linux-ext4 - Re: [PATCH 3/3] ext4: Pass DIO_SKIP_DIO_COUNT to dax_do

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Date:	Thu, 9 Jun 2016 02:02:26 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Toshimitsu Kani <toshi.kani@....com>
Subject: Re: [PATCH 3/3] ext4: Pass DIO_SKIP_DIO_COUNT to dax_do_io

On Fri, Jun 03, 2016 at 06:28:17PM -0400, Waiman Long wrote:
> Since all the DAX I/Os are synchronous, there is no need to update
> the DIO count in dax_do_io() when the count has already been updated
> or the i_rwsem lock (read or write) has or will be taken.
> 
> This patch passes in the DIO_SKIP_DIO_COUNT flag to dax_do_io() to
> disable two unneeded atomic operations that can slow thing down in
> fast storages like NVDIMM.
> 
> With a 38-threads fio I/O test with 2 shared files (on DAX-mount ext4
> formatted NVDIMM) running on a 4-socket Haswell-EX server with 4.6-rc1
> kernel, the aggregated bandwidths before and after the patch were:

Please do the right thing and remove the code to call inode_dio_begin /
inode_dio_end entirely.  There is nothing ext4 specific about the dax
code being synchronous.  Together with my previous suggestion
that also allows dropping the flags argument.

Then as a next step remove the end_io argument and just call it in
the callers which is perfectly safe again as dax is synchronous.
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