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From:   Paolo Valente <paolo.valente@linaro.org>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        ulf.hansson@linaro.org, linus.walleij@linaro.org,
        broonie@kernel.org, bfq-iosched@googlegroups.com,
        oleksandr@natalenko.name, mancha@tower-research.com,
        Paolo Valente <paolo.valente@linaro.org>
Subject: [PATCH BUGFIX IMPROVEMENT 11/14] block, bfq: reduce threshold for detecting command queueing
Date:   Tue, 29 Jan 2019 12:06:35 +0100
Message-Id: <20190129110638.12652-12-paolo.valente@linaro.org>
In-Reply-To: <20190129110638.12652-1-paolo.valente@linaro.org>
References: <20190129110638.12652-1-paolo.valente@linaro.org>
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bfq borrowed from cfq a simple heuristic for detecting whether the
drive performs command queueing: check whether the average number of
in-flight requests is above a given threshold. Unfortunately this
heuristic does fail to detect queueing (on drives with queueing) if
processes doing I/O are few and issue I/O with a low depth.

To reduce false negatives, this commit lowers the threshold.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
---
 block/bfq-iosched.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index bf585ad29bb5..48b579032d14 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -230,7 +230,7 @@ static struct kmem_cache *bfq_pool;
 #define BFQ_MIN_TT		(2 * NSEC_PER_MSEC)
 
 /* hw_tag detection: parallel requests threshold and min samples needed. */
-#define BFQ_HW_QUEUE_THRESHOLD	4
+#define BFQ_HW_QUEUE_THRESHOLD	3
 #define BFQ_HW_QUEUE_SAMPLES	32
 
 #define BFQQ_SEEK_THR		(sector_t)(8 * 100)
@@ -4798,7 +4798,7 @@ static void bfq_update_hw_tag(struct bfq_data *bfqd)
 	 * sum is not exact, as it's not taking into account deactivated
 	 * requests.
 	 */
-	if (bfqd->rq_in_driver + bfqd->queued < BFQ_HW_QUEUE_THRESHOLD)
+	if (bfqd->rq_in_driver + bfqd->queued <= BFQ_HW_QUEUE_THRESHOLD)
 		return;
 
 	if (bfqd->hw_tag_samples++ < BFQ_HW_QUEUE_SAMPLES)
-- 
2.20.1