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Commit abc3c744 authored by Shaohua Li's avatar Shaohua Li Committed by Jens Axboe
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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: default avatarShaohua Li <shaohua.li@intel.com>
Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
parent 9a8c28c8

block/cfq-iosched.c

+26 −4
Original line number Diff line number Diff line
@@ -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

		 */

		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;

	}



	/*