, regardless of whether or not
> it was necessary.  ext3 would wait, but still generate occasional checksum
> errors.  The network filesystems were left to do their own thing, so they'd
> wait too.
> 
> After this patchset, all the disk filesystems except ext3 and btrfs will wait
> only if the hardware requires it.  ext3 (if necessary) snapshots pages instead
> of blocking, and btrfs provides its own bdi so the mm will never wait.  Network
> filesystems haven't been touched, so either they provide their own wait code,
> or they don't block at all.  The blocking behavior is back to what it was
> before 3.0 if you don't have a disk requiring stable page writes.
> 
> (I will reconfirm this statement before sending out the next iteration.)
> 
> I will of course add all of this to the cover message.

OK, thanks, that sounds reasonable.

Do we generate nice kernel messages (at mount or device-probe time)
which will permit people to work out which strategy their device/fs is
using?
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