Re: [PATCH V6 2/8] block, bfq: forbid stable merging of queues associated with different actuators

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 11/4/22 01:26, Paolo Valente wrote:
> If queues associated with different actuators are merged, then control
> is lost on each actuator. Therefore some actuator may be
> underutilized, and throughput may decrease. This problem cannot occur
> with basic queue merging, because the latter is triggered by spatial
> locality, and sectors for different actuators are not close to each
> other. Yet it may happen with stable merging. To address this issue,
> this commit prevents stable merging from occurring among queues
> associated with different actuators.
> 
> Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
> ---
>  block/bfq-iosched.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 5c69394bbb65..ec4b0e70265f 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -5705,9 +5705,13 @@ static struct bfq_queue *bfq_do_or_sched_stable_merge(struct bfq_data *bfqd,
>  	 * it has been set already, but too long ago, then move it
>  	 * forward to bfqq. Finally, move also if bfqq belongs to a
>  	 * different group than last_bfqq_created, or if bfqq has a
> -	 * different ioprio or ioprio_class. If none of these
> -	 * conditions holds true, then try an early stable merge or
> -	 * schedule a delayed stable merge.
> +	 * different ioprio, ioprio_class or actuator_idx. If none of
> +	 * these conditions holds true, then try an early stable merge
> +	 * or schedule a delayed stable merge. As for the condition on
> +	 * actuator_idx, the reason is that, if queues associated with
> +	 * different actuators are merged, then control is lost on
> +	 * each actuator. Therefore some actuator may be
> +	 * underutilized, and throughput may decrease.
>  	 *
>  	 * A delayed merge is scheduled (instead of performing an
>  	 * early merge), in case bfqq might soon prove to be more
> @@ -5725,7 +5729,8 @@ static struct bfq_queue *bfq_do_or_sched_stable_merge(struct bfq_data *bfqd,
>  			bfqq->creation_time) ||
>  		bfqq->entity.parent != last_bfqq_created->entity.parent ||
>  		bfqq->ioprio != last_bfqq_created->ioprio ||
> -		bfqq->ioprio_class != last_bfqq_created->ioprio_class)
> +		bfqq->ioprio_class != last_bfqq_created->ioprio_class ||
> +		bfqq->actuator_idx != last_bfqq_created->actuator_idx)
>  		*source_bfqq = bfqq;
>  	else if (time_after_eq(last_bfqq_created->creation_time +
>  				 bfqd->bfq_burst_interval,

Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>

-- 
Damien Le Moal
Western Digital Research