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    Subject[PATCH V2 13/16] block, bfq: boost the throughput with random I/O on NCQ-capable HDDs
    Date
    This patch is basically the counterpart, for NCQ-capable rotational
    devices, of the previous patch. Exactly as the previous patch does on
    flash-based devices and for any workload, this patch disables device
    idling on rotational devices, but only for random I/O. In fact, only
    with these queues disabling idling boosts the throughput on
    NCQ-capable rotational devices. To not break service guarantees,
    idling is disabled for NCQ-enabled rotational devices only when the
    same symmetry conditions considered in the previous patches hold.

    Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
    Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
    ---
    block/bfq-iosched.c | 16 ++++++----------
    1 file changed, 6 insertions(+), 10 deletions(-)

    diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
    index ca1ae24..738fd5f 100644
    --- a/block/bfq-iosched.c
    +++ b/block/bfq-iosched.c
    @@ -6423,20 +6423,15 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
    * The next variable takes into account the cases where idling
    * boosts the throughput.
    *
    - * The value of the variable is computed considering that
    - * idling is usually beneficial for the throughput if:
    + * The value of the variable is computed considering, first, that
    + * idling is virtually always beneficial for the throughput if:
    * (a) the device is not NCQ-capable, or
    * (b) regardless of the presence of NCQ, the device is rotational
    - * and the request pattern for bfqq is I/O-bound (possible
    - * throughput losses caused by granting idling to seeky queues
    - * are mitigated by the fact that, in all scenarios where
    - * boosting throughput is the best thing to do, i.e., in all
    - * symmetric scenarios, only a minimal idle time is allowed to
    - * seeky queues).
    + * and the request pattern for bfqq is I/O-bound and sequential.
    *
    * Secondly, and in contrast to the above item (b), idling an
    * NCQ-capable flash-based device would not boost the
    - * throughput even with intense I/O; rather it would lower
    + * throughput even with sequential I/O; rather it would lower
    * the throughput in proportion to how fast the device
    * is. Accordingly, the next variable is true if any of the
    * above conditions (a) and (b) is true, and, in particular,
    @@ -6444,7 +6439,8 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
    * device.
    */
    idling_boosts_thr = !bfqd->hw_tag ||
    - (!blk_queue_nonrot(bfqd->queue) && bfq_bfqq_IO_bound(bfqq));
    + (!blk_queue_nonrot(bfqd->queue) && bfq_bfqq_IO_bound(bfqq) &&
    + bfq_bfqq_idle_window(bfqq));

    /*
    * The value of the next variable,
    --
    2.10.0
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