Fork of alistair23 Linux kernel for reMarkable from https://github.com/alistair23/linux
f0ba5ea2fe
If two processes do I/O close to each other, i.e., are cooperating processes in BFQ (and CFQ'S) nomenclature, then BFQ merges their associated bfq_queues, so as to get sequential I/O from the union of the I/O requests of the processes, and thus reach a higher throughput. A merged queue is then split if its I/O stops being sequential. In this respect, BFQ deems the I/O of a bfq_queue as (mostly) sequential only if less than 4 I/O requests are random, out of the last 32 requests inserted into the queue. Unfortunately, extensive testing (with the interleaved_io benchmark of the S suite [1], and with real applications spawning cooperating processes) has clearly shown that, with such a low threshold, only a rather low I/O throughput may be reached when several cooperating processes do I/O. In particular, the outcome of each test run was bimodal: if queue merging occurred and was stable during the test, then the throughput was close to the peak rate of the storage device, otherwise the throughput was arbitrarily low (usually around 1/10 of the peak rate with a rotational device). The probability to get the unlucky outcomes grew with the number of cooperating processes: it was already significant with 5 processes, and close to one with 7 or more processes. The cause of the low throughput in the unlucky runs was that the merged queues containing the I/O of these cooperating processes were soon split, because they contained more random I/O requests than those tolerated by the 4/32 threshold, but - that I/O would have however allowed the storage device to reach peak throughput or almost peak throughput; - in contrast, the I/O of these processes, if served individually (from separate queues) yielded a rather low throughput. So we repeated our tests with increasing values of the threshold, until we found the minimum value (19) for which we obtained maximum throughput, reliably, with at least up to 9 cooperating processes. Then we checked that the use of that higher threshold value did not cause any regression for any other benchmark in the suite [1]. This commit raises the threshold to such a higher value. [1] https://github.com/Algodev-github/S Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk> |
||
|---|---|---|
| Documentation | ||
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| drivers | ||
| firmware | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
README
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.