Re: [RFC]cfq-iosched: quantum check tweak

[Shaohua Li <shaohua.li@intel.com>]

On Wed, Jan 13, 2010 at 07:18:07PM +0800, Vivek Goyal wrote:
> On Wed, Jan 13, 2010 at 04:17:35PM +0800, Shaohua Li wrote:
> [..]
> > > >  static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq)
> > > >  {
> > > >  	unsigned int max_dispatch;
> > > > @@ -2258,7 +2273,10 @@ static bool cfq_may_dispatch(struct cfq_
> > > >  	if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq))
> > > >  		return false;
> > > >  
> > > > -	max_dispatch = cfqd->cfq_quantum;
> > > > +	max_dispatch = cfqd->cfq_quantum / 2;
> > > > +	if (max_dispatch < CFQ_SOFT_QUANTUM)
> > > 
> > > We don't have to hardcode CFQ_SOFT_QUANTUM or in fact we don't need it. We can
> > > derive the soft limit from hard limit (cfq_quantum). Say soft limit will be
> > > 50% of cfq_quantum value.
> > I'm hoping this doesn't give user a surprise. Say cfq_quantum sets to 7, then we
> > start doing throttling from 3 requests. Adding the CFQ_SOFT_QUANTUM gives a compatibility
> > against old behavior at least. Am I over thinking?
> >  
> 
> I would not worry too much about that. If you are really worried about
> that, then create one Documentation/block/cfq-iosched.txt and document
> how cfq_quantum works so that users know that cfq_quantum is upper hard
> limit and internal soft limit is cfq_quantum/2.
Good idea. Looks we don't document cfq tunnables, I'll try to do it later.

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.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
---
 block/cfq-iosched.c |   30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

Index: linux-2.6/block/cfq-iosched.c
===================================================================
--- linux-2.6.orig/block/cfq-iosched.c
+++ linux-2.6/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;
@@ -2215,6 +2215,19 @@ static int cfq_forced_dispatch(struct cf
 	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;
@@ -2231,7 +2244,7 @@ static bool cfq_may_dispatch(struct cfq_
 	if (cfqd->sync_flight && !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;
 
@@ -2248,13 +2261,22 @@ static bool cfq_may_dispatch(struct cfq_
 		/*
 		 * 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;
 	}
 
 	/*