From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763390Ab3ECT1F (ORCPT ); Fri, 3 May 2013 15:27:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:64128 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763076Ab3ECT1C (ORCPT ); Fri, 3 May 2013 15:27:02 -0400 Date: Fri, 3 May 2013 15:26:56 -0400 From: Vivek Goyal To: Tejun Heo Cc: Jens Axboe , lkml , Li Zefan , containers@lists.linux-foundation.org, Cgroups Subject: Re: [PATCHSET] blk-throttle: implement proper hierarchy support Message-ID: <20130503192656.GD6062@redhat.com> References: <20130502184514.GI30020@redhat.com> <20130502184953.GP19814@mtj.dyndns.org> <20130502190732.GK30020@redhat.com> <20130502193139.GL30020@redhat.com> <20130502231307.GT19814@mtj.dyndns.org> <20130503175652.GB6062@redhat.com> <20130503185751.GA22860@mtj.dyndns.org> <20130503190823.GC6062@redhat.com> <20130503191418.GC22860@mtj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130503191418.GC22860@mtj.dyndns.org> 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 On Fri, May 03, 2013 at 12:14:18PM -0700, Tejun Heo wrote: > On Fri, May 03, 2013 at 03:08:23PM -0400, Vivek Goyal wrote: > > T1 T2 T3 T4 T5 T6 T7 > > parent: b1 b2 b3 b4 b5 > > child: b1 b2 b3 b4 b5 > > > > > > So continuity breaks down because application is waiting for previous > > IO to finish. This forces expiry of existing time slices and new time > > slice start both in child and parent and penalty keep on increasing. > > It's a problem even in flat mode as the "child" above can easily be > just a process which is throttling itself and it won't be able to get > the configured bandwidth due to the scheduling bubbles introduced > whenever new slice is started. Shouldn't be too difficult to get rid > of, right? Key thing here is when to start a new slice. Generally when an IO has been dispatched from a group, we do not expire slice immediately. We kind of give group some grace period of throtl_slice (100ms). If next IO does not come with-in that duration, we start a fresh slice upon next IO arrival. I think similar problem should happen if there are two stacked devices and both are doing throttling and if delays between 2 IOs are big enough that it forces expirty of slice on each device. Atleast for the hiearchy case, we should be able to start a fresh time slice when child transfer bio to parent. I will write a patch and do some experiment. Thanks Vivek