cfq-iosched: quantum check tweak

Currently a queue can only dispatch up to 4 requests if there are other queues.
This isn't optimal, device can handle more requests, for example, AHCI can
handle 31 requests. I can understand the limit is for fairness, but we could
do a tweak: if the queue still has a lot of slice left, sounds we could
ignore the limit. Test shows this boost my workload (two thread randread of
a SSD) from 78m/s to 100m/s.
Thanks for suggestions from Corrado and Vivek for the patch.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 26 insertions, 4 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index f27e535ce26..0db07d7771b 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -19,7 +19,7 @@
  * tunables
  */
 /* max queue in one round of service */
-static const int cfq_quantum = 4;
+static const int cfq_quantum = 8;
 static const int cfq_fifo_expire[2] = { HZ / 4, HZ / 8 };
 /* maximum backwards seek, in KiB */
 static const int cfq_back_max = 16 * 1024;
@@ -2197,6 +2197,19 @@ static int cfq_forced_dispatch(struct cfq_data *cfqd)
         return dispatched;
 }
 
+static inline bool cfq_slice_used_soon(struct cfq_data *cfqd,
+        struct cfq_queue *cfqq)
+{
+        /* the queue hasn't finished any request, can't estimate */
+        if (cfq_cfqq_slice_new(cfqq))
+                return 1;
+        if (time_after(jiffies + cfqd->cfq_slice_idle * cfqq->dispatched,
+                cfqq->slice_end))
+                return 1;
+
+        return 0;
+}
+
 static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 {
         unsigned int max_dispatch;
@@ -2213,7 +2226,7 @@ static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq)
         if (cfqd->rq_in_flight[BLK_RW_SYNC] && !cfq_cfqq_sync(cfqq))
                 return false;
 
-        max_dispatch = cfqd->cfq_quantum;
+        max_dispatch = max_t(unsigned int, cfqd->cfq_quantum / 2, 1);
         if (cfq_class_idle(cfqq))
                 max_dispatch = 1;
 
@@ -2230,13 +2243,22 @@ static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq)
                 /*
                  * We have other queues, don't allow more IO from this one
                  */
-                if (cfqd->busy_queues > 1)
+                if (cfqd->busy_queues > 1 && cfq_slice_used_soon(cfqd, cfqq))
                         return false;
 
                 /*
                  * Sole queue user, no limit
                  */
-                max_dispatch = -1;
+                if (cfqd->busy_queues == 1)
+                        max_dispatch = -1;
+                else
+                        /*
+                         * Normally we start throttling cfqq when cfq_quantum/2
+                         * requests have been dispatched. But we can drive
+                         * deeper queue depths at the beginning of slice
+                         * subjected to upper limit of cfq_quantum.
+                         * */
+                        max_dispatch = cfqd->cfq_quantum;
         }
 
         /*