Re: [PATCH 2/5] xfs: open code inc_inode_iversion when logging an inode

[Ben Myers <bpm@sgi.com>]

On Mon, Nov 18, 2013 at 03:54:24PM -0600, Eric Sandeen wrote:
> On 10/31/13, 11:27 PM, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > 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.
> 
> Well, ok.  Maybe worth a note about why the unlocked read is 99.9999% ok...
> 
> Reviewed-by: Eric Sandeen <sandeen@redhat.com>

Ah, sorry Eric, I didn't realize you were still reviewing this guy.  I pulled
him in a bit earlier in the day.

Thanks,
	Ben

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