linux-kernel - Re: [RFC PATCH-tip 6/6] xfs: Enable reader optimistic spinning for DAX inodes

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Date:	Tue, 14 Jun 2016 11:24:23 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, linux-alpha@...r.kernel.org,
	linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-arch@...r.kernel.org, xfs@....sgi.com,
	Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip 6/6] xfs: Enable reader optimistic spinning for
 DAX inodes

On Tue, Jun 14, 2016 at 02:12:39PM -0400, Waiman Long wrote:
> This patch enables reader optimistic spinning for inodes that are
> under a DAX-based mount point.
> 
> On a 4-socket Haswell machine running on a 4.7-rc1 tip-based kernel,
> the fio test with multithreaded randrw and randwrite tests on the
> same file on a XFS partition on top of a NVDIMM with DAX were run,
> the aggregated bandwidths before and after the patch were as follows:

And why is this specific to DAX?  Many I/O operations already never
got out to disk, and ilock is mostly held for operations that have
nothing to do with disk I/O.