From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753778Ab2FSGV3 (ORCPT ); Tue, 19 Jun 2012 02:21:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:29462 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753271Ab2FSGV1 (ORCPT ); Tue, 19 Jun 2012 02:21:27 -0400 Message-ID: <4FE01A5F.6060201@redhat.com> Date: Tue, 19 Jun 2012 09:21:19 +0300 From: Dor Laor Reply-To: dlaor@redhat.com User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1 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> In-Reply-To: <4FDFE926.7030309@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 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? From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dor Laor Subject: Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk Date: Tue, 19 Jun 2012 09:21:19 +0300 Message-ID: <4FE01A5F.6060201@redhat.com> 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> Reply-To: dlaor@redhat.com Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Sasha Levin , Christoph Hellwig To: Asias He Return-path: In-Reply-To: <4FDFE926.7030309@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org List-Id: kvm.vger.kernel.org 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?