block, bfq: stress that low_latency must be off to get max throughput

The introduction of the BFQ and Kyber I/O schedulers has triggered a
new wave of I/O benchmarks. Unfortunately, comments and discussions on
these benchmarks confirm that there is still little awareness that it
is very hard to achieve, at the same time, a low latency and a high
throughput. In particular, virtually all benchmarks measure
throughput, or throughput-related figures of merit, but, for BFQ, they
use the scheduler in its default configuration. This configuration is
geared, instead, toward a low latency. This is evidently a sign that
BFQ documentation is still too unclear on this important aspect. This
commit addresses this issue by stressing how BFQ configuration must be
(easily) changed if the only goal is maximum throughput.

Signed-off-by: Paolo Valente <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

block/bfq-iosched.c

@@ -56,6 +56,11 @@
  * rotational or flash-based devices, and to get the job done quickly
  * for applications consisting in many I/O-bound processes.
  *
+ * NOTE: if the main or only goal, with a given device, is to achieve
+ * the maximum-possible throughput at all times, then do switch off
+ * all low-latency heuristics for that device, by setting low_latency
+ * to 0.
+ *
  * BFQ is described in [1], where also a reference to the initial, more
  * theoretical paper on BFQ can be found. The interested reader can find
  * in the latter paper full details on the main algorithm, as well as