From: Chunguang Xu <brookxu@xxxxxxxxxxx>
The IO depth of queues belong to CLASS_IDLE is limited to 1,
so that it can avoid introducing a larger tail latency under
a device with a larger IO depth. Although limiting the IO
depth may reduce the performance of idle_class, it is
generally not a big problem, because idle_class usually does
not have strict performance requirements.
Signed-off-by: Chunguang Xu <brookxu@xxxxxxxxxxx>
---
block/bfq-iosched.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index ec482e6..29940ec 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -4808,6 +4808,17 @@ static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
if (!bfqq)
goto exit;
+ /*
+ * Here, the IO depth of queues belong to CLASS_IDLE is limited
+ * to 1, so that it can avoid introducing a larger tail latency
+ * under a device with a larger IO depth. Although limiting the
+ * IO depth may reduce the performance of idle_class, it is
+ * generally not a big problem, because idle_class usually
+ * does not have strict performance requirements.
+ */
+ if (bfq_class_idle(bfqq) && bfqq->dispatched)
+ goto exit;
+
rq = bfq_dispatch_rq_from_bfqq(bfqd, bfqq);
if (rq) {
--
1.8.3.1
