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/IOSpeedReviewed-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>

Documentation/block/bfq-iosched.txt

@@ -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.