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

[Jeff Moyer <jmoyer@redhat.com>]

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>
> ---
[snip]
> @@ -1929,13 +1941,21 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
>  		return;
>  	}
>  
> +	/* There are other queues in the group, don't do group idle */
> +	if (group_idle && cfqq->cfqg->nr_cfqq > 1)
> +		return;
> +
>  	cfq_mark_cfqq_wait_request(cfqq);
>  
> -	sl = cfqd->cfq_slice_idle;
> +	if (group_idle)
> +		sl = cfqd->cfq_group_idle;
> +	else
> +		sl = cfqd->cfq_slice_idle;

What happens when both group_idle and slice_idle are set?  Is that a
sane thing to do from a user's perspective?  If not, please protect
against it in the configuration code.  If so, then explain why we prefer
group_idle here, but slice_idle in completed request for the extend_sl:

> @@ -3425,7 +3458,10 @@ 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;

Also, you'll need to add documentation for this new tunable.

Cheers,
Jeff