Re: [PATCH 2/3] cfq-iosched: Implement a new tunable group_idle

Vivek Goyal
2010-07-19T20:20:36Z

On Mon, Jul 19, 2010 at 02:58:41PM -0400, Jeff Moyer wrote:
> Vivek Goyal <vgoyal@redhat.com> writes:
>
> > o Implement a new tunable group_idle, which allows idling on the group
> > instead of a cfq queue. Hence one can set slice_idle = 0 and not idle
> > on the individual queues but idle on the group. This way on fast storage
> > we can get fairness between groups at the same time overall throughput
> > improves.
> >
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > ---
> > block/cfq-iosched.c | 60 +++++++++++++++++++++++++++++++++++++++++++++------
> > 1 files changed, 53 insertions(+), 7 deletions(-)
> >
> > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> > index f44064c..b23d7f4 100644
> > --- a/block/cfq-iosched.c
> > +++ b/block/cfq-iosched.c
> > @@ -30,6 +30,7 @@ static const int cfq_slice_sync = HZ / 10;
> > static int cfq_slice_async = HZ / 25;
> > static const int cfq_slice_async_rq = 2;
> > static int cfq_slice_idle = HZ / 125;
> > +static int cfq_group_idle = HZ / 125;
> > static const int cfq_target_latency = HZ * 3/10; /* 300 ms */
> > static const int cfq_hist_divisor = 4;
> >
> > @@ -198,6 +199,8 @@ struct cfq_group {
> > struct hlist_node cfqd_node;
> > atomic_t ref;
> > #endif
> > + /* number of requests that are on the dispatch list or inside driver */
> > + int dispatched;
> > };
> >
> > /*
> > @@ -271,6 +274,7 @@ struct cfq_data {
> > unsigned int cfq_slice[2];
> > unsigned int cfq_slice_async_rq;
> > unsigned int cfq_slice_idle;
> > + unsigned int cfq_group_idle;
> > unsigned int cfq_latency;
> > unsigned int cfq_group_isolation;
> >
> > @@ -1856,6 +1860,9 @@ static bool cfq_should_idle(struct cfq_data *cfqd, struct cfq_queue *cfqq)
> > BUG_ON(!service_tree);
> > BUG_ON(!service_tree->count);
> >
> > + if (!cfqd->cfq_slice_idle)
> > + return false;
> > +
> > /* We never do for idle class queues. */
> > if (prio == IDLE_WORKLOAD)
> > return false;
> > @@ -1880,7 +1887,7 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
> > {
> > struct cfq_queue *cfqq = cfqd->active_queue;
> > struct cfq_io_context *cic;
> > - unsigned long sl;
> > + unsigned long sl, group_idle = 0;
> >
> > /*
> > * SSD device without seek penalty, disable idling. But only do so
> > @@ -1896,8 +1903,13 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
> > /*
> > * idle is disabled, either manually or by past process history
> > */
> > - if (!cfqd->cfq_slice_idle || !cfq_should_idle(cfqd, cfqq))
> > - return;
> > + if (!cfqd->cfq_slice_idle || !cfq_should_idle(cfqd, cfqq)) {
>
> The check for cfqd->cfq_slice_idle is now redundant (as it's done in
> cfq_should_idle).

Yep. I will get rid of extra check.

>
> > @@ -2215,7 +2236,7 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
> > cfqq = NULL;
> > goto keep_queue;
> > } else
> > - goto expire;
> > + goto check_group_idle;
> > }
> >
> > /*
> > @@ -2249,6 +2270,17 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
> > goto keep_queue;
> > }
> >
> > + /*
> > + * If group idle is enabled and there are requests dispatched from
> > + * this group, wait for requests to complete.
> > + */
> > +check_group_idle:
> > + if (cfqd->cfq_group_idle && cfqq->cfqg->nr_cfqq == 1
> > + && cfqq->cfqg->dispatched) {
> > + cfqq = NULL;
> > + goto keep_queue;
> > + }
>
> I really wish we could roll all of this logic into cfq_should_idle.

Currently whether to idle on queue or not logic is also part of select_queue().
So to keep it same it makes sense to keep group logic also in select_queue().

>
> > @@ -3420,7 +3453,10 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
> > * the queue.
> > */
> > if (cfq_should_wait_busy(cfqd, cfqq)) {
> > - cfqq->slice_end = jiffies + cfqd->cfq_slice_idle;
> > + unsigned long extend_sl = cfqd->cfq_slice_idle;
> > + if (!cfqd->cfq_slice_idle)
> > + extend_sl = cfqd->cfq_group_idle;
> > + cfqq->slice_end = jiffies + extend_sl;
>
> So, if slice_idle and group_idle are both set, slice_idle trumps
> group_idle? Did you give that case any thought? If it doesn't make
> sense to configure both, then we should probably make sure they can't
> both be set.

Actually, the wait busy logic makes sense in case of slice_idle, where we
can give an extended slice to a queue and to make sure a queue/group does
not get deleted immediately after completing the slice and hence losing
the share.

Now in case of slice_idle=0, if there is only one queue in the group then
it becomes the same case as one queue in the group with slice_idle=8.

So yes, slice_idle trumps group_idle. group_idle kicks in only if
slice_idle=0. I think by default it being set it does not harm. What good
it will do if slice_idle=8 and group_idle=0. Instead it will become
more work for user to also set one more tunable if he plans to set
slice_idle=0.

Thanks
Vivek

>
> Cheers,
> Jeff