From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756370Ab2B1RBV (ORCPT ); Tue, 28 Feb 2012 12:01:21 -0500 Received: from mx1.redhat.com ([209.132.183.28]:9973 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753472Ab2B1RBU (ORCPT ); Tue, 28 Feb 2012 12:01:20 -0500 Date: Tue, 28 Feb 2012 09:01:03 -0800 From: Chris Wright To: Vivek Goyal Cc: Chris Wright , Tejun Heo , Kent Overstreet , axboe@kernel.dk, ctalbott@google.com, rni@google.com, linux-kernel@vger.kernel.org, Kevin Wolf , Stefan Hajnoczi Subject: Re: [PATCH 7/9] block: implement bio_associate_current() Message-ID: <20120228170103.GI14856@x200.localdomain> References: <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> <20120227231222.GF14856@x200.localdomain> <20120228141036.GE9920@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120228141036.GE9920@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 27, 2012 at 03:12:22PM -0800, Chris Wright wrote: > > [..] > > > > > > 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). > > Chris, > > Just to make sure I understand it right I am thinking loud. > > That means CLONE_IO is useful and ideally qemu would like to make use of it > but beacuse pthread interface does not support it, it is not used as of > today. It depends on the block I/O model in qemu. It can use either a pthread pool or native aio. All of the CFQ + idling problems we encountered were w/ a pthread pool (and IIRC, those would have predated preadv/pwritev, so the actual i/o vector from the guest was pulled apart and submitted as a bunch of discrete pread/pwrite requests), however in many cases simply using aio may be the better sol'n. Stefan, Kevin, have any thoughts re: qemu block I/O and making use of CLONE_IO in a modern qemu? thanks, -chris