Re: aio: bump i_count instead of using igrab

Chris Mason
2010-08-23T15:01:28Z

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.
>

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?

-chris