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cfq-iosched: Allow RT requests to pre-empt ongoing BE timeslice

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This patch adds the ability to pre-empt an ongoing BE timeslice when a RT
request is waiting for the current timeslice to complete. This reduces the
wait time to disk for RT requests from an upper bound of 4 (current value
of cfq_quantum) to 1 disk request.

Applied Jens' suggeested changes to avoid the rb lookup and use !cfq_class_rt()
and retested.

Latency(secs) for the RT task when doing sequential reads from 10G file.
                       | only RT | RT + BE | RT + BE + this patch
small (512 byte) reads | 143     | 163     | 145
large (1Mb) reads      | 142     | 158     | 146

Signed-off-by: Divyesh Shah <dpshah@google.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
This commit is contained in:
Divyesh Shah 2009-01-30 12:46:41 +01:00 committed by Jens Axboe
parent bc58ba9468
commit 3a9a3f6cc5
1 changed files with 38 additions and 1 deletions
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39
block/cfq-iosched.c
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@ -84,6 +84,11 @@ struct cfq_data {
*/
struct cfq_rb_root service_tree;
unsigned int busy_queues;
/*
* Used to track any pending rt requests so we can pre-empt current
* non-RT cfqq in service when this value is non-zero.
*/
unsigned int busy_rt_queues;
int rq_in_driver;
int sync_flight;
@ -562,6 +567,8 @@ static void cfq_add_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
BUG_ON(cfq_cfqq_on_rr(cfqq));
cfq_mark_cfqq_on_rr(cfqq);
cfqd->busy_queues++;
if (cfq_class_rt(cfqq))
cfqd->busy_rt_queues++;
cfq_resort_rr_list(cfqd, cfqq);
}
@ -581,6 +588,8 @@ static void cfq_del_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
BUG_ON(!cfqd->busy_queues);
cfqd->busy_queues--;
if (cfq_class_rt(cfqq))
cfqd->busy_rt_queues--;
}
/*
@ -1004,6 +1013,20 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
if (cfq_slice_used(cfqq))
goto expire;
/*
* If we have a RT cfqq waiting, then we pre-empt the current non-rt
* cfqq.
*/
if (!cfq_class_rt(cfqq) && cfqd->busy_rt_queues) {
/*
* We simulate this as cfqq timed out so that it gets to bank
* the remaining of its time slice.
*/
cfq_log_cfqq(cfqd, cfqq, "preempt");
cfq_slice_expired(cfqd, 1);
goto new_queue;
}
/*
* The active queue has requests and isn't expired, allow it to
* dispatch.
@ -1067,6 +1090,13 @@ __cfq_dispatch_requests(struct cfq_data *cfqd, struct cfq_queue *cfqq,
if (RB_EMPTY_ROOT(&cfqq->sort_list))
break;
/*
* If there is a non-empty RT cfqq waiting for current
* cfqq's timeslice to complete, pre-empt this cfqq
*/
if (!cfq_class_rt(cfqq) && cfqd->busy_rt_queues)
break;
} while (dispatched < max_dispatch);
/*
@ -1801,6 +1831,12 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
if (rq_is_meta(rq) && !cfqq->meta_pending)
return 1;
/*
* Allow an RT request to pre-empt an ongoing non-RT cfqq timeslice.
*/
if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))
return 1;
if (!cfqd->active_cic || !cfq_cfqq_wait_request(cfqq))
return 0;
@ -1870,7 +1906,8 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
/*
* not the active queue - expire current slice if it is
* idle and has expired it's mean thinktime or this new queue
* has some old slice time left and is of higher priority
* has some old slice time left and is of higher priority or
* this new queue is RT and the current one is BE
*/
cfq_preempt_queue(cfqd, cfqq);
cfq_mark_cfqq_must_dispatch(cfqq);