Subject: Re: [PATCH V3 4/6] vhost_net: determine whether or not to use zerocopy at one time

From: Jason Wang <[email-masked]>
Date: Sun, 29 Sep 2013 17:36:27 +0800

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 <[email-masked]>
>>>>>>>> >>>> > >> ---
>>>>>>>> >>>> > >>  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.