From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753040Ab2LQVRs (ORCPT ); Mon, 17 Dec 2012 16:17:48 -0500 Received: from mail-pb0-f46.google.com ([209.85.160.46]:45112 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751180Ab2LQVRq (ORCPT ); Mon, 17 Dec 2012 16:17:46 -0500 Date: Mon, 17 Dec 2012 13:17:38 -0800 From: Tejun Heo To: Vivek Goyal Cc: lizefan@huawei.com, axboe@kernel.dk, containers@lists.linux-foundation.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, ctalbott@google.com, rni@google.com Subject: Re: [PATCH 07/12] cfq-iosched: implement hierarchy-ready cfq_group charge scaling Message-ID: <20121217211738.GD1844@htj.dyndns.org> References: <1355524885-22719-1-git-send-email-tj@kernel.org> <1355524885-22719-8-git-send-email-tj@kernel.org> <20121217205317.GI7235@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121217205317.GI7235@redhat.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 Hello, On Mon, Dec 17, 2012 at 03:53:18PM -0500, Vivek Goyal wrote: > On Fri, Dec 14, 2012 at 02:41:20PM -0800, Tejun Heo wrote: > > Currently, cfqg charges are scaled directly according to cfqg->weight. > > Regardless of the number of active cfqgs or the amount of active > > weights, a given weight value always scales charge the same way. This > > works fine as long as all cfqgs are treated equally regardless of > > their positions in the hierarchy, which is what cfq currently > > implements. It can't work in hierarchical settings because the > > interpretation of a given weight value depends on where the weight is > > located in the hierarchy. > > I did not understand this. Why the current scheme will not work with > hierarchy? Because the meaning of a weight changes depending on where the weight exists in the hierarchy? > While we calculate the vdisktime, this is calculated with the help > of CFQ_DEFAULT_WEIGHT and cfqg->weight. So we scale used time slice > in proportion to CFQ_DEFAULT_WEIGTH/cfqg->weight. So higher the weight > lesser the charge and cfqg gets scheduled again faster and lower the > weight, higher the vdisktime and cfqg gets scheduled less frequently. > > As every cfqg does the same thing on service tree, they automatically > get fair share w.r.t their weight. > > And this mechanism should not be impacted by the hierarchy because we > have a separate service tree at separate level. This will not work > only if you come up with one compressed tree and then weights will > have to be adjusted. If we have a separate service tree in each group > then it should work just fine. Why would you create N service trees when you can almost trivially use one by calcualting the effective weight? You would have to be adjusting all trees above whenever something changes in a child. Thanks. -- tejun