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If the in-service bfq_queue is sync and remains temporarily idle, then I/O dispatching (from other queues) may be plugged. It may be dome for two reasons: either to boost throughput, or to preserve the bandwidth share of the in-service queue. In the first case, if the I/O of the in-service queue, when it finally arrives, consists only of one small I/O request, then it makes sense to plug even the I/O of the in-service queue. In fact, serving such a small request immediately is likely to lower throughput instead of boosting it, whereas waiting a little bit is likely to let that request grow, thanks to request merging, and become more profitable in terms of throughput (this is likely to happen exactly because the I/O of the queue has been detected to boost throughput). On the opposite end, if I/O dispatching is being plugged only to preserve the bandwidth of the in-service queue, then it would be better not to plug also the I/O of the in-service queue, because such a plugging is likely to cause only loss of bandwidth for the queue. Unfortunately, no distinction is made between the two cases, and the I/O of the in-service queue is always plugged in case just a small I/O request arrives. This commit draws this missing distinction and does not perform harmful plugging. Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
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.