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doc, block, bfq: update max IOPS sustainable with BFQ 

We have investigated more deeply the performance of BFQ, in terms of
number of IOPS that can be processed by the CPU when BFQ is used as
I/O scheduler. In more detail, using the script [1], we have measured
the number of IOPS reached on top of a null block device configured
with zero latency, as a function of the workload (sequential read,
sequential write, random read, random write) and of the system (we
considered desktops, laptops and embedded systems).

Basing on the resulting figures, with this commit we update the
current, conservative IOPS range reported in BFQ documentation. In
particular, the documentation now reports, for each of three different
systems, the lowest number of IOPS obtained for that system with the
above test (namely, the value obtained with the workload leading to
the lowest IOPS).

[1] https://github.com/Algodev-github/IOSpeed

Reviewed-by: Lee Tibbert <lee.tibbert@gmail.com>
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>
hifive-unleashed-5.1
Paolo Valente 2017-11-13 07:34:07 +01:00 committed by Jens Axboe
parent 4fc930896d
commit 68017e5d87
1 changed files with 11 additions and 6 deletions
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17
Documentation/block/bfq-iosched.txt
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@ -20,12 +20,17 @@ 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.
On average CPUs, the current version of BFQ can handle devices
performing at most ~30K IOPS; at most ~50 KIOPS on faster CPUs. As a
reference, 30-50 KIOPS correspond to very high bandwidths with
sequential I/O (e.g., 8-12 GB/s if I/O requests are 256 KB large), and
to 120-200 MB/s with 4KB random I/O. BFQ is currently being tested on
multi-queue devices too.
BFQ has a non-null overhead, which limits the maximum IOPS that the
CPU can process for a device scheduled with BFQ. To give an idea of
the limits on slow or average CPUs, here are BFQ limits 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):
- Intel i7-4850HQ: 250 KIOPS
- AMD A8-3850: 170 KIOPS
- ARM CortexTM-A53 Octa-core: 45 KIOPS
BFQ works for multi-queue devices too.
The table of contents follow. Impatients can just jump to Section 3.