Re: aio: bump i_count instead of using igrab

[Nick Piggin]

On Mon, Aug 23, 2010 at 11:00:23AM -0400, Chris Mason wrote:
> On Mon, Aug 23, 2010 at 10:50:31AM -0400, Christoph Hellwig wrote:
> > On Mon, Aug 23, 2010 at 10:47:55AM -0400, Chris Mason wrote:
> > > The aio batching code is using igrab to get an extra reference on the
> > > inode so it can safely batch.  igrab will go ahead and take the global
> > > inode spinlock, which can be a bottleneck on large machines doing lots
> > > of AIO.
> > > 
> > > In this case, igrab isn't required because we already have a reference
> > > on the file handle.  It is safe to just bump the i_count directly
> > > on the inode.
> > > 
> > > Benchmarking shows this patch brings IOP/s on tons of flash up by about
> > > 2.5X.
> > 
> > There's some places in XFS where we do the same, and it showed up as a
> > bottle neck before.  Instead of open coding the increment we have
> > a wrapper that includes and assert that the numbers is always positive.
> > 
> > I think we really want a proper helper for general use instead of
> > completly opencoding it.

Yeah igrab isn't nice. Several places are calling it without checking
return code too, which means they are either buggy or should be using
something else.


> Nick, this is about a 1 liner to fs/aio.c replacing igrab with
> atomic_inc directly on the inode reference count.
> 
> I know your scalability tree gets rid of the global, but in this case I
> think it still makes sense to avoid the locking completely when the
> caller knows it is safe.  Do you already have something similar hiding
> in the scalability tree?

Yes of course :) It has just simple refcounting increment helper that
can do some of the assertions.

I am hoping to be able to post the inode stuff soonish and hopefully
get it into Al's tree. It will pop up on fsdevel.


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