Re: aio: bump i_count instead of using igrab

[Christoph Hellwig]

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.

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