From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756138AbcECPWy (ORCPT ); Tue, 3 May 2016 11:22:54 -0400 Received: from mx2.suse.de ([195.135.220.15]:41877 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755920AbcECPWw (ORCPT ); Tue, 3 May 2016 11:22:52 -0400 Date: Tue, 3 May 2016 17:22:49 +0200 From: Jan Kara To: Jens Axboe Cc: Jan Kara , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, dchinner@redhat.com, sedat.dilek@gmail.com Subject: Re: [PATCH 7/8] wbt: add general throttling mechanism Message-ID: <20160503152249.GF25436@quack2.suse.cz> References: <1461686131-22999-1-git-send-email-axboe@fb.com> <1461686131-22999-8-git-send-email-axboe@fb.com> <20160428110559.GC17362@quack2.suse.cz> <57225C3E.7060504@fb.com> <20160503093410.GD12748@quack2.suse.cz> <5728B45F.6050200@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5728B45F.6050200@fb.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 03-05-16 08:23:27, Jens Axboe wrote: > On 05/03/2016 03:34 AM, Jan Kara wrote: > >On Thu 28-04-16 12:53:50, Jens Axboe wrote: > >>>2) As far as I can see in patch 8/8, you have plugged the throttling above > >>> the IO scheduler. When there are e.g. multiple cgroups with different IO > >>> limits operating, this throttling can lead to strange results (like a > >>> cgroup with low limit using up all available background "slots" and thus > >>> effectively stopping background writeback for other cgroups)? So won't > >>> it make more sense to plug this below the IO scheduler? Now I understand > >>> there may be other problems with this but I think we should put more > >>> though to that and provide some justification in changelogs. > >> > >>One complexity is that we have to do this early for blk-mq, since once you > >>get a request, you're already sitting on the hw tag. CoDel should actually > >>work fine at each hop, so hopefully this will as well. > > > >OK, I see. But then this suggests that any IO scheduling and / or > >cgroup-related throttling should happen before we get a request for blk-mq > >as well? And then we can still do writeback throttling below that layer? > > Not necessarily. For IO scheduling, basically we care about two parts: > > 1) Are you allowed to allocate the resources to queue some IO > 2) Are you allowed to dispatch But then it seems suboptimal to waste a relatively scarce resource (which HW tag is AFAIU) just because you happen to run from a cgroup that is bandwidth limited and thus are not allowed to dispatch? Honza -- Jan Kara SUSE Labs, CR