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Subject[PATCH v2 4/4] block, bfq: consider request size in bfq_asymmetric_scenario()
Date
There is a special case when bfq do not need to idle when more than
one groups is active:

1) all active queues have the same weight,
2) all active queues have the same request size.
3) all active queues belong to the same I/O-priority class,

Each time a request is dispatched, bfq can switch in service queue
safely, since the throughput of each active queue is guaranteed to
be equivalent.

Test procedure:
run "fio -numjobs=1 -ioengine=psync -bs=4k -direct=1 -rw=randread..." in
different cgroup(not root).

Test result: total bandwidth(Mib/s)
| total jobs | before this patch | after this patch |
| ---------- | ----------------- | --------------------- |
| 1 | 33.8 | 33.8 |
| 2 | 33.8 | 65.4 (32.7 each job) |
| 4 | 33.8 | 106.8 (26.7 each job) |
| 8 | 33.8 | 126.4 (15.8 each job) |

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
block/bfq-iosched.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 7df3fc0ef4ef..e5a07bd1fd84 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -268,6 +268,16 @@ static struct kmem_cache *bfq_pool;
*/
#define BFQ_RATE_SHIFT 16

+/*
+ * 1) bfq keep dispatching requests with same size for at least one second.
+ * 2) bfq dispatch at lease 1024 requests
+ *
+ * We think bfq are dispatching request with same size if the above two
+ * conditions hold true.
+ */
+#define VARIED_REQUEST_SIZE(bfqd) ((bfqd)->dispatch_count < 1024 ||\
+ time_before(jiffies, (bfqd)->dispatch_time + HZ))
+
/*
* When configured for computing the duration of the weight-raising
* for interactive queues automatically (see the comments at the
@@ -724,7 +734,8 @@ static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
bool multiple_classes_busy;

#ifdef CONFIG_BFQ_GROUP_IOSCHED
- if (bfqd->num_groups_with_pending_reqs > 1)
+ if (bfqd->num_groups_with_pending_reqs > 1 &&
+ VARIED_REQUEST_SIZE(bfqd))
return true;

if (bfqd->num_groups_with_pending_reqs &&
--
2.31.1
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