| From c3a84c1f6985798d2d0b11a8265a5f5592ec4b42 Mon Sep 17 00:00:00 2001 |
| From: Sasha Levin <sashal@kernel.org> |
| Date: Fri, 14 Sep 2018 16:23:09 +0200 |
| Subject: blok, bfq: do not plug I/O if all queues are weight-raised |
| |
| From: Paolo Valente <paolo.valente@linaro.org> |
| |
| [ Upstream commit c8765de0adfcaaf4ffb2d951e07444f00ffa9453 ] |
| |
| To reduce latency for interactive and soft real-time applications, bfq |
| privileges the bfq_queues containing the I/O of these |
| applications. These privileged queues, referred-to as weight-raised |
| queues, get a much higher share of the device throughput |
| w.r.t. non-privileged queues. To preserve this higher share, the I/O |
| of any non-weight-raised queue must be plugged whenever a sync |
| weight-raised queue, while being served, remains temporarily empty. To |
| attain this goal, bfq simply plugs any I/O (from any queue), if a sync |
| weight-raised queue remains empty while in service. |
| |
| Unfortunately, this plugging typically lowers throughput with random |
| I/O, on devices with internal queueing (because it reduces the filling |
| level of the internal queues of the device). |
| |
| This commit addresses this issue by restricting the cases where |
| plugging is performed: if a sync weight-raised queue remains empty |
| while in service, then I/O plugging is performed only if some of the |
| active bfq_queues are *not* weight-raised (which is actually the only |
| circumstance where plugging is needed to preserve the higher share of |
| the throughput of weight-raised queues). This restriction proved able |
| to boost throughput in really many use cases needing only maximum |
| throughput. |
| |
| Signed-off-by: Paolo Valente <paolo.valente@linaro.org> |
| Signed-off-by: Jens Axboe <axboe@kernel.dk> |
| Signed-off-by: Sasha Levin <sashal@kernel.org> |
| --- |
| block/bfq-iosched.c | 10 ++++++++-- |
| 1 file changed, 8 insertions(+), 2 deletions(-) |
| |
| diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c |
| index 35ddaa820737c..66b1ebc21ce4f 100644 |
| --- a/block/bfq-iosched.c |
| +++ b/block/bfq-iosched.c |
| @@ -3593,7 +3593,12 @@ static bool bfq_better_to_idle(struct bfq_queue *bfqq) |
| * whether bfqq is being weight-raised, because |
| * bfq_symmetric_scenario() does not take into account also |
| * weight-raised queues (see comments on |
| - * bfq_weights_tree_add()). |
| + * bfq_weights_tree_add()). In particular, if bfqq is being |
| + * weight-raised, it is important to idle only if there are |
| + * other, non-weight-raised queues that may steal throughput |
| + * to bfqq. Actually, we should be even more precise, and |
| + * differentiate between interactive weight raising and |
| + * soft real-time weight raising. |
| * |
| * As a side note, it is worth considering that the above |
| * device-idling countermeasures may however fail in the |
| @@ -3605,7 +3610,8 @@ static bool bfq_better_to_idle(struct bfq_queue *bfqq) |
| * to let requests be served in the desired order until all |
| * the requests already queued in the device have been served. |
| */ |
| - asymmetric_scenario = bfqq->wr_coeff > 1 || |
| + asymmetric_scenario = (bfqq->wr_coeff > 1 && |
| + bfqd->wr_busy_queues < bfqd->busy_queues) || |
| !bfq_symmetric_scenario(bfqd); |
| |
| /* |
| -- |
| 2.20.1 |
| |