Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932621Ab2FUJt3 (ORCPT ); Thu, 21 Jun 2012 05:49:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:7611 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759209Ab2FUJt1 (ORCPT ); Thu, 21 Jun 2012 05:49:27 -0400 Message-ID: <4FE2EE18.9050307@redhat.com> Date: Thu, 21 Jun 2012 12:49:12 +0300 From: Dor Laor Reply-To: dlaor@redhat.com User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120605 Thunderbird/13.0 MIME-Version: 1.0 To: Asias He CC: Sasha Levin , Rusty Russell , kvm@vger.kernel.org, "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Christoph Hellwig Subject: Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk References: <1340002390-3950-1-git-send-email-asias@redhat.com> <1340002390-3950-4-git-send-email-asias@redhat.com> <87hau9yse7.fsf@rustcorp.com.au> <4FDEE0CB.1030505@redhat.com> <87zk81x7dp.fsf@rustcorp.com.au> <4FDF0DA7.40604@redhat.com> <1340019575.22848.2.camel@lappy> <4FDFE926.7030309@redhat.com> <4FE01A5F.6060201@redhat.com> <4FE155B3.8020206@redhat.com> In-Reply-To: <4FE155B3.8020206@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2236 Lines: 55 On 06/20/2012 07:46 AM, Asias He wrote: > On 06/19/2012 02:21 PM, Dor Laor wrote: >> On 06/19/2012 05:51 AM, Asias He wrote: >>> On 06/18/2012 07:39 PM, Sasha Levin wrote: >>>> On Mon, 2012-06-18 at 14:14 +0300, Dor Laor wrote: >>>>> On 06/18/2012 01:05 PM, Rusty Russell wrote: >>>>>> On Mon, 18 Jun 2012 16:03:23 +0800, Asias He wrote: >>>>>>> On 06/18/2012 03:46 PM, Rusty Russell wrote: >>>>>>>> On Mon, 18 Jun 2012 14:53:10 +0800, Asias He >>>>>>>> wrote: >>>>>>>>> This patch introduces bio-based IO path for virtio-blk. >>>>>>>> >>>>>>>> Why make it optional? >>>>>>> >>>>>>> request-based IO path is useful for users who do not want to bypass >>>>>>> the >>>>>>> IO scheduler in guest kernel, e.g. users using spinning disk. For >>>>>>> users >>>>>>> using fast disk device, e.g. SSD device, they can use bio-based IO >>>>>>> path. >>>>>> >>>>>> Users using a spinning disk still get IO scheduling in the host >>>>>> though. >>>>>> What benefit is there in doing it in the guest as well? >>>>> >>>>> The io scheduler waits for requests to merge and thus batch IOs >>>>> together. It's not important w.r.t spinning disks since the host >>>>> can do >>>>> it but it causes much less vmexits which is the key issue for VMs. >>>> >>>> Is the amount of exits caused by virtio-blk significant at all with >>>> EVENT_IDX? >>> >>> Yes. EVENT_IDX saves the number of notify and interrupt. Let's take the >>> interrupt as an example, The guest fires 200K request to host, the >>> number of interrupt is about 6K thanks to EVENT_IDX. The ratio is 200K / >>> 6K = 33. The ratio of merging is 40000K / 200K = 20. >>> >> >> In this case, why don't you always recommend bio over request based? > > This case shows that IO scheduler's merging in guest saves a lot of > requests to host side. Why should I recommend bio over request based here? > Does it merge 20 request _on top_ of what event idx does? Of course if that's the case, we should keep that. Dor -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/