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bfq invokes various blkg_*stats_* functions to update the statistics contained in the special files blkio.bfq.* in the blkio controller groups, i.e., the I/O accounting related to the proportional-share policy provided by bfq. The execution of these functions takes a considerable percentage, about 40%, of the total per-request execution time of bfq (i.e., of the sum of the execution time of all the bfq functions that have to be executed to process an I/O request from its creation to its destruction). This reduces the request-processing rate sustainable by bfq noticeably, even on a multicore CPU. In fact, the bfq functions that invoke blkg_*stats_* functions cannot be executed in parallel with the rest of the code of bfq, because both are executed under the same same per-device scheduler lock. To reduce this slowdown, this commit moves, wherever possible, the invocation of these functions (more precisely, of the bfq functions that invoke blkg_*stats_* functions) outside the critical sections protected by the scheduler lock. With this change, and with all blkio.bfq.* statistics enabled, the throughput grows, e.g., from 250 to 310 KIOPS (+25%) on an Intel i7-4850HQ, in case of 8 threads doing random I/O in parallel on null_blk, with the latter configured with 0 latency. We obtained the same or higher throughput boosts, up to +30%, with other processors (some figures are reported in the documentation). For our tests, we used the script [1], with which our results can be easily reproduced. NOTE. This commit still protects the invocation of blkg_*stats_* functions with the request_queue lock, because the group these functions are invoked on may otherwise disappear before or while these functions are executed. Fortunately, tests without even this lock show, by difference, that the serialization caused by this lock has a little impact (at most ~5% of throughput reduction). [1] https://github.com/Algodev-github/IOSpeed Tested-by: Lee Tibbert <lee.tibbert@gmail.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Luca Miccio <lucmiccio@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.