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The round-robin path selector is inefficient in cases where there is a difference in latency between paths. In the presence of one or more high latency paths the round-robin selector continues to use the high latency path equally. This results in a bias towards the highest latency path and can cause a significant decrease in overall performance as IOs pile on the highest latency path. This problem is acute with NVMe-oF controllers. The queue-depth path selector sends I/O down the path with the lowest number of requests in its request queue. Paths with lower latency will clear requests more quickly and have less requests queued compared to higher latency paths. The goal of this path selector is to make more use of lower latency paths which will bring down overall IO latency and increase throughput and performance. Signed-off-by: Thomas Song <tsong@purestorage.com> [emilne: commandeered patch developed by Thomas Song @ Pure Storage] Co-developed-by: Ewan D. Milne <emilne@redhat.com> Signed-off-by: Ewan D. Milne <emilne@redhat.com> Co-developed-by: John Meneghini <jmeneghi@redhat.com> Signed-off-by: John Meneghini <jmeneghi@redhat.com> Link: https://lore.kernel.org/linux-nvme/20240509202929.831680-1-jmeneghi@redhat.com/ Tested-by: Marco Patalano <mpatalan@redhat.com> Tested-by: Jyoti Rani <jrani@purestorage.com> Tested-by: John Meneghini <jmeneghi@redhat.com> Reviewed-by: Randy Jennings <randyj@purestorage.com> Reviewed-by: Hannes Reinecke <hare@suse.de> Reviewed-by: Sagi Grimberg <sagi@grimberg.me> Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Keith Busch <kbusch@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 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.