Messages in this thread | | | Date | Mon, 21 Feb 2011 10:32:43 -0500 | From | Vivek Goyal <> | Subject | Re: [PATCH v4] Avoid preferential treatment of groups that aren't backlogged |
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On Fri, Feb 18, 2011 at 02:19:16PM -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 either the minimum vtime > of the tree or gets put at the "back of the line." 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 very small weight, it > can end up using more disk time than its fair share. Conversely, if it > has a very large weight, being put at the back of the vtime line is a > large penalty which prevents it consuming its 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@google.com>
Looks good to me.
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Vivek
> --- > block/cfq-iosched.c | 27 ++++++++++++++------------- > 1 files changed, 14 insertions(+), 13 deletions(-) > > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c > index 501ffdf..8a1b82f 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); > @@ -866,25 +870,19 @@ static void > cfq_group_service_tree_add(struct cfq_data *cfqd, struct cfq_group *cfqg) > { > struct cfq_rb_root *st = &cfqd->grp_service_tree; > - struct cfq_group *__cfqg; > - struct rb_node *n; > > cfqg->nr_cfqq++; > 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; > - > + else > + /* We assume that vdisktime was not modified when the task > + was off the service tree. > + */ > + cfqg->vdisktime = max_vdisktime(st->min_vdisktime, > + cfqg->vdisktime); > __cfq_group_service_tree_add(st, cfqg); > st->total_weight += cfqg->weight; > } > @@ -906,6 +904,9 @@ 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++; > cfq_blkiocg_update_dequeue_stats(&cfqg->blkg, 1); > } >
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