From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by dpdk.org (Postfix) with ESMTP id CF5421AFF0; Wed, 16 May 2018 17:00:54 +0200 (CEST) Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 58BA85BCB6; Wed, 16 May 2018 15:00:54 +0000 (UTC) Received: from [10.36.112.25] (ovpn-112-25.ams2.redhat.com [10.36.112.25]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EC6E8215CDA7; Wed, 16 May 2018 15:00:51 +0000 (UTC) To: Tiwei Bie Cc: dev@dpdk.org, jianfeng.tan@intel.com, mst@redhat.com, stable@dpdk.org References: <20180430155954.9939-1-maxime.coquelin@redhat.com> <20180503115634.feaimkzpnbodferd@debian> <20180516061021.GA25485@debian> From: Maxime Coquelin Message-ID: Date: Wed, 16 May 2018 17:00:50 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20180516061021.GA25485@debian> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.78 on 10.11.54.6 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.1]); Wed, 16 May 2018 15:00:54 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.1]); Wed, 16 May 2018 15:00:54 +0000 (UTC) for IP:'10.11.54.6' DOMAIN:'int-mx06.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'maxime.coquelin@redhat.com' RCPT:'' Subject: Re: [dpdk-dev] [PATCH] vhost: improve dirty pages logging performance X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 16 May 2018 15:00:55 -0000 On 05/16/2018 08:10 AM, Tiwei Bie wrote: > On Tue, May 15, 2018 at 03:50:54PM +0200, Maxime Coquelin wrote: >> Hi Tiwei, >> >> I just see I missed to reply to your comment on my commit message: >> >> On 05/03/2018 01:56 PM, Tiwei Bie wrote: >>> On Mon, Apr 30, 2018 at 05:59:54PM +0200, Maxime Coquelin wrote: >>>> This patch caches all dirty pages logging until the used ring index >>>> is updated. These dirty pages won't be accessed by the guest as >>>> long as the host doesn't give them back to it by updating the >>>> index. >>> Below sentence in above commit message isn't the reason why >>> we can cache the dirty page logging. Right? >>> >>> """ >>> These dirty pages won't be accessed by the guest as >>> long as the host doesn't give them back to it by updating the >>> index. >> >> That's my understanding. >> As long as the used index is not updated, the guest will not process >> the descs. >> If the migration converges between the time the descs are written, >> and the time the used index is updated on source side. Then the guest >> running on destination will not see the descriptors as used but as >> available, and so will be overwritten by the vhost backend on >> destination. > > If my understanding is correct, theoretically the vhost > backend can cache all the dirty page loggings before it > responds to the GET_VRING_BASE messages. Below are the > steps how QEMU live migration works (w/o postcopy): > > 1. Syncing dirty pages between src and dst; > 2. The dirty page sync converges; > 3. The src QEMU sends GET_VRING_BASE to vhost backend; > 4. Vhost backend still has a chance to log some dirty > pages before responding the GET_VRING_BASE messages; > 5. The src QEMU receives GET_VRING_BASE response (which > means the device has stopped); > 6. QEMU sync the remaining dirty pages; > 7. QEMU on the dst starts running. Thanks for the clarification. > (The steps 3~6 are the downtime which we want to minimize) Right, we want to minimize the downtime, but in the same time, be able to converge. > So I think the words in commit log isn't really related > to why we can cache the dirty page loggings. I'll try to improve it in v3. Thanks, Maxime > Best regards, > Tiwei Bie > >> >> Regards, >> Maxime