From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH] virtio_blk: merge S/G list entries by default Date: Wed, 10 Sep 2014 17:18:23 +0200 Message-ID: <54106BBF.7010907@redhat.com> References: <1410044994-14943-1-git-send-email-hch@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: 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 To: Ming Lei , Christoph Hellwig Cc: Jens Axboe , "Michael S. Tsirkin" , Linux Kernel Mailing List , Linux Virtualization List-Id: virtualization@lists.linuxfoundation.org Il 07/09/2014 12:32, Ming Lei ha scritto: > It is a good idea to disable SG merge for vq incapable of indirect because > there are very limited direct descriptors. I think you mean _enabling_ SG merge if indirect descriptors are not there. > For vq capable of indirect, it should be better to not do SG merge at default > because: > > - from hypervisor view, no matter how many segments one req has, all are > submitted to host kernel by one syscall, such as readv/io_submit > > - host kernel still need to do the same merge again Here we agree. Paolo > From my test(virtio-blk over null_blk), looks enabling SG merge may cause > throughput a little drop(~3%). From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751706AbaIJPS2 (ORCPT ); Wed, 10 Sep 2014 11:18:28 -0400 Received: from mail-wg0-f51.google.com ([74.125.82.51]:63664 "EHLO mail-wg0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750944AbaIJPS1 (ORCPT ); Wed, 10 Sep 2014 11:18:27 -0400 Message-ID: <54106BBF.7010907@redhat.com> Date: Wed, 10 Sep 2014 17:18:23 +0200 From: Paolo Bonzini User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.0 MIME-Version: 1.0 To: Ming Lei , Christoph Hellwig CC: Jens Axboe , Linux Virtualization , Linux Kernel Mailing List , "Michael S. Tsirkin" Subject: Re: [PATCH] virtio_blk: merge S/G list entries by default References: <1410044994-14943-1-git-send-email-hch@lst.de> In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Il 07/09/2014 12:32, Ming Lei ha scritto: > It is a good idea to disable SG merge for vq incapable of indirect because > there are very limited direct descriptors. I think you mean _enabling_ SG merge if indirect descriptors are not there. > For vq capable of indirect, it should be better to not do SG merge at default > because: > > - from hypervisor view, no matter how many segments one req has, all are > submitted to host kernel by one syscall, such as readv/io_submit > > - host kernel still need to do the same merge again Here we agree. Paolo > From my test(virtio-blk over null_blk), looks enabling SG merge may cause > throughput a little drop(~3%).