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block, bfq: do not overcharge writes in asymmetric scenarios 

Writes tend to starve reads. bfq counters this problem by overcharging
writes with an inflated service w.r.t. the actual service (number of
sector written) they receive.

Yet his overcharging is useless, and actually causes unfairness in the
opposite direction, when bfq happens to be enforcing strong I/O control.
bfq does this enforcing when the scenario is asymmetric, i.e., when some
bfq_queue or group of bfq_queues is to be granted a different bandwidth
than some other bfq_queue or group of bfq_queues. So, in such a
scenario, this commit disables write overcharging.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Paolo Valente 2019-01-29 12:06:37 +01:00 committed by Jens Axboe
parent b3c3498112
commit 02a6d787f4
1 changed files with 2 additions and 1 deletions
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3
block/bfq-iosched.c
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@ -888,7 +888,8 @@ static struct request *bfq_find_next_rq(struct bfq_data *bfqd,
static unsigned long bfq_serv_to_charge(struct request *rq,
struct bfq_queue *bfqq)
{
if (bfq_bfqq_sync(bfqq) || bfqq->wr_coeff > 1)
if (bfq_bfqq_sync(bfqq) || bfqq->wr_coeff > 1 ||
!bfq_symmetric_scenario(bfqq->bfqd))
return blk_rq_sectors(rq);
return blk_rq_sectors(rq) * bfq_async_charge_factor;