[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20100114041624.GA10276@sli10-desk.sh.intel.com>
Date: Thu, 14 Jan 2010 12:16:24 +0800
From: Shaohua Li <shaohua.li@...el.com>
To: Vivek Goyal <vgoyal@...hat.com>
Cc: Corrado Zoccolo <czoccolo@...il.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
"Zhang, Yanmin" <yanmin.zhang@...el.com>
Subject: Re: [RFC]cfq-iosched: quantum check tweak
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@...el.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;
}
/*
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists