From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754519Ab2B0XMf (ORCPT ); Mon, 27 Feb 2012 18:12:35 -0500 Received: from mx1.redhat.com ([209.132.183.28]:7086 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753002Ab2B0XMe (ORCPT ); Mon, 27 Feb 2012 18:12:34 -0500 Date: Mon, 27 Feb 2012 15:12:22 -0800 From: Chris Wright To: Vivek Goyal Cc: Tejun Heo , Kent Overstreet , axboe@kernel.dk, ctalbott@google.com, rni@google.com, linux-kernel@vger.kernel.org, Chris Wright Subject: Re: [PATCH 7/9] block: implement bio_associate_current() Message-ID: <20120227231222.GF14856@x200.localdomain> References: <1329431878-28300-8-git-send-email-tj@kernel.org> <20120217011907.GA15073@google.com> <20120217221406.GJ29414@google.com> <20120217223420.GJ26620@redhat.com> <20120217224103.GN29414@google.com> <20120217225125.GK26620@redhat.com> <20120217225735.GP29414@google.com> <20120220142233.GA10342@redhat.com> <20120220165922.GA7836@mtj.dyndns.org> <20120220191404.GB13423@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120220191404.GB13423@redhat.com> User-Agent: Mutt/1.5.21+46 (b01d63af6fea) (2011-07-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Vivek Goyal (vgoyal@redhat.com) wrote: > On Mon, Feb 20, 2012 at 08:59:22AM -0800, Tejun Heo wrote: > > On Mon, Feb 20, 2012 at 09:22:33AM -0500, Vivek Goyal wrote: > > > I guess you will first determine cfqq associated with cic and then do > > > > > > cfqq->cfqg->blkg->blkcg == bio_blkcg(bio) > > > > > > One can do that but still does not get rid of requirement of checking > > > for CGRPOUP_CHANGED as not every bio will have cgroup information stored > > > and you still will have to check whether submitting task has changed > > > the cgroup since it last did IO. > > > > Hmmm... but in that case task would be using a different blkg and the > > test would still work, wouldn't it? > > Oh.., forgot that bio_blkio_blkcg() returns the current tasks's blkcg if > bio->blkcg is not set. So if a task's cgroup changes, bio_blkcg() will > point to latest cgroup and cfqq->cfqg->blkg->blkcg will point to old > cgroup and test will indicate the discrepancy. So yes, it should work > for both the cases. > > > > > > > blkcg doesn't allow that anyway (it tries but is racy) and I actually > > > > was thinking about sending a RFC patch to kill CLONE_IO. > > > > > > I thought CLONE_IO is useful and it allows threads to share IO context. > > > qemu wanted to use it for its IO threads so that one virtual machine > > > does not get higher share of disk by just craeting more threads. In fact > > > if multiple threads are doing related IO, we would like them to use > > > same io context. > > > > I don't think that's true. Think of any multithreaded server program > > where each thread is working pretty much independently from others. > > If threads are working pretty much independently, then one does not have > to specify CLONE_IO. > > In case of qemu IO threads, I have debugged issues where an big IO range > is being splitted among its IO threads. Just do a sequential IO inside > guest, and I was seeing that few sector IO comes from one process, next > few sector come from other process and it goes on. A sequential range > of IO is some split among a bunch of threads and that does not work > well with CFQ if every IO is coming from its own IO context and IO > context is not shared. After a bunch of IO from one io context, CFQ > continues to idle on that io context thinking more IO will come soon. > Next IO does come but from a different thread and differnet context. > > CFQ now has employed some techniques to detect that case and try > to do preemption and try to reduce idling in such cases. But sometimes > these techniques work well and other times don't. So to me, CLONE_IO > can help in this case where application can specifically share > IO context and CFQ does not have to do all the tricks. > > That's a different thing that applications might not be making use > of CLONE_IO. > > > Virtualization *can* be a valid use case but are they actually using > > it? Aren't they better served by cgroup? > > cgroup can be very heavy weight when hundred's of virtual machines > are running. Why? because of idling. CFQ still has lots of tricks > to do preemption and cut down on idling across io contexts, but > across cgroup boundaries, isolation is much more stronger and very > little preemption (if any) is allowed. I suspect in current > implementation, if we create lots of blkio cgroup, it will be > bad for overall throughput of virtual machines (purely because of > idling). > > So I am not too excited about blkio cgroup solution because it might not > scale well. (Until and unless we find a better algorithm to cut down > on idling). > > I am ccing Chris Wright . He might have thoughts > on usage of CLONE_IO and qemu. Vivek, you summed it up pretty well. Also, for qemu, raw CLONE_IO is not an option because threads are created via pthread (we had done some local hacks to verify that CLONE_IO helped w/ the idling problem, and it did). thanks, -chris