Re: [PATCH] cfq: Make use of service count to estimate the rb_key offset

[Corrado Zoccolo]

Hi Gui, Jens
On Thu, Nov 26, 2009 at 7:20 AM, Gui Jianfeng
<guijianfeng@xxxxxxxxxxxxxx> wrote:
> Hi Jens, Czoccolo
>
> For the moment, different workload cfq queues are put into different
> service trees. But CFQ still uses "busy_queues" to estimate rb_key
> offset when inserting a cfq queue into a service tree. I think this
> isn't appropriate, and it should make use of service tree count to do
> this estimation. This patch is for for-2.6.33 branch.

In cfq_choose_wl, we rely on consistency of rb_keys across service
trees to compute the next workload to be serviced.
        for (i = 0; i < 3; ++i) {
                /* otherwise, select the one with lowest rb_key */
                queue = cfq_rb_first(service_tree_for(prio, i, cfqd));
                if (queue &&
                    (!key_valid || time_before(queue->rb_key, lowest_key))) {
                        lowest_key = queue->rb_key;
			cur_best = i;
			key_valid = true;
		}
        }

If you change how the rb_key is computed (so it is no longer
consistent across service trees) without changing how it is used can
introduce problems.

Thanks,
Corrado

>
> Signed-off-by: Gui Jianfeng <guijianfeng@xxxxxxxxxxxxxx>
> ---
> Âblock/cfq-iosched.c | Â Â8 ++++++--
> Â1 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 1bcbd8c..467981e 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -600,11 +600,15 @@ cfq_find_next_rq(struct cfq_data *cfqd, struct cfq_queue *cfqq,
> Âstatic unsigned long cfq_slice_offset(struct cfq_data *cfqd,
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âstruct cfq_queue *cfqq)
> Â{
> + Â Â Â struct cfq_rb_root *service_tree;
> +
> + Â Â Â service_tree = service_tree_for(cfqq_prio(cfqq), cfqq_type(cfqq), cfqd);
> +
> Â Â Â Â/*
> Â Â Â Â * just an approximation, should be ok.
> Â Â Â Â */
> - Â Â Â return (cfqd->busy_queues - 1) * (cfq_prio_slice(cfqd, 1, 0) -
> - Â Â Â Â Â Â Â Â Â Â Âcfq_prio_slice(cfqd, cfq_cfqq_sync(cfqq), cfqq->ioprio));
> + Â Â Â return Âservice_tree->count * (cfq_prio_slice(cfqd, 1, 0) -
> + Â Â Â Â Â Â Â Â Âcfq_prio_slice(cfqd, cfq_cfqq_sync(cfqq), cfqq->ioprio));
> Â}
>
> Â/*
> --
> 1.5.4.rc3
>
> --
> Regards
> Gui Jianfeng
>
>



-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@xxxxxxxxx
PhD - Department of Computer Science - University of Pisa, Italy
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