linux-kernel - Re: aio: bump i_count instead of using igrab

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Date:	Mon, 23 Aug 2010 17:18:41 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Chris Mason <chris.mason@...cle.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: aio: bump i_count instead of using igrab

Christoph Hellwig <hch@...radead.org> writes:

> 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.

Well, it would make detecting races or invalid assumptions a little
easier.  If Chris wants to code that up, that's fine with me.  Honestly,
though, I don't think it's necessary.

I've gone through the alloc/free paths for the inode and I'm convinced
this is safe.  I'm happy with this version of the patch.

Reviewed-by: Jeff Moyer <jmoyer@...hat.com>
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