Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755725Ab1BUPcz (ORCPT ); Mon, 21 Feb 2011 10:32:55 -0500 Received: from mx1.redhat.com ([209.132.183.28]:57027 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755481Ab1BUPcx (ORCPT ); Mon, 21 Feb 2011 10:32:53 -0500 Date: Mon, 21 Feb 2011 10:32:43 -0500 From: Vivek Goyal To: Chad Talbott Cc: jaxboe@fusionio.com, guijianfeng@cn.fujitsu.com, mrubin@google.com, teravest@google.com, jmoyer@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4] Avoid preferential treatment of groups that aren't backlogged Message-ID: <20110221153243.GE6428@redhat.com> References: <20110218221916.18235.88420.stgit@neat.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110218221916.18235.88420.stgit@neat.mtv.corp.google.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4488 Lines: 116 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 Looks good to me. Acked-by: Vivek Goyal 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); > } > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/