From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH V3 4/6] vhost_net: determine whether or not to use zerocopy at one time Date: Sun, 29 Sep 2013 17:36:27 +0800 Message-ID: <5247F49B.2020104@redhat.com> References: <1378111261-14826-1-git-send-email-jasowang@redhat.com> <1378111261-14826-5-git-send-email-jasowang@redhat.com> <20130904115929.GA9393@redhat.com> <5227F274.9040506@redhat.com> <20130923071620.GB31886@redhat.com> <5243B859.3070302@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org To: "Michael S. Tsirkin" Return-path: In-Reply-To: <5243B859.3070302@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: netdev.vger.kernel.org On 09/26/2013 12:30 PM, Jason Wang wrote: > On 09/23/2013 03:16 PM, Michael S. Tsirkin wrote: >> > On Thu, Sep 05, 2013 at 10:54:44AM +0800, Jason Wang wrote: >>>> >> > On 09/04/2013 07:59 PM, Michael S. Tsirkin wrote: >>>>>> >>> > > On Mon, Sep 02, 2013 at 04:40:59PM +0800, Jason Wang wrote: >>>>>>>> >>>> > >> Currently, even if the packet length is smaller than VHOST_GOODCOPY_LEN, if >>>>>>>> >>>> > >> upend_idx != done_idx we still set zcopy_used to true and rollback this choice >>>>>>>> >>>> > >> later. This could be avoided by determining zerocopy once by checking all >>>>>>>> >>>> > >> conditions at one time before. >>>>>>>> >>>> > >> >>>>>>>> >>>> > >> Signed-off-by: Jason Wang >>>>>>>> >>>> > >> --- >>>>>>>> >>>> > >> drivers/vhost/net.c | 47 ++++++++++++++++++++--------------------------- >>>>>>>> >>>> > >> 1 files changed, 20 insertions(+), 27 deletions(-) >>>>>>>> >>>> > >> >>>>>>>> >>>> > >> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c >>>>>>>> >>>> > >> index 8a6dd0d..3f89dea 100644 >>>>>>>> >>>> > >> --- a/drivers/vhost/net.c >>>>>>>> >>>> > >> +++ b/drivers/vhost/net.c >>>>>>>> >>>> > >> @@ -404,43 +404,36 @@ static void handle_tx(struct vhost_net *net) >>>>>>>> >>>> > >> iov_length(nvq->hdr, s), hdr_size); >>>>>>>> >>>> > >> break; >>>>>>>> >>>> > >> } >>>>>>>> >>>> > >> - zcopy_used = zcopy && (len >= VHOST_GOODCOPY_LEN || >>>>>>>> >>>> > >> - nvq->upend_idx != nvq->done_idx); >>>>>>>> >>>> > >> + >>>>>>>> >>>> > >> + zcopy_used = zcopy && len >= VHOST_GOODCOPY_LEN >>>>>>>> >>>> > >> + && (nvq->upend_idx + 1) % UIO_MAXIOV != >>>>>>>> >>>> > >> + nvq->done_idx >>>>>> >>> > > Thinking about this, this looks strange. >>>>>> >>> > > The original idea was that once we start doing zcopy, we keep >>>>>> >>> > > using the heads ring even for short packets until no zcopy is outstanding. >>>> >> > >>>> >> > What's the reason for keep using the heads ring? >> > To keep completions in order. > Ok, I will do some test to see the impact. Since the our of order completion will happen when switching between zero copy and non zero copy. I test this by using two sessions of netperf in burst mode, one with 1 byte TCP_RR another with 512 bytes of TCP_RR. There's no difference with the patch applied.