Re: [PATCH 7/8] cfq-iosched: Add flat mode and switch between two modes by "use_hierarchy"

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Dec 13, 2010 at 09:45:14AM +0800, Gui Jianfeng wrote:
[..]
>  
> +	/* Service tree for cfq group flat scheduling mode. */
> +	struct cfq_rb_root flat_service_tree;
> +
>  	/*
>  	 * The priority currently being served
>  	 */
> @@ -647,12 +650,16 @@ cfq_group_slice(struct cfq_data *cfqd, struct cfq_group *cfqg)
>  	struct cfq_entity *cfqe = &cfqg->cfqe;
>  	struct cfq_rb_root *st = cfqe->service_tree;
>  
> -	if (st)
> -		return cfq_target_latency * cfqe->weight
> -			/ st->total_weight;
> -	else
> -		/* If this is the root group, give it a full slice. */
> -		return cfq_target_latency;
> +	if (cfqd->use_hierarchy) {
> +		if (st)
> +			return cfq_target_latency * cfqe->weight
> +				/ st->total_weight;
> +		else
> +			/* If this is the root group, give it a full slice. */
> +			return cfq_target_latency;
> +	} else {
> +		return cfq_target_latency * cfqe->weight / st->total_weight;
> +	}
>  }

Once you have moved the notion of entity and weight of the entity, I think
you can simplify things a bit and come up with a notion of entity slice
in a hieararhy and we can avoid using separate mechanisms for queues and
groups.

There can be multiple ways of doing this and you shall have to see what
is a simple way which works. For queues was keeping a track of average
number of queues and that way estimating the slice length. You can
try keeping track of average number of entities in a group or something
like that. But do think everything now in terms of entities and simplify
the logic a bit.

Thanks
Vivek