linux-kernel - Re: Performance testing of various barrier reduction patches [was: Re: [RFC v4] ext4: Coordinate fsync requests]

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Date:	Fri, 15 Oct 2010 17:02:08 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Ric Wheeler <rwheeler@...hat.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	"Ted Ts'o" <tytso@....edu>, Mingming Cao <cmm@...ibm.com>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Keith Mannthey <kmannth@...ibm.com>,
	Mingming Cao <mcao@...ibm.com>, Tejun Heo <tj@...nel.org>,
	Josef Bacik <josef@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>
Subject: Re: Performance testing of various barrier reduction patches [was:
	Re: [RFC v4] ext4: Coordinate fsync requests]

On Sat, Oct 16, 2010 at 01:40:41AM +0200, Christoph Hellwig wrote:
> On Fri, Oct 15, 2010 at 04:39:04PM -0700, Darrick J. Wong wrote:
> > On Tue, Oct 12, 2010 at 04:14:55PM +0200, Christoph Hellwig wrote:
> > > I still think adding code to every filesystem to optimize for a rather
> > > stupid use case is not a good idea.  I dropped out a bit from the
> > > thread in the middle, but what was the real use case for lots of
> > > concurrent fsyncs on the same inode again?
> > 
> > The use case I'm looking at is concurrent fsyncs on /different/ inodes,
> > actually.  We have _n_ different processes, each writing (and fsyncing) its own
> > separate file on the same filesystem.
> > 
> > iirc, ext4_sync_file is called with the inode mutex held, which prevents
> > concurrent fsyncs on the same inode.
> 
> Indeed.  Although we could drop it at least for the cache flush
> call.  We already do this for block devices.

<nod>

Unfortunately, the patch immediately triggers the BUG at
drivers/scsi/scsi_lib.c:1064:
 /*
  * BLOCK_PC requests may transfer data, in which case they must
  * a bio attached to them.  Or they might contain a SCSI command
  * that does not transfer data, in which case they may optionally
  * submit a request without an attached bio.
  */
 if (req->bio) {
         int ret;

         BUG_ON(!req->nr_phys_segments);

--D
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