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doc, block, bfq: add information on bfq execution time

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The execution time of BFQ has been slightly lowered. Report the new
execution time in BFQ documentation.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Paolo Valente 2019-03-12 09:59:35 +01:00 committed by Jens Axboe
parent fffca087d5
commit 4438cf50e7
1 changed files with 22 additions and 7 deletions
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29
Documentation/block/bfq-iosched.txt
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@ -20,13 +20,26 @@ for that device, by setting low_latency to 0. See Section 3 for
details on how to configure BFQ for the desired tradeoff between
latency and throughput, or on how to maximize throughput.
BFQ has a non-null overhead, which limits the maximum IOPS that a CPU
can process for a device scheduled with BFQ. To give an idea of the
limits on slow or average CPUs, here are, first, the limits of BFQ for
three different CPUs, on, respectively, an average laptop, an old
desktop, and a cheap embedded system, in case full hierarchical
support is enabled (i.e., CONFIG_BFQ_GROUP_IOSCHED is set), but
CONFIG_DEBUG_BLK_CGROUP is not set (Section 4-2):
As every I/O scheduler, BFQ adds some overhead to per-I/O-request
processing. To give an idea of this overhead, the total,
single-lock-protected, per-request processing time of BFQ---i.e., the
sum of the execution times of the request insertion, dispatch and
completion hooks---is, e.g., 1.9 us on an Intel Core i7-2760QM@2.40GHz
(dated CPU for notebooks; time measured with simple code
instrumentation, and using the throughput-sync.sh script of the S
suite [1], in performance-profiling mode). To put this result into
context, the total, single-lock-protected, per-request execution time
of the lightest I/O scheduler available in blk-mq, mq-deadline, is 0.7
us (mq-deadline is ~800 LOC, against ~10500 LOC for BFQ).
Scheduling overhead further limits the maximum IOPS that a CPU can
process (already limited by the execution of the rest of the I/O
stack). To give an idea of the limits with BFQ, on slow or average
CPUs, here are, first, the limits of BFQ for three different CPUs, on,
respectively, an average laptop, an old desktop, and a cheap embedded
system, in case full hierarchical support is enabled (i.e.,
CONFIG_BFQ_GROUP_IOSCHED is set), but CONFIG_DEBUG_BLK_CGROUP is not
set (Section 4-2):
- Intel i7-4850HQ: 400 KIOPS
- AMD A8-3850: 250 KIOPS
- ARM CortexTM-A53 Octa-core: 80 KIOPS
@ -566,3 +579,5 @@ applications. Unset this tunable if you need/want to control weights.
Slightly extended version:
http://algogroup.unimore.it/people/paolo/disk_sched/bfq-v1-suite-
results.pdf
[3] https://github.com/Algodev-github/S