linux-kernel - Re: [PATCH] cfq-iosched: queue groups more gracefully

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Date:	Thu, 23 Jun 2011 13:51:18 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cfq-iosched: queue groups more gracefully

On Thu, Jun 23, 2011 at 08:22:06PM +0400, Konstantin Khlebnikov wrote:
> This patch queue awakened cfq-groups according its current vdisktime,
> it try to save upto one group timeslice from unused virtual disk time.
> Thus group does not loses everything, if it was not continuously backlogged.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...nvz.org>

I think this patch is not required till we start preemption across
groups? Any more details of actual use will help.

> ---
>  block/cfq-iosched.c |   36 ++++++++++++++++++++++++++++++------
>  1 files changed, 30 insertions(+), 6 deletions(-)
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index c71533e..d5c7c79 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -592,6 +592,26 @@ cfq_group_slice(struct cfq_data *cfqd, struct cfq_group *cfqg)
>  	return cfq_target_latency * cfqg->weight / st->total_weight;
>  }
>  
> +static inline u64
> +cfq_group_vslice(struct cfq_data *cfqd, struct cfq_group *cfqg)
> +{
> +	struct cfq_rb_root *st = &cfqd->grp_service_tree;
> +	u64 vslice;
> +
> +	/* There no group slices in iops mode */
> +	if (iops_mode(cfqd))
> +		return 0;
> +
> +	/*
> +	 * Equal to cfq_scale_slice(cfq_group_slice(cfqd, cfqg), cfqg).
> +	 * Add group weight beacuse it currently not in service tree.
> +	 */
> +	vslice = (u64)cfq_target_latency << CFQ_SERVICE_SHIFT;
> +	vslice *= BLKIO_WEIGHT_DEFAULT;
> +	do_div(vslice, st->total_weight + cfqg->weight);

Above is not equivalent to cfq_scale_slice(cfq_group_slice(cfqd, cfqg),
cfqg) as comment says.

you are not calculating cfq_group_slice(). Instead using cfq_target_latency.

Also it does not make sense. A higher weight group gets lower vslice
and in turn gets put further away on the tree. This is reverse of what
you want.

> +	return vslice;
> +}
> +
>  static inline unsigned
>  cfq_scaled_cfqq_slice(struct cfq_data *cfqd, struct cfq_queue *cfqq)
>  {
> @@ -884,16 +904,20 @@ cfq_group_notify_queue_add(struct cfq_data *cfqd, struct cfq_group *cfqg)
>  		return;
>  
>  	/*
> -	 * Currently put the group at the end. Later implement something
> -	 * so that groups get lesser vtime based on their weights, so that
> -	 * if group does not loose all if it was not continuously backlogged.
> +	 * Bump vdisktime to be greater or equal min_vdisktime.
> +	 */
> +	cfqg->vdisktime = max_vdisktime(cfqg->vdisktime, st->min_vdisktime);
> +

why do we need to do this?

> +	/*
> +	 * Put the group at the end, but save one slice from unused time.
>  	 */
>  	n = rb_last(&st->rb);
>  	if (n) {
>  		__cfqg = rb_entry_cfqg(n);
> -		cfqg->vdisktime = __cfqg->vdisktime + CFQ_IDLE_DELAY;
> -	} else
> -		cfqg->vdisktime = st->min_vdisktime;
> +		cfqg->vdisktime = max_vdisktime(cfqg->vdisktime,
						 ^^^^^^^
I think you meant st->min_vdisktime here?
> +				__cfqg->vdisktime -
> +					cfq_group_vslice(cfqd, cfqg));
> +	}
>  	cfq_group_service_tree_add(st, cfqg);
>  }
> 

Thanks
Vivek 
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