Messages in this thread | | | Date | Wed, 28 Jan 2009 11:47:22 +0530 | Subject | Re: [PATCH v2][RESEND] Allow RT requests to pre-empt ongoing BE timeslice in CFQ. | From | Divyesh Shah <> |
| |
+ Jens Axboe
On Wed, Jan 28, 2009 at 11:35 AM, Divyesh Shah <dpshah@google.com> wrote: > 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. > > Signed-off-by: Divyesh Shah <dpshah@google.com> > --- > 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 > > block/cfq-iosched.c | 39 ++++++++++++++++++++++++++++++++++++++- > 1 files changed, 38 insertions(+), 1 deletions(-) > > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c > index e8525fa..5fec9e5 100644 > --- a/block/cfq-iosched.c > +++ b/block/cfq-iosched.c > @@ -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--; > } > > /* > @@ -1005,6 +1014,20 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd) > 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); > >
| |