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A mirror of the official Linux kernel repository just in case
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The AIL pushing code spends a huge amount of time skipping over items that are already marked as flushing. It is not uncommon to see hundreds of thousands of items skipped every second due to inode clustering marking all the inodes in a cluster as flushing when the first one is flushed. However, to discover an item is already flushing and should be skipped we have to call the iop_push() method for it to try to flush the item. For inodes (where this matters most), we have to first check that inode is flushable first. We can optimise this overhead away by tracking whether the log item is flushing internally. This allows xfsaild_push() to check the log item directly for flushing state and immediately skip the log item. Whilst this doesn't remove the CPU cache misses for loading the log item, it does avoid the overhead of an indirect function call and the cache misses involved in accessing inode and backing cluster buffer structures to determine flushing state. When trying to flush hundreds of thousands of inodes each second, this CPU overhead saving adds up quickly. It's so noticeable that the biggest issue with pushing on the AIL on fast storage becomes the 10ms back-off wait when we hit enough pinned buffers to break out of the push loop but not enough for the AIL pushing to be considered stuck. This limits the xfsaild to about 70% total CPU usage, and on fast storage this isn't enough to keep the storage 100% busy. The xfsaild will block on IO submission on slow storage and so is self throttling - it does not need a backoff in the case where we are really just breaking out of the walk to submit the IO we have gathered. Further with no backoff we don't need to gather huge delwri lists to mitigate the impact of backoffs, so we can submit IO more frequently and reduce the time log items spend in flushing state by breaking out of the item push loop once we've gathered enough IO to batch submission effectively. Signed-off-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Darrick J. Wong <djwong@kernel.org> Signed-off-by: Chandan Babu R <chandanbabu@kernel.org> |
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| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
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| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
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| .editorconfig | ||
| .get_maintainer.ignore | ||
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| .mailmap | ||
| .rustfmt.toml | ||
| COPYING | ||
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| Kbuild | ||
| Kconfig | ||
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| README | ||
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 reStructuredText 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.