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.