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bd11b3a391
Hrvoje reports that when a large fixed buffer is registered and IO is being done to the latter pages of said buffer, the IO submission time is much worse: reading to the start of the buffer: 11238 ns reading to the end of the buffer: 1039879 ns In fact, it's worse by two orders of magnitude. The reason for that is how io_uring figures out how to setup the iov_iter. We point the iter at the first bvec, and then use iov_iter_advance() to fast-forward to the offset within that buffer we need. However, that is abysmally slow, as it entails iterating the bvecs that we setup as part of buffer registration. There's really no need to use this generic helper, as we know it's a BVEC type iterator, and we also know that each bvec is PAGE_SIZE in size, apart from possibly the first and last. Hence we can just use a shift on the offset to find the right index, and then adjust the iov_iter appropriately. After this fix, the timings are: reading to the start of the buffer: 10135 ns reading to the end of the buffer: 1377 ns Or about an 755x improvement for the tail page. Reported-by: Hrvoje Zeba <zeba.hrvoje@gmail.com> Tested-by: Hrvoje Zeba <zeba.hrvoje@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk> |
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| fs | ||
| include | ||
| init | ||
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| kernel | ||
| lib | ||
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| net | ||
| samples | ||
| scripts | ||
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| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
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| Kbuild | ||
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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 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.