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docs: block/bfq: describe per-device weight 

The functionality of setting per-device weight for BFQ was added
in v5.4 (commit 795fe54c2a), but the documentation was never
updated.

While at it, improve formatting a bit.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Link: https://lore.kernel.org/r/20210614214109.207430-1-kolyshkin@gmail.com
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Kir Kolyshkin 2021-06-14 14:41:09 -07:00 committed by Jens Axboe
parent a72c374f97
commit fda0b5ba9d
1 changed files with 27 additions and 11 deletions
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38
Documentation/block/bfq-iosched.rst
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@ -553,20 +553,36 @@ throughput sustainable with bfq, because updating the blkio.bfq.*
stats is rather costly, especially for some of the stats enabled by
CONFIG_BFQ_CGROUP_DEBUG.
Parameters to set
-----------------
Parameters
----------
For each group, there is only the following parameter to set.
For each group, the following parameters can be set:
weight (namely blkio.bfq.weight or io.bfq-weight): the weight of the
group inside its parent. Available values: 1..1000 (default 100). The
linear mapping between ioprio and weights, described at the beginning
of the tunable section, is still valid, but all weights higher than
IOPRIO_BE_NR*10 are mapped to ioprio 0.
weight
This specifies the default weight for the cgroup inside its parent.
Available values: 1..1000 (default: 100).
Recall that, if low-latency is set, then BFQ automatically raises the
weight of the queues associated with interactive and soft real-time
applications. Unset this tunable if you need/want to control weights.
For cgroup v1, it is set by writing the value to `blkio.bfq.weight`.
For cgroup v2, it is set by writing the value to `io.bfq.weight`.
(with an optional prefix of `default` and a space).
The linear mapping between ioprio and weights, described at the beginning
of the tunable section, is still valid, but all weights higher than
IOPRIO_BE_NR*10 are mapped to ioprio 0.
Recall that, if low-latency is set, then BFQ automatically raises the
weight of the queues associated with interactive and soft real-time
applications. Unset this tunable if you need/want to control weights.
weight_device
This specifies a per-device weight for the cgroup. The syntax is
`minor:major weight`. A weight of `0` may be used to reset to the default
weight.
For cgroup v1, it is set by writing the value to `blkio.bfq.weight_device`.
For cgroup v2, the file name is `io.bfq.weight`.
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