Re: [PATCH] Avoid preferential treatment of groups that aren'tbacklogged

[Vivek Goyal]

On Wed, Feb 09, 2011 at 05:32:11PM -0800, Chad Talbott wrote:
> Problem: If a group isn't backlogged, we remove it from the service
> tree. When it becomes active again, it gets the minimum vtime of the
> tree. That is true even when the group was idle for a very small time,
> and it consumed some IO time right before it became idle.  If group
> has small weight, it can end up using more disk time than its fair
> share.
> 
> Solution: We solve the problem by assigning the group its old vtime if
> it has not been idle long enough. Otherise we assign it the service
> tree's min vtime.
> 
> Complications: When an entire service tree becomes completely idle, we
> lose the vtime state. All the old vtime values are not relevant any
> more. For example, consider the case when the service tree is idle and
> a brand new group sends IO. That group would have an old vtime value
> of zero, but the service tree's vtime would become closer to zero. In
> such a case, it would be unfair for the older groups to get a much
> higher old vtime stored in them.
> 
> We solve that issue by keeping a generation number that counts the
> number of instances when the service tree becomes completely
> empty. The generation number is stored in each group too. If a group
> becomes backlogged after a service tree has been empty, we compare its
> stored generation number with the current service tree generation
> number, and discard the old vtime if the group generation number is
> stale.
> 
> The preemption specific code is taken care of automatically because we
> allow preemption checks after inserting a group back into the service
> tree and assigning it an appropriate vtime.
> 
> Signed-off-by: Chad Talbott <ctalbott@xxxxxxxxxx>
> ---
>  block/cfq-iosched.c |   24 +++++++++++++-----------
>  1 files changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 501ffdf..f09f3fe 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -178,6 +178,7 @@ struct cfq_group {
>  
>  	/* group service_tree key */
>  	u64 vdisktime;
> +	u64 generation_num;
>  	unsigned int weight;
>  
>  	/* number of cfqq currently on this group */
> @@ -300,6 +301,9 @@ struct cfq_data {
>  	/* List of cfq groups being managed on this device*/
>  	struct hlist_head cfqg_list;
>  	struct rcu_head rcu;
> +
> +	/* Generation number, counts service tree empty events */
> +	u64 active_generation;
>  };
>  
>  static struct cfq_group *cfq_get_next_cfqg(struct cfq_data *cfqd);
> @@ -873,18 +877,13 @@ cfq_group_service_tree_add(struct cfq_data *cfqd, struct cfq_group *cfqg)
>  	if (!RB_EMPTY_NODE(&cfqg->rb_node))
>  		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 continously backlogged.
> -	 */
> -	n = rb_last(&st->rb);
> -	if (n) {
> -		__cfqg = rb_entry_cfqg(n);
> -		cfqg->vdisktime = __cfqg->vdisktime + CFQ_IDLE_DELAY;
> -	} else
> +	if (cfqd->active_generation > cfqg->generation_num)
>  		cfqg->vdisktime = st->min_vdisktime;

- What happens when cfqd->active_generation wraps over? Should we use
  functions which take care of wrapping.

- So when generation number changes you want to put the newly backlogged
  group at the front of tree and not at the end of it? Though it kind
  of make sense as any continuously backlogged groups will be on service
  tree for long time and newly backlogged groups are either new IO
  starting or some application which high think time and which does IO
  one in a while and does not keep disk occupied for very long time. In
  such cases it probably is not a bad idea to put newly backlogged 
  groups at the beginning of the tree.

> -
> +	else
> +		/* We assume that vdisktime was not modified when the task
> +		   was off the service tree.
> +		 */
> +		cfqg->vdisktime = max(st->min_vdisktime, cfqg->vdisktime);
>  	__cfq_group_service_tree_add(st, cfqg);
>  	st->total_weight += cfqg->weight;
>  }
> @@ -906,6 +905,9 @@ cfq_group_service_tree_del(struct cfq_data *cfqd, struct cfq_group *cfqg)
>  	if (!RB_EMPTY_NODE(&cfqg->rb_node))
>  		cfq_rb_erase(&cfqg->rb_node, st);
>  	cfqg->saved_workload_slice = 0;
> +	cfqg->generation_num = cfqd->active_generation;
> +	if (RB_EMPTY_ROOT(&st->rb))
> +		cfqd->active_generation++;

I don't understand that what is the significance behind chaning generation
number when tree is idle? When tree is idle does not mean that few
recently deleted groups will not get backlogged soon? 

Thanks
Vivek
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