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When joining a log transaction we acquire the root's log mutex, then increment the root's log batch and log writers counters while holding the mutex. However we don't need to increment the log batch there, because we are holding the mutex and incremented the log writers counter as well, so any other task trying to sync log will wait for the current task to finish its logging and still achieve the desired log batching. Since the log batch counter is an atomic counter and is incremented twice at the very beginning of the fsync callback (btrfs_sync_file()), once before flushing delalloc and once again after waiting for writeback to complete, eliminating its increment when joining the log transaction may provide some performance gains in case we have multiple concurrent tasks doing fsyncs against different files in the same subvolume, as it reduces contention on the atomic (locking the cacheline and bouncing it). When testing fio with 32 jobs, on a 8 cores VM, doing fsyncs against different files of the same subvolume, on top of a zram device, I could consistently see gains (higher throughput) between 1% to 2%, which is a very low value and possibly hard to be observed with a real device (I couldn't observe consistent gains with my low/mid end NVMe device). So this change is mostly motivated to just simplify the logic, as updating the log batch counter is only relevant when an fsync starts and while not holding the root's log mutex. Reviewed-by: Josef Bacik <josef@toxicpanda.com> Signed-off-by: Filipe Manana <fdmanana@suse.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
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.