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Because this happens at high thread counts on high IOPS devices
doing mixed read/write AIO-DIO to a single file at about a million
iops:
64.09% 0.21% [kernel] [k] io_submit_one
- 63.87% io_submit_one
- 44.33% aio_write
- 42.70% xfs_file_write_iter
- 41.32% xfs_file_dio_write_aligned
- 25.51% xfs_file_write_checks
- 21.60% _raw_spin_lock
- 21.59% do_raw_spin_lock
- 19.70% __pv_queued_spin_lock_slowpath
This also happens of the IO completion IO path:
22.89% 0.69% [kernel] [k] xfs_dio_write_end_io
- 22.49% xfs_dio_write_end_io
- 21.79% _raw_spin_lock
- 20.97% do_raw_spin_lock
- 20.10% __pv_queued_spin_lock_slowpath
IOWs, fio is burning ~14 whole CPUs on this spin lock.
So, do an unlocked check against inode size first, then if we are
at/beyond EOF, take the spinlock and recheck. This makes the
spinlock disappear from the overwrite fastpath.
I'd like to report that fixing this makes things go faster. It
doesn't - it just exposes the the XFS_ILOCK as the next severe
contention point doing extent mapping lookups, and that now burns
all the 14 CPUs this spinlock was burning.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.