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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: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
This commit is contained in:
Shaohua Li 2010-03-01 09:20:54 +01:00 committed by Jens Axboe
parent 9a8c28c831
commit abc3c744d0
1 changed files with 26 additions and 4 deletions
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30
block/cfq-iosched.c
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@ -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;
}
/*