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Re: [PATCH 4/4] xfs: open code inc_inode_iversion when logging an inode

To: Eric Sandeen <sandeen@xxxxxxxxxxx>, Jean Noel Cordenner <jean-noel.cordenner@xxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 4/4] xfs: open code inc_inode_iversion when logging an inode
From: Ben Myers <bpm@xxxxxxx>
Date: Mon, 30 Sep 2013 17:39:46 -0500
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <5249FA36.1070609@xxxxxxxxxxx>
References: <1380497826-13474-1-git-send-email-david@xxxxxxxxxxxxx> <1380497826-13474-5-git-send-email-david@xxxxxxxxxxxxx> <5249FA36.1070609@xxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
Hi Gents,

On Mon, Sep 30, 2013 at 05:24:54PM -0500, Eric Sandeen wrote:
> On 9/29/13 6:37 PM, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@xxxxxxxxxx>
> > 
> > Michael L Semon reported that generic/069 runtime increased on v5
> > superblocks by 100% compared to v4 superblocks. his perf-based
> > analysis pointed directly at the timestamp updates being done by the
> > write path in this workload. The append writers are doing 4-byte
> > writes, so there are lots of timestamp updates occurring.
> > 
> > The thing is, they aren't being triggered by timestamp changes -
> > they are being triggered by the inode change counter needing to be
> > updated. That is, every write(2) system call needs to bump the inode
> > version count, and it does that through the timestamp update
> > mechanism. Hence for v5 filesystems, test generic/069 is running 3
> > orders of magnitude more timestmap update transactions on v5
> > filesystems due to the fact it does a huge number of *4 byte*
> > write(2) calls.
> > 
> > This isn't a real world scenario we really need to address - anyone
> > doing such sequential IO should be using fwrite(3), not write(2).
> > i.e. fwrite(3) buffers the writes in userspace to minimise the
> > number of write(2) syscalls, and the problem goes away.
> > 
> > However, there is a small change we can make to improve the
> > situation - removing the expensive lock operation on the change
> > counter update.  All inode version counter changes in XFS occur
> > under the ip->i_ilock during a transaction, and therefore we
> > don't actually need the spin lock that provides exclusive access to
> > it through inc_inode_iversion().
> > 
> > Hence avoid the lock and just open code the increment ourselves when
> > logging the inode.
> > 
> > Reported-by: Michael L. Semon <mlsemon35@xxxxxxxxx>
> > Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> > ---
> >  fs/xfs/xfs_trans_inode.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_trans_inode.c b/fs/xfs/xfs_trans_inode.c
> > index 53dfe46..e6601c1 100644
> > --- a/fs/xfs/xfs_trans_inode.c
> > +++ b/fs/xfs/xfs_trans_inode.c
> > @@ -118,8 +118,7 @@ xfs_trans_log_inode(
> >      */
> >     if (!(ip->i_itemp->ili_item.li_desc->lid_flags & XFS_LID_DIRTY) &&
> >         IS_I_VERSION(VFS_I(ip))) {
> > -           inode_inc_iversion(VFS_I(ip));
> > -           ip->i_d.di_changecount = VFS_I(ip)->i_version;
> 
> comment about the reason for the open-code might be good, too?
> 
> otherwise some semantic patcher might "fix" it for you again later...
> 
> -Eric
> 
> > +           ip->i_d.di_changecount = ++VFS_I(ip)->i_version;
> >             flags |= XFS_ILOG_CORE;
> >     }
> >  
> > 

Adding a comment strikes me as a good idea too... But isn't that lock there for
a reason?  I suspect that will break i_version like i_size on 32 bit systems.
Jean added this function, hopefully he can shed some light.

Thanks,
        Ben

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